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\input texinfo   @c -*-texinfo-*-
@c %**start of header

@c oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
@c                                                                            o
@c                            GNAT DOCUMENTATION                              o
@c                                                                            o
@c                              G N A T _ U G                                 o
@c                                                                            o
@c          Copyright (C) 1992-2002 Ada Core Technologies, Inc.               o
@c                                                                            o
@c  GNAT is free software;  you can  redistribute it  and/or modify it under  o
@c  terms of the  GNU General Public License as published  by the Free Soft-  o
@c  ware  Foundation;  either version 2,  or (at your option) any later ver-  o
@c  sion.  GNAT is distributed in the hope that it will be useful, but WITH-  o
@c  OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY  o
@c  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License  o
@c  for  more details.  You should have  received  a copy of the GNU General  o
@c  Public License  distributed with GNAT;  see file COPYING.  If not, write  o
@c  to  the Free Software Foundation,  59 Temple Place - Suite 330,  Boston,  o
@c  MA 02111-1307, USA.                                                       o
@c                                                                            o
@c  GNAT is maintained by Ada Core Technologies Inc (http://www.gnat.com).    o
@c                                                                            o
@c oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

@c oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
@c
@c                           GNAT_UG Style Guide
@c
@c  1. Always put a @noindent on the line before the first paragraph
@c     after any of these commands:
@c
@c          @chapter
@c          @section
@c          @subsection
@c          @subsubsection
@c          @subsubsubsection
@c
@c          @end smallexample
@c          @end itemize
@c          @end enumerate
@c
@c  2. DO NOT use @example. Use @smallexample instead.
@c
@c  3. Each @chapter, @section, @subsection, @subsubsection, etc.
@c     command must be preceded by two empty lines
@c
@c  4. The @item command must be on a line of its own if it is in an
@c     @itemize or @enumerate command.
@c
@c  5. When talking about ALI files use "ALI" (all uppercase), not "Ali"
@c     or "ali".
@c
@c oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

@setfilename gnat_ug_vms.info
@settitle GNAT User's Guide for OpenVMS Alpha




@include gcc-common.texi

@setchapternewpage odd
@syncodeindex fn cp
@c %**end of header

@titlepage

@title GNAT User's Guide
@center @titlefont{for OpenVMS Alpha}




@subtitle GNAT, The GNU Ada 95 Compiler
@subtitle GNAT Version for GCC @value{version-GCC}

@author Ada Core Technologies, Inc.

@page
@vskip 0pt plus 1filll

Copyright @copyright{} 1995-2002, Free Software Foundation

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being ``GNU Free Documentation License'', with the
Front-Cover Texts being
``GNAT User's Guide for OpenVMS Alpha'',
and with no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

@end titlepage

@ifnottex
@node Top, About This Guide, (dir), (dir)
@top GNAT User's Guide

GNAT User's Guide for OpenVMS Alpha




GNAT, The GNU Ada 95 Compiler

GNAT Version for GCC @value{version-GCC}

Ada Core Technologies, Inc.

Copyright @copyright{} 1995-2002, Free Software Foundation

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being ``GNU Free Documentation License'', with the
Front-Cover Texts being
``GNAT User's Guide for OpenVMS Alpha'',
and with no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

@menu
* About This Guide::
* Getting Started with GNAT::
* The GNAT Compilation Model::
* Compiling Using GNAT COMPILE::
* Binding Using GNAT BIND::
* Linking Using GNAT LINK::
* The GNAT Make Program GNAT MAKE::
* Renaming Files Using GNAT CHOP::
* Configuration Pragmas::
* Handling Arbitrary File Naming Conventions Using gnatname::
* GNAT Project Manager::
* Elaboration Order Handling in GNAT::
* The Cross-Referencing Tools GNAT XREF and GNAT FIND::
* File Name Krunching Using GNAT KRUNCH::
* Preprocessing Using GNAT PREPROCESS::
* The GNAT Run-Time Library Builder GNAT LIBRARY::
* The GNAT Library Browser GNAT LIST::
* Finding Memory Problems with GNAT Debug Pool::
* Creating Sample Bodies Using GNAT STUB::
* Reducing the Size of Ada Executables with GNAT ELIM::
* Other Utility Programs::
* Compatibility with DEC Ada::
* Running and Debugging Ada Programs::
* Inline Assembler::
* Performance Considerations::
* GNU Free Documentation License::
* Index::

 --- The Detailed Node Listing ---

About This Guide

* What This Guide Contains::
* What You Should Know before Reading This Guide::
* Related Information::
* Conventions::


Getting Started with GNAT

* Running GNAT::
* Running a Simple Ada Program::
* Running a Program with Multiple Units::
* Using the GNAT MAKE Utility::
* Editing with EMACS::

The GNAT Compilation Model

* Source Representation::
* Foreign Language Representation::
* File Naming Rules::
* Using Other File Names::
* Alternative File Naming Schemes::
* Generating Object Files::
* Source Dependencies::
* The Ada Library Information Files::
* Binding an Ada Program::
* Mixed Language Programming::
* Building Mixed Ada & C++ Programs::
* Comparison between GNAT and C/C++ Compilation Models::
* Comparison between GNAT and Conventional Ada Library Models::

Foreign Language Representation

* Latin-1::
* Other 8-Bit Codes::
* Wide Character Encodings::

Compiling Ada Programs With GNAT COMPILE

* Compiling Programs::
* Qualifiers for GNAT COMPILE::
* Search Paths and the Run-Time Library (RTL)::
* Order of Compilation Issues::
* Examples::

Qualifiers for GNAT COMPILE

* Output and Error Message Control::
* Debugging and Assertion Control::
* Run-Time Checks::
* Stack Overflow Checking::
* Run-Time Control::
* Validity Checking::
* Style Checking::
* Using GNAT COMPILE for Syntax Checking::
* Using GNAT COMPILE for Semantic Checking::
* Compiling Ada 83 Programs::
* Character Set Control::
* File Naming Control::
* Subprogram Inlining Control::
* Auxiliary Output Control::
* Debugging Control::
* Units to Sources Mapping Files::

Binding Ada Programs With GNAT BIND

* Running GNAT BIND::
* Generating the Binder Program in C::
* Consistency-Checking Modes::
* Binder Error Message Control::
* Elaboration Control::
* Output Control::
* Binding with Non-Ada Main Programs::
* Binding Programs with No Main Subprogram::
* Summary of Binder Qualifiers::
* Command-Line Access::
* Search Paths for GNAT BIND::
* Examples of GNAT BIND Usage::

Linking Using GNAT LINK

* Running GNAT LINK::
* Qualifiers for GNAT LINK::
* Setting Stack Size from GNAT LINK::
* Setting Heap Size from GNAT LINK::

The GNAT Make Program GNAT MAKE

* Running GNAT MAKE::
* Qualifiers for GNAT MAKE::
* Mode Qualifiers for GNAT MAKE::
* Notes on the Command Line::
* How GNAT MAKE Works::
* Examples of GNAT MAKE Usage::

Renaming Files Using GNAT CHOP

* Handling Files with Multiple Units::
* Operating GNAT CHOP in Compilation Mode::
* Command Line for GNAT CHOP::
* Qualifiers for GNAT CHOP::
* Examples of GNAT CHOP Usage::

Configuration Pragmas

* Handling of Configuration Pragmas::
* The Configuration Pragmas Files::

Handling Arbitrary File Naming Conventions Using gnatname

* Arbitrary File Naming Conventions::
* Running gnatname::
* Qualifiers for gnatname::
* Examples of gnatname Usage::

GNAT Project Manager

* Introduction::
* Examples of Project Files::
* Project File Syntax::
* Objects and Sources in Project Files::
* Importing Projects::
* Project Extension::
* External References in Project Files::
* Packages in Project Files::
* Variables from Imported Projects::
* Naming Schemes::
* Library Projects::
* Qualifiers Related to Project Files::
* Tools Supporting Project Files::
* An Extended Example::
* Project File Complete Syntax::

Elaboration Order Handling in GNAT

* Elaboration Code in Ada 95::
* Checking the Elaboration Order in Ada 95::
* Controlling the Elaboration Order in Ada 95::
* Controlling Elaboration in GNAT - Internal Calls::
* Controlling Elaboration in GNAT - External Calls::
* Default Behavior in GNAT - Ensuring Safety::
* Elaboration Issues for Library Tasks::
* Mixing Elaboration Models::
* What to Do If the Default Elaboration Behavior Fails::
* Elaboration for Access-to-Subprogram Values::
* Summary of Procedures for Elaboration Control::
* Other Elaboration Order Considerations::

The Cross-Referencing Tools GNAT XREF and GNAT FIND

* GNAT XREF Qualifiers::
* GNAT FIND Qualifiers::
* Project Files for GNAT XREF and GNAT FIND::
* Regular Expressions in GNAT FIND and GNAT XREF::
* Examples of GNAT XREF Usage::
* Examples of GNAT FIND Usage::

File Name Krunching Using GNAT KRUNCH

* About GNAT KRUNCH::
* Using GNAT KRUNCH::
* Krunching Method::
* Examples of GNAT KRUNCH Usage::

Preprocessing Using GNAT PREPROCESS

* Using GNAT PREPROCESS::
* Qualifiers for GNAT PREPROCESS::
* Form of Definitions File::
* Form of Input Text for GNAT PREPROCESS::

The GNAT Run-Time Library Builder GNAT LIBRARY

* Running GNAT LIBRARY::
* Qualifiers for GNAT LIBRARY::
* Examples of GNAT LIBRARY Usage::

The GNAT Library Browser GNAT LIST

* Running GNAT LIST::
* Qualifiers for GNAT LIST::
* Examples of GNAT LIST Usage::


Finding Memory Problems with GNAT Debug Pool

Creating Sample Bodies Using GNAT STUB

* Running GNAT STUB::
* Qualifiers for GNAT STUB::

Reducing the Size of Ada Executables with GNAT ELIM

* About GNAT ELIM::
* Eliminate Pragma::
* Tree Files::
* Preparing Tree and Bind Files for GNAT ELIM::
* Running GNAT ELIM::
* Correcting the List of Eliminate Pragmas::
* Making Your Executables Smaller::
* Summary of the GNAT ELIM Usage Cycle::

Other Utility Programs

* Using Other Utility Programs with GNAT::
* The GNAT STANDARD Utility Program::
* The External Symbol Naming Scheme of GNAT::
* Ada Mode for Glide::
* Converting Ada Files to html with gnathtml::
* LSE::

Compatibility with DEC Ada

* Ada 95 Compatibility::
* Differences in the Definition of Package System::
* Language-Related Features::
* The Package STANDARD::
* The Package SYSTEM::
* Tasking and Task-Related Features::
* Implementation of Tasks in DEC Ada for OpenVMS Alpha Systems::
* Pragmas and Pragma-Related Features::
* Library of Predefined Units::
* Bindings::
* Main Program Definition::
* Implementation-Defined Attributes::
* Compiler and Run-Time Interfacing::
* Program Compilation and Library Management::
* Input-Output::
* Implementation Limits::
* Tools::

Language-Related Features

* Integer Types and Representations::
* Floating-Point Types and Representations::
* Pragmas Float_Representation and Long_Float::
* Fixed-Point Types and Representations::
* Record and Array Component Alignment::
* Address Clauses::
* Other Representation Clauses::

Implementation of Tasks in DEC Ada for OpenVMS Alpha Systems

* Assigning Task IDs::
* Task IDs and Delays::
* Task-Related Pragmas::
* Scheduling and Task Priority::
* The Task Stack::
* External Interrupts::

Pragmas and Pragma-Related Features

* Restrictions on the Pragma INLINE::
* Restrictions on the Pragma INTERFACE::
* Restrictions on the Pragma SYSTEM_NAME::

Library of Predefined Units

* Changes to DECLIB::

Bindings

* Shared Libraries and Options Files::
* Interfaces to C::

Running and Debugging Ada Programs

* The GNAT Debugger GDB::
* Running GDB::
* Introduction to GDB Commands::
* Using Ada Expressions::
* Calling User-Defined Subprograms::
* Using the Next Command in a Function::
* Ada Exceptions::
* Ada Tasks::
* Debugging Generic Units::
* GNAT Abnormal Termination or Failure to Terminate::
* Naming Conventions for GNAT Source Files::
* Getting Internal Debugging Information::
* Stack Traceback::

Inline Assembler

* Basic Assembler Syntax::
* A Simple Example of Inline Assembler::
* Output Variables in Inline Assembler::
* Input Variables in Inline Assembler::
* Inlining Inline Assembler Code::
* Other Asm Functionality::
* A Complete Example::



Performance Considerations

* Controlling Run-Time Checks::
* Optimization Levels::
* Debugging Optimized Code::
* Inlining of Subprograms::
* Coverage Analysis::

* Index::
@end menu
@end ifnottex

@node About This Guide
@unnumbered About This Guide

@noindent
This guide describes the use of of GNAT, a full language compiler for the Ada
95 programming language, implemented on DIGITAL OpenVMS Alpha Systems.
It describes the features of the compiler and tools, and details
how to use them to build Ada 95 applications.

@menu
* What This Guide Contains::
* What You Should Know before Reading This Guide::
* Related Information::
* Conventions::
@end menu

@node What This Guide Contains
@unnumberedsec What This Guide Contains

@noindent
This guide contains the following chapters:
@itemize @bullet
@item
@ref{Getting Started with GNAT}, describes how to get started compiling
and running Ada programs with the GNAT Ada programming environment.
@item
@ref{The GNAT Compilation Model}, describes the compilation model used
by GNAT.
@item
@ref{Compiling Using GNAT COMPILE}, describes how to compile
Ada programs with @code{GNAT COMPILE}, the Ada compiler.
@item
@ref{Binding Using GNAT BIND}, describes how to
perform binding of Ada programs with @code{GNAT BIND}, the GNAT binding
utility.
@item
@ref{Linking Using GNAT LINK},
describes @code{GNAT LINK}, a
program that provides for linking using the GNAT run-time library to
construct a program. @code{GNAT LINK} can also incorporate foreign language
object units into the executable.
@item
@ref{The GNAT Make Program GNAT MAKE}, describes @code{GNAT MAKE}, a
utility that automatically determines the set of sources
needed by an Ada compilation unit, and executes the necessary compilations
binding and link.
@item
@ref{Renaming Files Using GNAT CHOP}, describes
@code{GNAT CHOP}, a utility that allows you to preprocess a file that
contains Ada source code, and split it into one or more new files, one
for each compilation unit.
@item
@ref{Configuration Pragmas}, describes the configuration pragmas handled by GNAT.
@item
@ref{Handling Arbitrary File Naming Conventions Using gnatname}, shows how to override
the default GNAT file naming conventions, either for an individual unit or globally.
@item
@ref{GNAT Project Manager}, describes how to use project files to organize large projects.
@item
@ref{Elaboration Order Handling in GNAT}, describes how GNAT helps you deal with
elaboration order issues.
@item
@ref{The Cross-Referencing Tools GNAT XREF and GNAT FIND}, discusses
@code{GNAT XREF} and @code{GNAT FIND}, two tools that provide an easy
way to navigate through sources.
@item
@ref{File Name Krunching Using GNAT KRUNCH}, describes the @code{GNAT KRUNCH}
file name krunching utility, used to handle shortened
file names on operating systems with a limit on the length of names.
@item
@ref{Preprocessing Using GNAT PREPROCESS}, describes @code{GNAT PREPROCESS}, a
preprocessor utility that allows a single source file to be used to
generate multiple or parameterized source files, by means of macro
substitution.
@item
@ref{The GNAT Library Browser GNAT LIST}, describes @code{GNAT LIST}, a
utility that displays information about compiled units, including dependences
on the corresponding sources files, and consistency of compilations.
@item
@ref{Finding Memory Problems with GNAT Debug Pool}, describes how to
use the GNAT-specific Debug Pool in order to detect as early as possible
the use of incorrect memory references.

@item
@ref{Creating Sample Bodies Using GNAT STUB}, discusses @code{GNAT STUB},
a utility that generates empty but compilable bodies for library units.

@item
@ref{Reducing the Size of Ada Executables with GNAT ELIM}, describes
@code{GNAT ELIM}, a tool which detects unused subprograms and helps
the compiler to create a smaller executable for the program.

@item
@ref{Other Utility Programs}, discusses several other GNAT utilities,
including @code{GNAT STANDARD}.

@item
@ref{Running and Debugging Ada Programs}, describes how to run and debug
Ada programs.

@item
@ref{Inline Assembler}, shows how to use the inline assembly facility in an Ada program.


@item
@ref{Performance Considerations}, reviews the trade offs between using
defaults or options in program development.
@item
@ref{Compatibility with DEC Ada}, details the compatibility of GNAT with
DEC Ada 83 for OpenVMS Alpha.
@end itemize

@node What You Should Know before Reading This Guide
@unnumberedsec What You Should Know before Reading This Guide

@cindex Ada 95 Language Reference Manual
@noindent
This user's guide assumes that you are familiar with Ada 95 language, as
described in the International Standard ANSI/ISO/IEC-8652:1995, Jan
1995.

@node Related Information
@unnumberedsec Related Information

@noindent
For further information about related tools, refer to the following
documents:

@itemize @bullet
@item
@cite{GNAT Reference Manual}, which contains all reference
material for the GNAT implementation of Ada 95.

@item
@cite{Ada 95 Language Reference Manual}, which contains all reference
material for the Ada 95 programming language.

@item
@cite{Debugging with GDB}
, located in the GNU:[DOCS] directory,
contains all details on the use of the GNU source-level debugger.

@item
@cite{GNU EMACS Manual}
, located in the GNU:[DOCS] directory if the EMACS kit is installed,
contains full information on the extensible editor and programming
environment EMACS.

@end itemize

@node Conventions
@unnumberedsec Conventions
@cindex Conventions
@cindex Typographical conventions

@noindent
Following are examples of the typographical and graphic conventions used
in this guide:

@itemize @bullet
@item
@code{Functions}, @code{utility program names}, @code{standard names},
and @code{classes}.

@item
@samp{Option flags}

@item
@file{File Names}, @file{button names}, and @file{field names}.

@item
@var{Variables}.

@item
@emph{Emphasis}.

@item
[optional information or parameters]

@item
Examples are described by text
@smallexample
and then shown this way.
@end smallexample
@end itemize

@noindent
Commands that are entered by the user are preceded in this manual by the
characters @w{"@code{$ }"} (dollar sign followed by space). If your system
uses this sequence as a prompt, then the commands will appear exactly as
you see them in the manual. If your system uses some other prompt, then
the command will appear with the @code{$} replaced by whatever prompt
character you are using.


@node Getting Started with GNAT
@chapter Getting Started with GNAT

@noindent
This chapter describes some simple ways of using GNAT to build
executable Ada programs.

@menu
* Running GNAT::
* Running a Simple Ada Program::

* Running a Program with Multiple Units::

* Using the GNAT MAKE Utility::
* Editing with EMACS::
@end menu

@node Running GNAT
@section Running GNAT

@noindent
Three steps are needed to create an executable file from an Ada source
file:

@enumerate
@item
The source file(s) must be compiled.
@item
The file(s) must be bound using the GNAT binder.
@item
All appropriate object files must be linked to produce an executable.
@end enumerate

@noindent
All three steps are most commonly handled by using the @code{GNAT MAKE}
utility program that, given the name of the main program, automatically
performs the necessary compilation, binding and linking steps.

@node Running a Simple Ada Program
@section Running a Simple Ada Program

@noindent
Any text editor may be used to prepare an Ada program. If @code{Glide} is
used, the optional Ada mode may be helpful in laying out the program. The
program text is a normal text file. We will suppose in our initial
example that you have used your editor to prepare the following
standard format text file:

@smallexample
@group
@cartouche
@b{with} Ada.Text_IO; @b{use} Ada.Text_IO;
@b{procedure} Hello @b{is}
@b{begin}
   Put_Line ("Hello WORLD!");
@b{end} Hello;
@end cartouche
@end group
@end smallexample

@noindent
This file should be named @file{HELLO.ADB}.
With the normal default file naming conventions, GNAT requires
that each file
contain a single compilation unit whose file name is the
unit name,
with periods replaced by hyphens; the
extension is @file{ads} for a
spec and @file{adb} for a body.
You can override this default file naming convention by use of the
special pragma @code{Source_File_Name} (@pxref{Using Other File Names}).
Alternatively, if you want to rename your files according to this default
convention, which is probably more convenient if you will be using GNAT
for all your compilations, then the @code{GNAT CHOP} utility
can be used to generate correctly-named source files
(@pxref{Renaming Files Using GNAT CHOP}).

You can compile the program using the following command (@code{$} is used
as the command prompt in the examples in this document):

@smallexample
$ GNAT COMPILE HELLO.ADB
@end smallexample


@noindent
@code{GNAT COMPILE} is the command used to run the compiler. This compiler is
capable of compiling programs in several languages, including Ada 95 and
C. It assumes that you have given it an Ada program if the file extension is
either @file{.ADS} or @file{.ADB}, and it will then call the GNAT compiler to compile
the specified file.


This compile command generates a file
@file{HELLO.OBJ}, which is the object
file corresponding to your Ada program. It also generates an "Ada Library Information" file
@file{HELLO.ALI},
which contains additional information used to check
that an Ada program is consistent.
To build an executable file,
use @code{GNAT BIND} to bind the program
and @code{GNAT LINK} to link it. The
argument to both @code{GNAT BIND} and @code{GNAT LINK} is the name of the
@file{ali} file, but the default extension of @file{.ALI} can
be omitted. This means that in the most common case, the argument
is simply the name of the main program:

@smallexample
$ GNAT BIND hello
$ GNAT LINK hello
@end smallexample


@noindent
A simpler method of carrying out these steps is to use
@command{GNAT MAKE},
a master program that invokes all the required
compilation, binding and linking tools in the correct order. In particular,
@command{GNAT MAKE} automatically recompiles any sources that have been modified
since they were last compiled, or sources that depend
on such modified sources, so that "version skew" is avoided.
@cindex Version skew (avoided by @command{GNAT MAKE})

@smallexample
$ GNAT MAKE HELLO.ADB
@end smallexample


@noindent
The result is an executable program called @file{hello}, which can be
run by entering:

@c The following should be removed (BMB 2001-01-23)
@c @smallexample
@c $ $ RUN HELLO
@c @end smallexample

@smallexample
$ hello
@end smallexample

@noindent
assuming that the current directory is on the search path for executable programs.

@noindent
and, if all has gone well, you will see

@smallexample
Hello WORLD!
@end smallexample

@noindent
appear in response to this command.




@node Running a Program with Multiple Units
@section Running a Program with Multiple Units

@noindent
Consider a slightly more complicated example that has three files: a
main program, and the spec and body of a package:

@smallexample
@cartouche
@group
@b{package} Greetings @b{is}
   @b{procedure} Hello;
   @b{procedure} Goodbye;
@b{end} Greetings;

@b{with} Ada.Text_IO; @b{use} Ada.Text_IO;
@b{package} @b{body} Greetings @b{is}
   @b{procedure} Hello @b{is}
   @b{begin}
      Put_Line ("Hello WORLD!");
   @b{end} Hello;

   @b{procedure} Goodbye @b{is}
   @b{begin}
      Put_Line ("Goodbye WORLD!");
   @b{end} Goodbye;
@b{end} Greetings;
@end group

@group
@b{with} Greetings;
@b{procedure} Gmain @b{is}
@b{begin}
   Greetings.Hello;
   Greetings.Goodbye;
@b{end} Gmain;
@end group
@end cartouche
@end smallexample

@noindent
Following the one-unit-per-file rule, place this program in the
following three separate files:

@table @file
@item GREETINGS.ADS
spec of package @code{Greetings}

@item GREETINGS.ADB
body of package @code{Greetings}

@item GMAIN.ADB
body of main program
@end table

@noindent
To build an executable version of
this program, we could use four separate steps to compile, bind, and link
the program, as follows:

@smallexample
$ GNAT COMPILE GMAIN.ADB
$ GNAT COMPILE GREETINGS.ADB
$ GNAT BIND gmain
$ GNAT LINK gmain
@end smallexample


@noindent
Note that there is no required order of compilation when using GNAT.
In particular it is perfectly fine to compile the main program first.
Also, it is not necessary to compile package specs in the case where
there is an accompanying body; you only need to compile the body. If you want
to submit these files to the compiler for semantic checking and not code generation,
then use the
@option{/NOLOAD} qualifier:

@smallexample
   $ GNAT COMPILE GREETINGS.ADS /NOLOAD
@end smallexample


@noindent
Although the compilation can be done in separate steps as in the
above example, in practice it is almost always more convenient
to use the @code{GNAT MAKE} tool. All you need to know in this case
is the name of the main program's source file. The effect of the above four
commands can be achieved with a single one:

@smallexample
$ GNAT MAKE GMAIN.ADB
@end smallexample


@noindent
In the next section we discuss the advantages of using @code{GNAT MAKE} in
more detail.

@node Using the GNAT MAKE Utility
@section Using the @command{GNAT MAKE} Utility

@noindent
If you work on a program by compiling single components at a time using
@code{GNAT COMPILE}, you typically keep track of the units you modify. In order to
build a consistent system, you compile not only these units, but also any
units that depend on the units you have modified.
For example, in the preceding case,
if you edit @file{GMAIN.ADB}, you only need to recompile that file. But if
you edit @file{GREETINGS.ADS}, you must recompile both
@file{GREETINGS.ADB} and @file{GMAIN.ADB}, because both files contain
units that depend on @file{GREETINGS.ADS}.

@code{GNAT BIND} will warn you if you forget one of these compilation
steps, so that it is impossible to generate an inconsistent program as a
result of forgetting to do a compilation. Nevertheless it is tedious and
error-prone to keep track of dependencies among units.
One approach to handle the dependency-bookkeeping is to use a
makefile. However, makefiles present maintenance problems of their own:
if the dependencies change as you change the program, you must make
sure that the makefile is kept up-to-date manually, which is also an
error-prone process.

The @code{GNAT MAKE} utility takes care of these details automatically.
Invoke it using either one of the following forms:

@smallexample
$ GNAT MAKE GMAIN.ADB
$ GNAT MAKE GMAIN
@end smallexample


@noindent
The argument is the name of the file containing the main program;
you may omit the extension. @code{GNAT MAKE}
examines the environment, automatically recompiles any files that need
recompiling, and binds and links the resulting set of object files,
generating the executable file, @file{GMAIN.EXE}.
In a large program, it
can be extremely helpful to use @code{GNAT MAKE}, because working out by hand
what needs to be recompiled can be difficult.

Note that @code{GNAT MAKE}
takes into account all the Ada 95 rules that
establish dependencies among units. These include dependencies that result
from inlining subprogram bodies, and from
generic instantiation. Unlike some other
Ada make tools, @code{GNAT MAKE} does not rely on the dependencies that were
found by the compiler on a previous compilation, which may possibly
be wrong when sources change. @code{GNAT MAKE} determines the exact set of
dependencies from scratch each time it is run.

@node Editing with EMACS
@section Editing with EMACS
@cindex EMACS

@noindent
EMACS is an extensible self-documenting text editor that is available in a
separate VMSINSTAL kit.

Invoke EMACS by typing "EMACS" at the command prompt. To get started,
click on the EMACS Help menu and run the EMACS Tutorial.
In a character cell terminal, EMACS help is invoked with "Ctrl-h" (also written
as "C-h"), and the tutorial by "C-h t".

Documentation on EMACS and other tools is available in EMACS under the
pull-down menu button: Help - Info. After selecting Info, use the middle
mouse button to select a topic (e.g. EMACS).

In a character cell terminal, do "C-h i" to invoke info, and then "m"
(stands for menu) followed by the menu item desired, as in "m EMACS", to get
to the EMACS manual.
Help on EMACS is also available by typing "HELP EMACS" at the DCL command
prompt.

The tutorial is highly recommended in order to learn the intricacies of EMACS,
which is sufficiently extensible to provide for a complete programming
environment and shell for the sophisticated user.


@node The GNAT Compilation Model
@chapter The GNAT Compilation Model
@cindex GNAT compilation model
@cindex Compilation model

@menu
* Source Representation::
* Foreign Language Representation::
* File Naming Rules::
* Using Other File Names::
* Alternative File Naming Schemes::
* Generating Object Files::
* Source Dependencies::
* The Ada Library Information Files::
* Binding an Ada Program::
* Mixed Language Programming::
* Building Mixed Ada & C++ Programs::
* Comparison between GNAT and C/C++ Compilation Models::
* Comparison between GNAT and Conventional Ada Library Models::
@end menu

@noindent
This chapter describes the compilation model used by GNAT. Although
similar to that used by other languages, such as C and C++, this model
is substantially different from the traditional Ada compilation models,
which are based on a library. The model is initially described without
reference to the library-based model. If you have not previously used an
Ada compiler, you need only read the first part of this chapter. The
last section describes and discusses the differences between the GNAT
model and the traditional Ada compiler models. If you have used other
Ada compilers, this section will help you to understand those
differences, and the advantages of the GNAT model.

@node Source Representation
@section Source Representation
@cindex Latin-1

@noindent
Ada source programs are represented in standard text files, using
Latin-1 coding. Latin-1 is an 8-bit code that includes the familiar
7-bit ASCII set, plus additional characters used for
representing foreign languages (@pxref{Foreign Language Representation}
for support of non-USA character sets). The format effector characters
are represented using their standard ASCII encodings, as follows:

@table @code
@item VT
@findex VT
Vertical tab, @code{16#0B#}

@item HT
@findex HT
Horizontal tab, @code{16#09#}

@item CR
@findex CR
Carriage return, @code{16#0D#}

@item LF
@findex LF
Line feed, @code{16#0A#}

@item FF
@findex FF
Form feed, @code{16#0C#}
@end table

@noindent
Source files are in standard text file format. In addition, GNAT will
recognize a wide variety of stream formats, in which the end of physical
physical lines is marked by any of the following sequences:
@code{LF}, @code{CR}, @code{CR-LF}, or @code{LF-CR}. This is useful
in accommodating files that are imported from other operating systems.

@cindex End of source file
@cindex Source file, end
@findex SUB
The end of a source file is normally represented by the physical end of
file. However, the control character @code{16#1A#} (@code{SUB}) is also
recognized as signalling the end of the source file. Again, this is
provided for compatibility with other operating systems where this
code is used to represent the end of file.

Each file contains a single Ada compilation unit, including any pragmas
associated with the unit. For example, this means you must place a
package declaration (a package @dfn{spec}) and the corresponding body in
separate files. An Ada @dfn{compilation} (which is a sequence of
compilation units) is represented using a sequence of files. Similarly,
you will place each subunit or child unit in a separate file.

@node Foreign Language Representation
@section Foreign Language Representation

@noindent
GNAT supports the standard character sets defined in Ada 95 as well as
several other non-standard character sets for use in localized versions
of the compiler (@pxref{Character Set Control}).
@menu
* Latin-1::
* Other 8-Bit Codes::
* Wide Character Encodings::
@end menu

@node Latin-1
@subsection Latin-1
@cindex Latin-1

@noindent
The basic character set is Latin-1. This character set is defined by ISO
standard 8859, part 1. The lower half (character codes @code{16#00#}
... @code{16#7F#)} is identical to standard ASCII coding, but the upper half is
used to represent additional characters. These include extended letters
used by European languages, such as French accents, the vowels with umlauts
used in German, and the extra letter A-ring used in Swedish.

@findex Ada.Characters.Latin_1
For a complete list of Latin-1 codes and their encodings, see the source
file of library unit @code{Ada.Characters.Latin_1} in file
@file{A-CHLAT1.ADS}.
You may use any of these extended characters freely in character or
string literals. In addition, the extended characters that represent
letters can be used in identifiers.

@node Other 8-Bit Codes
@subsection Other 8-Bit Codes

@noindent
GNAT also supports several other 8-bit coding schemes:

@table @asis
@cindex Latin-2
@item Latin-2
Latin-2 letters allowed in identifiers, with uppercase and lowercase
equivalence.

@item Latin-3
@cindex Latin-3
Latin-3 letters allowed in identifiers, with uppercase and lowercase
equivalence.

@item Latin-4
@cindex Latin-4
Latin-4 letters allowed in identifiers, with uppercase and lowercase
equivalence.

@item Latin-5
@cindex Latin-5
@cindex Cyrillic
Latin-4 letters (Cyrillic) allowed in identifiers, with uppercase and lowercase
equivalence.

@item IBM PC (code page 437)
@cindex code page 437
This code page is the normal default for PCs in the U.S. It corresponds
to the original IBM PC character set. This set has some, but not all, of
the extended Latin-1 letters, but these letters do not have the same
encoding as Latin-1. In this mode, these letters are allowed in
identifiers with uppercase and lowercase equivalence.

@item IBM PC (code page 850)
@cindex code page 850
This code page is a modification of 437 extended to include all the
Latin-1 letters, but still not with the usual Latin-1 encoding. In this
mode, all these letters are allowed in identifiers with uppercase and
lowercase equivalence.

@item Full Upper 8-bit
Any character in the range 80-FF allowed in identifiers, and all are
considered distinct. In other words, there are no uppercase and lowercase
equivalences in this range. This is useful in conjunction with
certain encoding schemes used for some foreign character sets (e.g.
the typical method of representing Chinese characters on the PC).

@item No Upper-Half
No upper-half characters in the range 80-FF are allowed in identifiers.
This gives Ada 83 compatibility for identifier names.
@end table

@noindent
For precise data on the encodings permitted, and the uppercase and lowercase
equivalences that are recognized, see the file @file{CSETS.ADB} in
the GNAT compiler sources. You will need to obtain a full source release
of GNAT to obtain this file.

@node Wide Character Encodings
@subsection Wide Character Encodings

@noindent
GNAT allows wide character codes to appear in character and string
literals, and also optionally in identifiers, by means of the following
possible encoding schemes:

@table @asis

@item Hex Coding
In this encoding, a wide character is represented by the following five
character sequence:

@smallexample
ESC a b c d
@end smallexample

@noindent
Where @code{a}, @code{b}, @code{c}, @code{d} are the four hexadecimal
characters (using uppercase letters) of the wide character code. For
example, ESC A345 is used to represent the wide character with code
@code{16#A345#}.
This scheme is compatible with use of the full Wide_Character set.

@item Upper-Half Coding
@cindex Upper-Half Coding
The wide character with encoding @code{16#abcd#} where the upper bit is on (in
other words, "a" is in the range 8-F) is represented as two bytes,
@code{16#ab#} and @code{16#cd#}. The second byte cannot be a format control
character, but is not required to be in the upper half. This method can
be also used for shift-JIS or EUC, where the internal coding matches the
external coding.

@item Shift JIS Coding
@cindex Shift JIS Coding
A wide character is represented by a two-character sequence,
@code{16#ab#} and
@code{16#cd#}, with the restrictions described for upper-half encoding as
described above. The internal character code is the corresponding JIS
character according to the standard algorithm for Shift-JIS
conversion. Only characters defined in the JIS code set table can be
used with this encoding method.

@item EUC Coding
@cindex EUC Coding
A wide character is represented by a two-character sequence
@code{16#ab#} and
@code{16#cd#}, with both characters being in the upper half. The internal
character code is the corresponding JIS character according to the EUC
encoding algorithm. Only characters defined in the JIS code set table
can be used with this encoding method.

@item UTF-8 Coding
A wide character is represented using
UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO
10646-1/Am.2. Depending on the character value, the representation
is a one, two, or three byte sequence:
@smallexample
@iftex
@leftskip=.7cm
@end iftex
16#0000#-16#007f#: 2#0xxxxxxx#
16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#

@end smallexample

@noindent
where the xxx bits correspond to the left-padded bits of the
16-bit character value. Note that all lower half ASCII characters
are represented as ASCII bytes and all upper half characters and
other wide characters are represented as sequences of upper-half
(The full UTF-8 scheme allows for encoding 31-bit characters as
6-byte sequences, but in this implementation, all UTF-8 sequences
of four or more bytes length will be treated as illegal).
@item Brackets Coding
In this encoding, a wide character is represented by the following eight
character sequence:

@smallexample
[ " a b c d " ]
@end smallexample

@noindent
Where @code{a}, @code{b}, @code{c}, @code{d} are the four hexadecimal
characters (using uppercase letters) of the wide character code. For
example, ["A345"] is used to represent the wide character with code
@code{16#A345#}. It is also possible (though not required) to use the
Brackets coding for upper half characters. For example, the code
@code{16#A3#} can be represented as @code{["A3"]}.

This scheme is compatible with use of the full Wide_Character set,
and is also the method used for wide character encoding in the standard
ACVC (Ada Compiler Validation Capability) test suite distributions.

@end table

@noindent
Note: Some of these coding schemes do not permit the full use of the
Ada 95 character set. For example, neither Shift JIS, nor EUC allow the
use of the upper half of the Latin-1 set.

@node File Naming Rules
@section File Naming Rules

@noindent
The default file name is determined by the name of the unit that the
file contains. The name is formed by taking the full expanded name of
the unit and replacing the separating dots with hyphens and using
uppercase for all letters.

An exception arises if the file name generated by the above rules starts
with one of the characters
A,G,I, or S,
and the second character is a
minus. In this case, the character dollar sign is used in place
of the minus. The reason for this special rule is to avoid clashes with
the standard names for child units of the packages System, Ada,
Interfaces, and GNAT, which use the prefixes
S- A- I- and G-
respectively.

The file extension is @file{.ADS} for a spec and
@file{.ADB} for a body. The following list shows some
examples of these rules.

@table @file
@item MAIN.ADS
Main (spec)
@item MAIN.ADB
Main (body)
@item ARITH_FUNCTIONS.ADS
Arith_Functions (package spec)
@item ARITH_FUNCTIONS.ADB
Arith_Functions (package body)
@item FUNC-SPEC.ADS
Func.Spec (child package spec)
@item FUNC-SPEC.ADB
Func.Spec (child package body)
@item MAIN-SUB.ADB
Sub (subunit of Main)
@item A$BAD.ADB
A.Bad (child package body)
@end table

@noindent
Following these rules can result in excessively long
file names if corresponding
unit names are long (for example, if child units or subunits are
heavily nested). An option is available to shorten such long file names
(called file name "krunching"). This may be particularly useful when
programs being developed with GNAT are to be used on operating systems
with limited file name lengths. @xref{Using GNAT KRUNCH}.

Of course, no file shortening algorithm can guarantee uniqueness over
all possible unit names; if file name krunching is used, it is your
responsibility to ensure no name clashes occur. Alternatively you
can specify the exact file names that you want used, as described
in the next section. Finally, if your Ada programs are migrating from a
compiler with a different naming convention, you can use the GNAT CHOP
utility to produce source files that follow the GNAT naming conventions.
(For details @pxref{Renaming Files Using GNAT CHOP}.)

@node Using Other File Names
@section Using Other File Names
@cindex File names

@noindent
In the previous section, we have described the default rules used by
GNAT to determine the file name in which a given unit resides. It is
often convenient to follow these default rules, and if you follow them,
the compiler knows without being explicitly told where to find all
the files it needs.

However, in some cases, particularly when a program is imported from
another Ada compiler environment, it may be more convenient for the
programmer to specify which file names contain which units. GNAT allows
arbitrary file names to be used by means of the Source_File_Name pragma.
The form of this pragma is as shown in the following examples:
@cindex Source_File_Name pragma

@smallexample
@group
@cartouche
@b{pragma} Source_File_Name (My_Utilities.Stacks,
  Spec_File_Name => "MYUTILST_A.ADA");
@b{pragma} Source_File_name (My_Utilities.Stacks,
  Body_File_Name => "MYUTILST.ADA");
@end cartouche
@end group
@end smallexample

@noindent
As shown in this example, the first argument for the pragma is the unit
name (in this example a child unit). The second argument has the form
of a named association. The identifier
indicates whether the file name is for a spec or a body;
the file name itself is given by a string literal.

The source file name pragma is a configuration pragma, which means that
normally it will be placed in the @file{GNAT.ADC}
file used to hold configuration
pragmas that apply to a complete compilation environment.
For more details on how the @file{GNAT.ADC} file is created and used
@pxref{Handling of Configuration Pragmas}
@cindex @file{GNAT.ADC}


@noindent
@code{GNAT MAKE} handles non-standard file names in the usual manner (the
non-standard file name for the main program is simply used as the
argument to GNAT MAKE). Note that if the extension is also non-standard,
then it must be included in the GNAT MAKE command, it may not be omitted.

@node Alternative File Naming Schemes
@section Alternative File Naming Schemes
@cindex File naming schemes, alternative
@cindex File names

In the previous section, we described the use of the @code{Source_File_Name}
pragma to allow arbitrary names to be assigned to individual source files.
However, this approach requires one pragma for each file, and especially in
large systems can result in very long @file{GNAT.ADC} files, and also create
a maintenance problem.

GNAT also provides a facility for specifying systematic file naming schemes
other than the standard default naming scheme previously described. An
alternative scheme for naming is specified by the use of
@code{Source_File_Name} pragmas having the following format:
@cindex Source_File_Name pragma

@smallexample
pragma Source_File_Name (
   Spec_File_Name  => FILE_NAME_PATTERN
 [,Casing          => CASING_SPEC]
 [,Dot_Replacement => STRING_LITERAL]);

pragma Source_File_Name (
   Body_File_Name  => FILE_NAME_PATTERN
 [,Casing          => CASING_SPEC]
 [,Dot_Replacement => STRING_LITERAL]);

pragma Source_File_Name (
   Subunit_File_Name  => FILE_NAME_PATTERN
 [,Casing             => CASING_SPEC]
 [,Dot_Replacement    => STRING_LITERAL]);

FILE_NAME_PATTERN ::= STRING_LITERAL
CASING_SPEC ::= Lowercase | Uppercase | Mixedcase

@end smallexample

@noindent
The @code{FILE_NAME_PATTERN} string shows how the file name is constructed.
It contains a single asterisk character, and the unit name is substituted
systematically for this asterisk. The optional parameter
@code{Casing} indicates
whether the unit name is to be all upper-case letters, all lower-case letters,
or mixed-case. If no
@code{Casing} parameter is used, then the default is all
upper-case.

The optional @code{Dot_Replacement} string is used to replace any periods
that occur in subunit or child unit names. If no @code{Dot_Replacement}
argument is used then separating dots appear unchanged in the resulting
file name.
Although the above syntax indicates that the
@code{Casing} argument must appear
before the @code{Dot_Replacement} argument, but it
is also permissible to write these arguments in the opposite order.

As indicated, it is possible to specify different naming schemes for
bodies, specs, and subunits. Quite often the rule for subunits is the
same as the rule for bodies, in which case, there is no need to give
a separate @code{Subunit_File_Name} rule, and in this case the
@code{Body_File_name} rule is used for subunits as well.

The separate rule for subunits can also be used to implement the rather
unusual case of a compilation environment (e.g. a single directory) which
contains a subunit and a child unit with the same unit name. Although
both units cannot appear in the same partition, the Ada Reference Manual
allows (but does not require) the possibility of the two units coexisting
in the same environment.

The file name translation works in the following steps:

@itemize @bullet

@item
If there is a specific @code{Source_File_Name} pragma for the given unit,
then this is always used, and any general pattern rules are ignored.

@item
If there is a pattern type @code{Source_File_Name} pragma that applies to
the unit, then the resulting file name will be used if the file exists. If
more than one pattern matches, the latest one will be tried first, and the
first attempt resulting in a reference to a file that exists will be used.

@item
If no pattern type @code{Source_File_Name} pragma that applies to the unit
for which the corresponding file exists, then the standard GNAT default
naming rules are used.

@end itemize

@noindent
As an example of the use of this mechanism, consider a commonly used scheme
in which file names are all lower case, with separating periods copied
unchanged to the resulting file name, and specs end with ".1.ADA", and
bodies end with ".2.ADA". GNAT will follow this scheme if the following
two pragmas appear:

@smallexample
pragma Source_File_Name
  (Spec_File_Name => "*.1.ADA");
pragma Source_File_Name
  (Body_File_Name => "*.2.ADA");
@end smallexample

@noindent
The default GNAT scheme is actually implemented by providing the following
default pragmas internally:

@smallexample
pragma Source_File_Name
  (Spec_File_Name => "*.ADS", Dot_Replacement => "-");
pragma Source_File_Name
  (Body_File_Name => "*.ADB", Dot_Replacement => "-");
@end smallexample

@noindent
Our final example implements a scheme typically used with one of the
Ada 83 compilers, where the separator character for subunits was "__"
(two underscores), specs were identified by adding @file{_.ADA}, bodies
by adding @file{.ADA}, and subunits by
adding @file{.SEP}. All file names were
upper case. Child units were not present of course since this was an
Ada 83 compiler, but it seems reasonable to extend this scheme to use
the same double underscore separator for child units.

@smallexample
pragma Source_File_Name
  (Spec_File_Name => "*_.ADA",
   Dot_Replacement => "__",
   Casing = Uppercase);
pragma Source_File_Name
  (Body_File_Name => "*.ADA",
   Dot_Replacement => "__",
   Casing = Uppercase);
pragma Source_File_Name
  (Subunit_File_Name => "*.SEP",
   Dot_Replacement => "__",
   Casing = Uppercase);
@end smallexample

@node Generating Object Files
@section Generating Object Files

@noindent
An Ada program consists of a set of source files, and the first step in
compiling the program is to generate the corresponding object files.
These are generated by compiling a subset of these source files.
The files you need to compile are the following:

@itemize @bullet
@item
If a package spec has no body, compile the package spec to produce the
object file for the package.

@item
If a package has both a spec and a body, compile the body to produce the
object file for the package. The source file for the package spec need
not be compiled in this case because there is only one object file, which
contains the code for both the spec and body of the package.

@item
For a subprogram, compile the subprogram body to produce the object file
for the subprogram. The spec, if one is present, is as usual in a
separate file, and need not be compiled.

@item
@cindex Subunits
In the case of subunits, only compile the parent unit. A single object
file is generated for the entire subunit tree, which includes all the
subunits.

@item
Compile child units independently of their parent units
(though, of course, the spec of all the ancestor unit must be present in order
to compile a child unit).

@item
@cindex Generics
Compile generic units in the same manner as any other units. The object
files in this case are small dummy files that contain at most the
flag used for elaboration checking. This is because GNAT always handles generic
instantiation by means of macro expansion. However, it is still necessary to
compile generic units, for dependency checking and elaboration purposes.
@end itemize

@noindent
The preceding rules describe the set of files that must be compiled to
generate the object files for a program. Each object file has the same
name as the corresponding source file, except that the extension is
@file{.OBJ} as usual.

You may wish to compile other files for the purpose of checking their
syntactic and semantic correctness. For example, in the case where a
package has a separate spec and body, you would not normally compile the
spec. However, it is convenient in practice to compile the spec to make
sure it is error-free before compiling clients of this spec, because such
compilations will fail if there is an error in the spec.

GNAT provides an option for compiling such files purely for the
purposes of checking correctness; such compilations are not required as
part of the process of building a program. To compile a file in this
checking mode, use the @option{/NOLOAD} qualifier.

@node Source Dependencies
@section Source Dependencies

@noindent
A given object file clearly depends on the source file which is compiled
to produce it. Here we are using @dfn{depends} in the sense of a typical
@code{make} utility; in other words, an object file depends on a source
file if changes to the source file require the object file to be
recompiled.
In addition to this basic dependency, a given object may depend on
additional source files as follows:

@itemize @bullet
@item
If a file being compiled @code{with}'s a unit @var{X}, the object file
depends on the file containing the spec of unit @var{X}. This includes
files that are @code{with}'ed implicitly either because they are parents
of @code{with}'ed child units or they are run-time units required by the
language constructs used in a particular unit.

@item
If a file being compiled instantiates a library level generic unit, the
object file depends on both the spec and body files for this generic
unit.

@item
If a file being compiled instantiates a generic unit defined within a
package, the object file depends on the body file for the package as
well as the spec file.

@item
@findex Inline
@cindex @option{/INLINE=PRAGMA} qualifier
If a file being compiled contains a call to a subprogram for which
pragma @code{Inline} applies and inlining is activated with the
@option{/INLINE=PRAGMA} qualifier, the object file depends on the file containing the
body of this subprogram as well as on the file containing the spec. Note
that for inlining to actually occur as a result of the use of this qualifier,
it is necessary to compile in optimizing mode.

@cindex @option{-gnatN} qualifier
The use of @option{-gnatN} activates a more extensive inlining optimization
that is performed by the front end of the compiler. This inlining does
not require that the code generation be optimized. Like @option{/INLINE=PRAGMA},
the use of this qualifier generates additional dependencies.

@item
If an object file O  depends on the proper body of a subunit through inlining
or instantiation, it depends on the parent unit of the subunit. This means that
any modification of the parent unit or one of its subunits affects the
compilation of O.

@item
The object file for a parent unit depends on all its subunit body files.

@item
The previous two rules meant that for purposes of computing dependencies and
recompilation, a body and all its subunits are treated as an indivisible whole.

@noindent
These rules are applied transitively: if unit @code{A} @code{with}'s
unit @code{B}, whose elaboration calls an inlined procedure in package
@code{C}, the object file for unit @code{A} will depend on the body of
@code{C}, in file @file{C.ADB}.

The set of dependent files described by these rules includes all the
files on which the unit is semantically dependent, as described in the
Ada 95 Language Reference Manual. However, it is a superset of what the
ARM describes, because it includes generic, inline, and subunit dependencies.

An object file must be recreated by recompiling the corresponding source
file if any of the source files on which it depends are modified. For
example, if the @code{make} utility is used to control compilation,
the rule for an Ada object file must mention all the source files on
which the object file depends, according to the above definition.
The determination of the necessary
recompilations is done automatically when one uses @code{GNAT MAKE}.
@end itemize

@node The Ada Library Information Files
@section The Ada Library Information Files
@cindex Ada Library Information files
@cindex @file{ali} files

@noindent
Each compilation actually generates two output files. The first of these
is the normal object file that has a @file{.OBJ} extension. The second is a
text file containing full dependency information. It has the same
name as the source file, but an @file{.ALI} extension.
This file is known as the Ada Library Information (@file{ali}) file.
The following information is contained in the @file{ali} file.

@itemize @bullet
@item
Version information (indicates which version of GNAT was used to compile
the unit(s) in question)

@item
Main program information (including priority and time slice settings,
as well as the wide character encoding used during compilation).

@item
List of arguments used in the @code{GNAT COMPILE} command for the compilation

@item
Attributes of the unit, including configuration pragmas used, an indication
of whether the compilation was successful, exception model used etc.

@item
A list of relevant restrictions applying to the unit (used for consistency)
checking.

@item
Categorization information (e.g. use of pragma @code{Pure}).

@item
Information on all @code{with}'ed units, including presence of
@code{Elaborate} or @code{Elaborate_All} pragmas.

@item
Information from any @code{Linker_Options} pragmas used in the unit

@item
Information on the use of @code{Body_Version} or @code{Version}
attributes in the unit.

@item
Dependency information. This is a list of files, together with
time stamp and checksum information. These are files on which
the unit depends in the sense that recompilation is required
if any of these units are modified.

@item
Cross-reference data. Contains information on all entities referenced
in the unit. Used by tools like @code{GNAT XREF} and @code{GNAT FIND} to
provide cross-reference information.

@end itemize

@noindent
For a full detailed description of the format of the @file{ali} file,
see the source of the body of unit @code{Lib.Writ}, contained in file
@file{LIB-WRIT.ADB} in the GNAT compiler sources.

@node Binding an Ada Program
@section Binding an Ada Program

@noindent
When using languages such as C and C++, once the source files have been
compiled the only remaining step in building an executable program
is linking the object modules together. This means that it is possible to
link an inconsistent version of a program, in which two units have
included different versions of the same header.

The rules of Ada do not permit such an inconsistent program to be built.
For example, if two clients have different versions of the same package,
it is illegal to build a program containing these two clients.
These rules are enforced by the GNAT binder, which also determines an
elaboration order consistent with the Ada rules.

The GNAT binder is run after all the object files for a program have
been created. It is given the name of the main program unit, and from
this it determines the set of units required by the program, by reading the
corresponding ALI files. It generates error messages if the program is
inconsistent or if no valid order of elaboration exists.

If no errors are detected, the binder produces a main program, in Ada by
default, that contains calls to the elaboration procedures of those
compilation unit that require them, followed by
a call to the main program. This Ada program is compiled to generate the
object file for the main program. The name of
the Ada file is @file{B$@var{xxx}.ADB} (with the corresponding spec
@file{B$@var{xxx}.ADS}) where @var{xxx} is the name of the
main program unit.

Finally, the linker is used to build the resulting executable program,
using the object from the main program from the bind step as well as the
object files for the Ada units of the program.

@node Mixed Language Programming
@section Mixed Language Programming
@cindex Mixed Language Programming

@menu
* Interfacing to C::
* Calling Conventions::
@end menu

@node Interfacing to C
@subsection Interfacing to C
@noindent
There are two ways to
build a program that contains some Ada files and some other language
files depending on whether the main program is in Ada or not.
If the main program is in Ada, you should proceed as follows:

@enumerate
@item
Compile the other language files to generate object files. For instance:
@smallexample
GNAT COMPILE FILE1.C
GNAT COMPILE FILE2.C
@end smallexample

@item
Compile the Ada units to produce a set of object files and ALI
files. For instance:
@smallexample
GNAT MAKE /ACTIONS=COMPILE MY_MAIN.ADB
@end smallexample

@item
Run the Ada binder on the Ada main program. For instance:
@smallexample
GNAT BIND MY_MAIN.ALI
@end smallexample

@item
Link the Ada main program, the Ada objects and the other language
objects. For instance:
@smallexample
GNAT LINK MY_MAIN.ALI FILE1.OBJ FILE2.OBJ
@end smallexample
@end enumerate

The three last steps can be grouped in a single command:
@smallexample
GNAT MAKE MY_MAIN.ADB /LINKER_QUALIFIERS FILE1.OBJ FILE2.OBJ
@end smallexample

@cindex Binder output file
@noindent
If the main program is in some language other than Ada, Then you may
have more than one entry point in the Ada subsystem. You must use a
special option of the binder to generate callable routines to initialize
and finalize the Ada units (@pxref{Binding with Non-Ada Main Programs}).
Calls to the initialization and finalization routines must be inserted in
the main program, or some other appropriate point in the code. The call to
initialize the Ada units must occur before the first Ada subprogram is
called, and the call to finalize the Ada units must occur after the last
Ada subprogram returns. You use the same procedure for building the
program as described previously. In this case, however, the binder
only places the initialization and finalization subprograms into file
@file{B$@var{xxx}.ADB} instead of the main program.
So, if the main program is not in Ada, you should proceed as follows:

@enumerate
@item
Compile the other language files to generate object files. For instance:
@smallexample
GNAT COMPILE FILE1.C
GNAT COMPILE FILE2.C
@end smallexample

@item
Compile the Ada units to produce a set of object files and ALI
files. For instance:
@smallexample
GNAT MAKE /ACTIONS=COMPILE ENTRY_POINT1.ADB
GNAT MAKE /ACTIONS=COMPILE ENTRY_POINT2.ADB
@end smallexample

@item
Run the Ada binder on the Ada main program. For instance:
@smallexample
GNAT BIND /NOMAIN ENTRY_POINT1.ALI ENTRY_POINT2.ALI
@end smallexample

@item
Link the Ada main program, the Ada objects and the other language
objects. You only need to give the last entry point here. For instance:
@smallexample
GNAT LINK ENTRY_POINT2.ALI FILE1.OBJ FILE2.OBJ
@end smallexample
@end enumerate

@node Calling Conventions
@subsection Calling Conventions
@cindex Foreign Languages
@cindex Calling Conventions
GNAT follows standard calling sequence conventions and will thus interface
to any other language that also follows these conventions. The following
Convention identifiers are recognized by GNAT:

@itemize @bullet
@cindex Interfacing to Ada
@cindex Other Ada compilers
@cindex Convention Ada
@item
Ada. This indicates that the standard Ada calling sequence will be
used and all Ada data items may be passed without any limitations in the
case where GNAT is used to generate both the caller and callee. It is also
possible to mix GNAT generated code and code generated by another Ada
compiler. In this case, the data types should be restricted to simple
cases, including primitive types. Whether complex data types can be passed
depends on the situation. Probably it is safe to pass simple arrays, such
as arrays of integers or floats. Records may or may not work, depending
on whether both compilers lay them out identically. Complex structures
involving variant records, access parameters, tasks, or protected types,
are unlikely to be able to be passed.

Note that in the case of GNAT running
on a platform that supports DEC Ada 83, a higher degree of compatibility
can be guaranteed, and in particular records are layed out in an identical
manner in the two compilers. Note also that if output from two different
compilers is mixed, the program is responsible for dealing with elaboration
issues. Probably the safest approach is to write the main program in the
version of Ada other than GNAT, so that it takes care of its own elaboration
requirements, and then call the GNAT-generated adainit procedure to ensure
elaboration of the GNAT components. Consult the documentation of the other
Ada compiler for further details on elaboration.

However, it is not possible to mix the tasking run time of GNAT and
DEC Ada 83, All the tasking operations must either be entirely within
GNAT compiled sections of the program, or entirely within DEC Ada 83
compiled sections of the program.

@cindex Interfacing to Assembly
@cindex Convention Assembler
@item
Assembler. Specifies assembler as the convention. In practice this has the
same effect as convention Ada (but is not equivalent in the sense of being
considered the same convention).

@cindex Convention Asm
@findex Asm
@item
Asm. Equivalent to Assembler.

@cindex Convention Asm
@findex Asm
@item
Asm. Equivalent to Assembly.

@cindex Interfacing to COBOL
@cindex Convention COBOL
@findex COBOL
@item
COBOL. Data will be passed according to the conventions described
in section B.4 of the Ada 95 Reference Manual.

@findex C
@cindex Interfacing to C
@cindex Convention C
@item
C. Data will be passed according to the conventions described
in section B.3 of the Ada 95 Reference Manual.

@cindex Convention Default
@findex Default
@item
Default. Equivalent to C.

@cindex Convention External
@findex External
@item
External. Equivalent to C.

@findex C++
@cindex Interfacing to C++
@cindex Convention C++
@item
CPP. This stands for C++. For most purposes this is identical to C.
See the separate description of the specialized GNAT pragmas relating to
C++ interfacing for further details.

@findex Fortran
@cindex Interfacing to Fortran
@cindex Convention Fortran
@item
Fortran. Data will be passed according to the conventions described
in section B.5 of the Ada 95 Reference Manual.

@item
Intrinsic. This applies to an intrinsic operation, as defined in the Ada 95
Reference Manual. If a a pragma Import (Intrinsic) applies to a subprogram,
this means that the body of the subprogram is provided by the compiler itself,
usually by means of an efficient code sequence, and that the user does not
supply an explicit body for it. In an application program, the pragma can only
be applied to the following two sets of names, which the GNAT compiler
recognizes.
@itemize @bullet
@item
Rotate_Left, Rotate_Right, Shift_Left, Shift_Right, Shift_Right_-
Arithmetic.  The corresponding subprogram declaration must have
two formal parameters. The
first one must be a signed integer type or a modular type with a binary
modulus, and the second parameter must be of type Natural.
The return type must be the same as the type of the first argument. The size
of this type can only be 8, 16, 32, or 64.
@item binary arithmetic operators: "+", "-", "*", "/"
The corresponding operator declaration must have parameters and result type
that have the same root numeric type (for example, all three are long_float
types). This simplifies the definition of operations that use type checking
to perform dimensional checks:
@smallexample
type Distance is new Long_Float;
type Time     is new Long_Float;
type Velocity is new Long_Float;
function "/" (D : Distance; T : Time)
  return Velocity;
pragma Import (Intrinsic, "/");
@end smallexample
@noindent
This common idiom is often programmed with a generic definition and an explicit
body. The pragma makes it simpler to introduce such declarations. It incurs
no overhead in compilation time or code size, because it is implemented as a
single machine instruction.
@end itemize
@noindent

@findex Stdcall
@cindex Convention Stdcall
@item
Stdcall. This is relevant only to NT/Win95 implementations of GNAT,
and specifies that the Stdcall calling sequence will be used, as defined
by the NT API.

@findex DLL
@cindex Convention DLL
@item
DLL. This is equivalent to Stdcall.

@findex Win32
@cindex Convention Win32
@item
Win32. This is equivalent to Stdcall.

@findex Stubbed
@cindex Convention Stubbed
@item
Stubbed. This is a special convention that indicates that the compiler
should provide a stub body that raises @code{Program_Error}.
@end itemize

@noindent
GNAT additionally provides a useful pragma @code{Convention_Identifier}
that can be used to parametrize conventions and allow additional synonyms
to be specified. For example if you have legacy code in which the convention
identifier Fortran77 was used for Fortran, you can use the configuration
pragma:

@smallexample
   pragma Convention_Identifier (Fortran77, Fortran);
@end smallexample

@noindent
And from now on the identifier Fortran77 may be used as a convention
identifier (for example in an @code{Import} pragma) with the same
meaning as Fortran.

@node Building Mixed Ada & C++ Programs
@section Building Mixed Ada & C++ Programs

@noindent
Building a mixed application containing both Ada and C++ code may be a
challenge for the unaware programmer. As a matter of fact, this
interfacing has not been standardized in the Ada 95 reference manual due
to the immaturity and lack of standard of C++ at the time. This
section gives a few hints that should make this task easier. In
particular the first section addresses the differences with
interfacing with C. The second section looks into the delicate problem
of linking the complete application from its Ada and C++ parts. The last
section give some hints on how the GNAT run time can be adapted in order
to allow inter-language dispatching with a new C++ compiler.

@menu
* Interfacing to C++::
* Linking a Mixed C++ & Ada Program::
* A Simple Example::
* Adapting the Run Time to a New C++ Compiler::
@end menu

@node Interfacing to C++
@subsection Interfacing to C++

@noindent
GNAT supports interfacing with C++ compilers generating code that is
compatible with the standard Application Binary Interface of the given
platform.

@noindent
Interfacing can be done at 3 levels: simple data, subprograms and
classes. In the first 2 cases, GNAT offer a specific @var{Convention
CPP} that behaves exactly like @var{Convention C}. Usually C++ mangle
names of subprograms and currently GNAT does not provide any help to
solve the demangling problem. This problem can be addressed in 2 ways:
@itemize @bullet
@item
by modifying the C++ code in order to force a C convention using
the @var{extern "C"} syntax.

@item
by figuring out the mangled name and use it as the Link_Name argument of
the pragma import.
@end itemize

@noindent
Interfacing at the class level can be achieved by using the GNAT specific
pragmas such as @code{CPP_Class} and @code{CPP_Virtual}. See the GNAT
Reference Manual for additional information.

@node Linking a Mixed C++ & Ada Program
@subsection Linking a Mixed C++ & Ada Program

@noindent
Usually the linker of the C++ development system must be used to link
mixed applications because most C++ systems will resolve elaboration
issues (such as calling constructors on global class instances)
transparently during the link phase. GNAT has been adapted to ease the
use of a foreign linker for the last phase. Three cases can be
considered:
@enumerate

@item
Using GNAT and G++ (GNU C++ compiler) from the same GCC
installation. The c++ linker can simply be called by using the c++
specific driver called @code{c++}. Note that this setup is not
very common because it may request recompiling the whole GCC
tree from sources and it does not allow to upgrade easily to a new
version of one compiler for one of the two languages without taking the
risk of destabilizing the other.

@smallexample
$ c++ -c file1.C
$ c++ -c file2.C
$ GNAT MAKE ada_unit /LINKER_QUALIFIERS FILE1.OBJ FILE2.OBJ --LINK=c++
@end smallexample

@item
Using GNAT and G++ from 2 different GCC installations. If both compilers
are on the PATH, the same method can be used. It is important to be
aware that environment variables such as C_INCLUDE_PATH,
GCC_EXEC_PREFIX, BINUTILS_ROOT or GCC_ROOT will affect both compilers at
the same time and thus may make one of the 2 compilers operate
improperly if they are set for the other. In particular it is important
that the link command has access to the proper GNAT COMPILE library @file{libgcc.a},
that is to say the one that is part of the C++ compiler
installation. The implicit link command as suggested in the GNAT MAKE
command from the former example can be replaced by an explicit link
command with full verbosity in order to verify which library is used:
@smallexample
$ GNAT BIND ada_unit
$ GNAT LINK -v -v ada_unit FILE1.OBJ FILE2.OBJ --LINK=c++
@end smallexample
If there is a problem due to interfering environment variables, it can
be workaround by using an intermediate script. The following example
shows the proper script to use when GNAT has not been installed at its
default location and g++ has been installed at its default location:

@smallexample
$ GNAT LINK -v -v ada_unit FILE1.OBJ FILE2.OBJ --LINK=./my_script
$ cat ./my_script
#!/bin/sh
unset BINUTILS_ROOT
unset GCC_ROOT
c++ $*
@end smallexample

@item
Using a non GNU C++ compiler. The same set of command as previously
described can be used to insure that the c++ linker is
used. Nonetheless, you need to add the path to libgcc explicitely, since some
libraries needed by GNAT are located in this directory:

@smallexample

$ GNAT LINK ada_unit FILE1.OBJ FILE2.OBJ --LINK=./my_script
$ cat ./my_script
#!/bin/sh
CC $* `GNAT COMPILE -print-libgcc-file-name`

@end smallexample

Where CC is the name of the non GNU C++ compiler.

@end enumerate

@node A Simple Example
@subsection  A Simple Example
@noindent
The following example, provided as part of the GNAT examples, show how
to achieve procedural interfacing between Ada and C++ in both
directions. The C++ class A has 2 methods. The first method is exported
to Ada by the means of an extern C wrapper function. The second method
calls an Ada subprogram. On the Ada side, The C++ calls is modelized by
a limited record with a layout comparable to the C++ class. The Ada
subprogram, in turn, calls the c++ method. So from the C++ main program
the code goes back and forth between the 2 languages.

@noindent
Here are the compilation commands
for native configurations:
@smallexample
$ GNAT MAKE -c simple_cpp_interface
$ c++ -c cpp_main.C
$ c++ -c ex7.C
$ GNAT BIND -n simple_cpp_interface
$ GNAT LINK simple_cpp_interface -o cpp_main --LINK=$(CPLUSPLUS)
      -lstdc++ EX7.OBJ CPP_MAIN.OBJ
@end smallexample
@noindent
Here are the corresponding sources:
@smallexample

//cpp_main.C

#include "ex7.h"

extern "C" @{
  void adainit (void);
  void adafinal (void);
  void method1 (A *t);
@}

void method1 (A *t)
@{
  t->method1 ();
@}

int main ()
@{
  A obj;
  adainit ();
  obj.method2 (3030);
  adafinal ();
@}

//ex7.h

class Origin @{
 public:
  int o_value;
@};
class A : public Origin @{
 public:
  void method1 (void);
  virtual void method2 (int v);
  A();
  int   a_value;
@};

//ex7.C

#include "ex7.h"
#include <stdio.h>

extern "C" @{ void ada_method2 (A *t, int v);@}

void A::method1 (void)
@{
  a_value = 2020;
  printf ("in A::method1, a_value = %d \n",a_value);

@}

void A::method2 (int v)
@{
   ada_method2 (this, v);
   printf ("in A::method2, a_value = %d \n",a_value);

@}

A::A(void)
@{
   a_value = 1010;
  printf ("in A::A, a_value = %d \n",a_value);
@}

-- Ada sources
@b{package} @b{body} Simple_Cpp_Interface @b{is}

   @b{procedure} Ada_Method2 (This : @b{in} @b{out} A; V : Integer) @b{is}
   @b{begin}
      Method1 (This);
      This.A_Value := V;
   @b{end} Ada_Method2;

@b{end} Simple_Cpp_Interface;

@b{package} Simple_Cpp_Interface @b{is}
   @b{type} A @b{is} @b{limited}
      @b{record}
         O_Value : Integer;
         A_Value : Integer;
      @b{end} @b{record};
   @b{pragma} Convention (C, A);

   @b{procedure} Method1 (This : @b{in} @b{out} A);
   @b{pragma} Import (C, Method1);

   @b{procedure} Ada_Method2 (This : @b{in} @b{out} A; V : Integer);
   @b{pragma} Export (C, Ada_Method2);

@b{end} Simple_Cpp_Interface;
@end smallexample

@node Adapting the Run Time to a New C++ Compiler
@subsection Adapting the Run Time to a New C++ Compiler
@noindent
GNAT offers the capability to derive Ada 95 tagged types directly from
preexisting C++ classes and . See "Interfacing with C++" in the GNAT
reference manual. The mechanism used by GNAT for achieving such a goal
has been made user configurable through a GNAT library unit
@code{Interfaces.CPP}. The default version of this file is adapted to
the GNU c++ compiler. Internal knowledge of the virtual
table layout used by the new C++ compiler is needed to configure
properly this unit. The Interface of this unit is known by the compiler
and cannot be changed except for the value of the constants defining the
characteristics of the virtual table: CPP_DT_Prologue_Size, CPP_DT_Entry_Size,
CPP_TSD_Prologue_Size, CPP_TSD_Entry_Size. Read comments in the source
of this unit for more details.

@node Comparison between GNAT and C/C++ Compilation Models
@section Comparison between GNAT and C/C++ Compilation Models

@noindent
The GNAT model of compilation is close to the C and C++ models. You can
think of Ada specs as corresponding to header files in C. As in C, you
don't need to compile specs; they are compiled when they are used. The
Ada @code{with} is similar in effect to the @code{#include} of a C
header.

One notable difference is that, in Ada, you may compile specs separately
to check them for semantic and syntactic accuracy. This is not always
possible with C headers because they are fragments of programs that have
less specific syntactic or semantic rules.

The other major difference is the requirement for running the binder,
which performs two important functions. First, it checks for
consistency. In C or C++, the only defense against assembling
inconsistent programs lies outside the compiler, in a makefile, for
example. The binder satisfies the Ada requirement that it be impossible
to construct an inconsistent program when the compiler is used in normal
mode.

@cindex Elaboration order control
The other important function of the binder is to deal with elaboration
issues. There are also elaboration issues in C++ that are handled
automatically. This automatic handling has the advantage of being
simpler to use, but the C++ programmer has no control over elaboration.
Where @code{GNAT BIND} might complain there was no valid order of
elaboration, a C++ compiler would simply construct a program that
malfunctioned at run time.

@node Comparison between GNAT and Conventional Ada Library Models
@section Comparison between GNAT and Conventional Ada Library Models

@noindent
This section is intended to be useful to Ada programmers who have
previously used an Ada compiler implementing the traditional Ada library
model, as described in the Ada 95 Language Reference Manual. If you
have not used such a system, please go on to the next section.

@cindex GNAT library
In GNAT, there is no @dfn{library} in the normal sense. Instead, the set of
source files themselves acts as the library. Compiling Ada programs does
not generate any centralized information, but rather an object file and
a ALI file, which are of interest only to the binder and linker.
In a traditional system, the compiler reads information not only from
the source file being compiled, but also from the centralized library.
This means that the effect of a compilation depends on what has been
previously compiled. In particular:

@itemize @bullet
@item
When a unit is @code{with}'ed, the unit seen by the compiler corresponds
to the version of the unit most recently compiled into the library.

@item
Inlining is effective only if the necessary body has already been
compiled into the library.

@item
Compiling a unit may obsolete other units in the library.
@end itemize

@noindent
In GNAT, compiling one unit never affects the compilation of any other
units because the compiler reads only source files. Only changes to source
files can affect the results of a compilation. In particular:

@itemize @bullet
@item
When a unit is @code{with}'ed, the unit seen by the compiler corresponds
to the source version of the unit that is currently accessible to the
compiler.

@item
@cindex Inlining
Inlining requires the appropriate source files for the package or
subprogram bodies to be available to the compiler. Inlining is always
effective, independent of the order in which units are complied.

@item
Compiling a unit never affects any other compilations. The editing of
sources may cause previous compilations to be out of date if they
depended on the source file being modified.
@end itemize

@noindent
The most important result of these differences is that order of compilation
is never significant in GNAT. There is no situation in which one is
required to do one compilation before another. What shows up as order of
compilation requirements in the traditional Ada library becomes, in
GNAT, simple source dependencies; in other words, there is only a set
of rules saying what source files must be present when a file is
compiled.

@node Compiling Using GNAT COMPILE
@chapter Compiling Using @code{GNAT COMPILE}

@noindent
This chapter discusses how to compile Ada programs using the @code{GNAT COMPILE}
command. It also describes the set of qualifiers
that can be used to control the behavior of the compiler.
@menu
* Compiling Programs::
* Qualifiers for GNAT COMPILE::
* Search Paths and the Run-Time Library (RTL)::
* Order of Compilation Issues::
* Examples::
@end menu

@node Compiling Programs
@section Compiling Programs

@noindent
The first step in creating an executable program is to compile the units
of the program using the @code{GNAT COMPILE} command. You must compile the
following files:

@itemize @bullet
@item
the body file (@file{.ADB}) for a library level subprogram or generic
subprogram

@item
the spec file (@file{.ADS}) for a library level package or generic
package that has no body

@item
the body file (@file{.ADB}) for a library level package
or generic package that has a body

@end itemize

@noindent
You need @emph{not} compile the following files

@itemize @bullet

@item
the spec of a library unit which has a body

@item
subunits
@end itemize

@noindent
because they are compiled as part of compiling related units. GNAT
package specs
when the corresponding body is compiled, and subunits when the parent is
compiled.
@cindex No code generated
If you attempt to compile any of these files, you will get one of the
following error messages (where fff is the name of the file you compiled):

@smallexample
No code generated for file @var{fff} (@var{package spec})
No code generated for file @var{fff} (@var{subunit})
@end smallexample

@noindent
The basic command for compiling a file containing an Ada unit is

@smallexample
$ GNAT COMPILE [@var{qualifiers}] @file{file name}
@end smallexample

@noindent
where @var{file name} is the name of the Ada file (usually
having an extension
@file{.ADS} for a spec or @file{.ADB} for a body).
The result of a successful compilation is an object file, which has the
same name as the source file but an extension of @file{.OBJ} and an Ada
Library Information (ALI) file, which also has the same name as the
source file, but with @file{.ALI} as the extension. GNAT creates these
two output files in the current directory, but you may specify a source
file in any directory using an absolute or relative path specification
containing the directory information.

@findex GNAT1
@code{GNAT COMPILE} is actually a driver program that looks at the extensions of
the file arguments and loads the appropriate compiler. For example, the
GNU C compiler is @file{CC1}, and the Ada compiler is @file{GNAT1}.
These programs are in directories known to the driver program (in some
configurations via environment variables you set), but need not be in
your path. The @code{GNAT COMPILE} driver also calls the assembler and any other
utilities needed to complete the generation of the required object
files.

It is possible to supply several file names on the same @code{GNAT COMPILE}
command. This causes @code{GNAT COMPILE} to call the appropriate compiler for
each file. For example, the following command lists three separate
files to be compiled:

@smallexample
$ GNAT COMPILE X.ADB Y.ADB Z.C
@end smallexample

@noindent
calls @code{GNAT1} (the Ada compiler) twice to compile @file{X.ADB} and
@file{Y.ADB}, and @code{CC1} (the C compiler) once to compile @file{Z.C}.
The compiler generates three object files @file{X.OBJ}, @file{Y.OBJ} and
@file{Z.OBJ} and the two ALI files @file{X.ALI} and @file{Y.ALI} from the
Ada compilations. Any qualifiers apply to all the files listed.

@node Qualifiers for GNAT COMPILE
@section Qualifiers for @code{GNAT COMPILE}

@noindent
The @code{GNAT COMPILE} command accepts qualifiers that control the
compilation process. These qualifiers are fully described in this section.
First we briefly list all the qualifiers, in alphabetical order, then we
describe the qualifiers in more detail in functionally grouped sections.

@menu
* Output and Error Message Control::
* Debugging and Assertion Control::
* Run-Time Checks::
* Stack Overflow Checking::
* Run-Time Control::
* Validity Checking::
* Style Checking::
* Using GNAT COMPILE for Syntax Checking::
* Using GNAT COMPILE for Semantic Checking::
* Compiling Ada 83 Programs::
* Character Set Control::
* File Naming Control::
* Subprogram Inlining Control::
* Auxiliary Output Control::
* Debugging Control::
* Units to Sources Mapping Files::
@end menu

@table @code

@item /DEBUG
@cindex @code{/DEBUG} (@code{GNAT COMPILE})
Generate debugging information. This information is stored in the object
file and copied from there to the final executable file by the linker,
where it can be read by the debugger. You must use the
@code{/DEBUG} qualifier if you plan on using the debugger.

@item /SEARCH=@var{dir}
@cindex @code{/SEARCH} (@code{GNAT COMPILE})
@cindex RTL
Direct GNAT to search the @var{dir} directory for source files needed by
the current compilation
(@pxref{Search Paths and the Run-Time Library (RTL)}).

@item /NOCURRENT_DIRECTORY
@cindex @code{/NOCURRENT_DIRECTORY} (@code{GNAT COMPILE})
@cindex RTL
Except for the source file named in the command line, do not look for source files
in the directory containing the source file named in the command line
(@pxref{Search Paths and the Run-Time Library (RTL)}).



@item  /NOOPTIMIZE (default)
@itemx /OPTIMIZE[=(keyword[,...])]
Selects the level of optimization for your program. The supported
keywords are as follows:
@table @code
@item   ALL (default)
Perform most optimizations, including those that
be expensive.

@item   NONE
Do not do any optimizations. Same as @code{/NOOPTIMIZE}.

@item SOME
Perform some optimizations, but omit ones that are costly.

@item   DEVELOPMENT
Same as @code{SOME}.

@item   INLINING
Full optimization, and also attempt automatic inlining of small
subprograms within a unit (@pxref{Inlining of Subprograms}).

@item   UNROLL_LOOPS
Try to unroll loops. This keyword may be specified together with
any keyword above other than @code{NONE}. Loop unrolling
usually, but not always, improves the performance of programs.
@end table

@item /RUNTIME_SYSTEM=@var{rts-path}
@cindex @code{/RUNTIME_SYSTEM} (@code{GNAT COMPILE})
Specifies the default location of the runtime library. Same meaning as the
equivalent @code{GNAT MAKE} flag (see @ref{Qualifiers for GNAT MAKE}).

@item /ASM
@cindex @code{/ASM} (@code{GNAT COMPILE})
Used to
cause the assembler source file to be
generated, using @file{.S} as the extension,
instead of the object file.
This may be useful if you need to examine the generated assembly code.

@item /VERBOSE
@cindex @code{/VERBOSE} (@code{GNAT COMPILE})
Show commands generated by the @code{GNAT COMPILE} driver. Normally used only for
debugging purposes or if you need to be sure what version of the
compiler you are executing.


@item /CHECKS=ASSERTIONS
Assertions enabled. @code{Pragma Assert} and @code{pragma Debug} to be
activated.

@item -gnatA
Avoid processing @file{GNAT.ADC}. If a GNAT.ADC file is present, it will be ignored.

@item /WARNINGS=BRIEF
Generate brief messages to @file{SYS$ERROR} even if verbose mode set.

@item /NOLOAD
Check syntax and semantics only (no code generation attempted).

@item /COMPRESS_NAMES
Compress debug information and external symbol name table entries.

@item /XDEBUG
Output expanded source files for source level debugging. This qualifier
also suppress generation of cross-reference information (see /XREF=SUPPRESS).

@item -gnatec@var{path}
Specify a configuration pragma file. (see @ref{The Configuration Pragmas Files})

@item -gnatem@var{path}
Specify a mapping file. (see @ref{Units to Sources Mapping Files})

@item /CHECKS=ELABORATION
Full dynamic elaboration checks.

@item /REPORT_ERRORS=FULL
Full errors. Multiple errors per line, all undefined references.

@item /UPPERCASE_EXTERNALS
Externals names are folded to all uppercase.

@item /STYLE=GNAT
Internal GNAT implementation mode. This should not be used for
applications programs, it is intended only for use by the compiler
and its run-time library. For documentation, see the GNAT sources.

@item /EXPAND_SOURCE
List generated expanded code in source form.

@item /IDENTIFIER_CHARACTER_SET=@var{c}
Identifier character set
For details of the possible selections for @var{c},
see @xref{Character Set Control}.

@item /HELP
Output usage information. The output is written to @file{SYS$OUTPUT}.

@item /FILE_NAME_MAX_LENGTH=@var{n}
Limit file names to @var{n} (1-999) characters .

@item /LIST
Output full source listing with embedded error messages.

@item /ERROR_LIMIT=@var{n}
Limit number of detected errors to @var{n} (1-999).

@item /INLINE=PRAGMA
Activate inlining across unit boundaries for subprograms for which
pragma @code{inline} is specified.

@item -gnatN
Activate front end inlining.

@item /INLINE=SUPPRESS
Suppresses all inlining, even if other optimization or inlining qualifiers
are set.


@item /CHECKS=OVERFLOW
Enable numeric overflow checking (which is not normally enabled by
default). Not that division by zero is a separate check that is not
controlled by this qualifier (division by zero checking is on by default).

@item /CHECKS=SUPPRESS_ALL
Suppress all checks.

@item /TRY_SEMANTICS
Don't quit; try semantics, even if parse errors.

@item /FORCE_ALI
Don't quit; generate @file{ali} and tree files even if illegalities.

@item /POLLING_ENABLE
Enable polling. This is required on some systems (notably Windows NT) to
obtain asynchronous abort and asynchronous transfer of control capability.
See the description of pragma Polling in the GNAT Reference Manual for
full details.

@item /REPRESENTATION_INFO[0/1/2/3][s]
Output representation information for declared types and objects.

@item /SYNTAX_ONLY
Syntax check only.

@item /TREE_OUTPUT
Tree output file to be generated.

@item -gnatT nnn
Set time slice to specified number of microseconds

@item /UNITS_LIST
List units for this compilation.

@item /UNIQUE_ERROR_TAG
Tag all error messages with the unique string "error:"

@item /REPORT_ERRORS=VERBOSE
Verbose mode. Full error output with source lines to @file{SYS$OUTPUT}.

@item /VALIDITY_CHECKING
Control level of validity checking. See separate section describing
this feature.

@item /WARNINGS=@var{xxx}
Warning mode where
@var{xxx} is a string of options describing the exact warnings that
are enabled or disabled. See separate section on warning control.

@item /WIDE_CHARACTER_ENCODING=@var{e}
Wide character encoding method
(@var{e}=@code{BRACKETS, NONE, HEX, UPPER, SHIFT_JIS, EUC, UTF8})

@item /XREF=SUPPRESS
Suppress generation of cross-reference information.

@item /STYLE_CHECKS=(option,option..)
Enable built-in style checks. See separate section describing this feature.

@item /DISTRIBUTION_STUBS=@var{m}
Distribution stub generation and compilation
(@var{m}=@code{RECEIVER} or @code{CALLER} to specify the type of stubs
to be generated and compiled).

@item /83
Enforce Ada 83 restrictions.

@end table


@noindent
The following restrictions apply to the combination of qualifiers
in this manner:

@itemize @bullet
@item
The qualifier @option{/NOLOAD} if combined with other qualifiers must come
first in the string.

@item
The qualifier @option{/SYNTAX_ONLY} if combined with other qualifiers must come
first in the string.

@item
Once a "y" appears in the string (that is a use of the @option{/STYLE=}
qualifier), then all further characters in the qualifier are interpreted
as style modifiers (see description of @option{/STYLE=}).

@item
Once a "d" appears in the string (that is a use of the @option{-gnatd}
qualifier), then all further characters in the qualifier are interpreted
as debug flags (see description of @option{-gnatd}).

@item
Once a "w" appears in the string (that is a use of the @option{-gnatw}
qualifier), then all further characters in the qualifier are interpreted
as warning mode modifiers (see description of @option{-gnatw}).

@item
Once a "V" appears in the string (that is a use of the @option{/VALIDITY_CHECKING}
qualifier), then all further characters in the qualifier are interpreted
as validity checking options (see description of @option{/VALIDITY_CHECKING}).

@end itemize

@node Output and Error Message Control
@subsection Output and Error Message Control
@findex SYS$ERROR

@noindent
The standard default format for error messages is called "brief format."
Brief format messages are written to @file{SYS$ERROR} (the standard error
file) and have the following form:

@smallexample
@iftex
@leftskip=.7cm
@end iftex
E.ADB:3:04: Incorrect spelling of keyword "function"
E.ADB:4:20: ";" should be "is"
@end smallexample

@noindent
The first integer after the file name is the line number in the file,
and the second integer is the column number within the line.
@code{glide} can parse the error messages
and point to the referenced character.
The following qualifiers provide control over the error message
format:

@table @code
@item /REPORT_ERRORS=VERBOSE
@cindex @option{/REPORT_ERRORS=VERBOSE} (@code{GNAT COMPILE})
@findex SYS$OUTPUT
The effect of this setting is to write long-format error
messages to @file{SYS$OUTPUT} (the standard output file.
The same program compiled with the
@option{/REPORT_ERRORS=VERBOSE} qualifier would generate:

@smallexample
@group
@cartouche
3. funcion X (Q : Integer)
   |
>>> Incorrect spelling of keyword "function"
4. return Integer;
                 |
>>> ";" should be "is"
@end cartouche
@end group
@end smallexample

@noindent
The vertical bar indicates the location of the error, and the @samp{>>>}
prefix can be used to search for error messages. When this qualifier is
used the only source lines output are those with errors.

@item /LIST
@cindex @option{/LIST} (@code{GNAT COMPILE})
This qualifier causes a full listing of
the file to be generated. The output might look as follows:

@smallexample
@group
@cartouche
 1. procedure E is
 2.    V : Integer;
 3.    funcion X (Q : Integer)
       |
    >>> Incorrect spelling of keyword "function"
 4.     return Integer;
                      |
    >>> ";" should be "is"
 5.    begin
 6.       return Q + Q;
 7.    end;
 8. begin
 9.    V := X + X;
10.end E;
@end cartouche
@end group
@end smallexample

@noindent
@findex SYS$ERROR
When you specify the @option{/REPORT_ERRORS=VERBOSE} or @option{/LIST} qualifiers and
standard output is redirected, a brief summary is written to
@file{SYS$ERROR} (standard error) giving the number of error messages and
warning messages generated.

@item /UNIQUE_ERROR_TAG
@cindex @option{/UNIQUE_ERROR_TAG} (@code{GNAT COMPILE})
This qualifier forces all error messages to be preceded by the unique
string "error:". This means that error messages take a few more
characters in space, but allows easy searching for and identification
of error messages.

@item /WARNINGS=BRIEF
@cindex @option{/WARNINGS=BRIEF} (@code{GNAT COMPILE})
This qualifier causes GNAT to generate the
brief format error messages to @file{SYS$ERROR} (the standard error
file) as well as the verbose
format message or full listing (which as usual is written to
@file{SYS$OUTPUT} (the standard output file).

@item /ERROR_LIMIT=@var{n}
@cindex @option{/ERROR_LIMIT} (@code{GNAT COMPILE})
@var{n} is a decimal integer in the
range of 1 to 999 and limits the number of error messages to be
generated. For example, using @option{/ERROR_LIMIT=2} might yield

@smallexample
@iftex
@leftskip=.7cm
@end iftex
E.ADB:3:04: Incorrect spelling of keyword "function"
E.ADB:5:35: missing ".."
fatal error: maximum errors reached
compilation abandoned
@end smallexample

@item /REPORT_ERRORS=FULL
@cindex @option{/REPORT_ERRORS=FULL} (@code{GNAT COMPILE})
@cindex Error messages, suppressing
Normally, the compiler suppresses error messages that are likely to be
redundant. This qualifier causes all error
messages to be generated. In particular, in the case of
references to undefined variables. If a given variable is referenced
several times, the normal format of messages is
@smallexample
@iftex
@leftskip=.7cm
@end iftex
E.ADB:7:07: "V" is undefined (more references follow)
@end smallexample

@noindent
where the parenthetical comment warns that there are additional
references to the variable @code{V}. Compiling the same program with the
@option{/REPORT_ERRORS=FULL} qualifier yields

@smallexample
E.ADB:7:07: "V" is undefined
E.ADB:8:07: "V" is undefined
E.ADB:8:12: "V" is undefined
E.ADB:8:16: "V" is undefined
E.ADB:9:07: "V" is undefined
E.ADB:9:12: "V" is undefined
@end smallexample

@item /TRY_SEMANTICS
@cindex @option{/TRY_SEMANTICS} (@code{GNAT COMPILE})
In normal operation mode, the compiler first parses the program and
determines if there are any syntax errors. If there are, appropriate
error messages are generated and compilation is immediately terminated.
This qualifier tells
GNAT to continue with semantic analysis even if syntax errors have been
found. This may enable the detection of more errors in a single run. On
the other hand, the semantic analyzer is more likely to encounter some
internal fatal error when given a syntactically invalid tree.

@item /FORCE_ALI
In normal operation mode, the @file{ali} file is not generated if any
illegalities are detected in the program. The use of @option{/FORCE_ALI} forces
generation of the @file{ali} file. This file is marked as being in
error, so it cannot be used for binding purposes, but it does contain
reasonably complete cross-reference information, and thus may be useful
for use by tools (e.g. semantic browsing tools or integrated development
environments) that are driven from the @file{ali} file.

In addition, if @option{/TREE_OUTPUT} is also specified, then the tree file is
generated even if there are illegalities. It may be useful in this case
to also specify @option{/TRY_SEMANTICS} to ensure that full semantic processing
occurs. The resulting tree file can be processed by ASIS, for the purpose
of providing partial information about illegal units, but if the error
causes the tree to be badly malformed, then ASIS may crash during the
analysis.

@end table

@noindent
In addition to error messages, which correspond to illegalities as defined
in the Ada 95 Reference Manual, the compiler detects two kinds of warning
situations.

@cindex Warning messages
First, the compiler considers some constructs suspicious and generates a
warning message to alert you to a possible error. Second, if the
compiler detects a situation that is sure to raise an exception at
run time, it generates a warning message. The following shows an example
of warning messages:
@smallexample
@iftex
@leftskip=.2cm
@end iftex
E.ADB:4:24: warning: creation of object may raise Storage_Error
E.ADB:10:17: warning: static value out of range
E.ADB:10:17: warning: "Constraint_Error" will be raised at run time

@end smallexample

@noindent
GNAT considers a large number of situations as appropriate
for the generation of warning messages. As always, warnings are not
definite indications of errors. For example, if you do an out-of-range
assignment with the deliberate intention of raising a
@code{Constraint_Error} exception, then the warning that may be
issued does not indicate an error. Some of the situations for which GNAT
issues warnings (at least some of the time) are given in the following
list, which is not necessarily complete.

@itemize @bullet
@item
Possible infinitely recursive calls

@item
Out-of-range values being assigned

@item
Possible order of elaboration problems

@item
Unreachable code

@item
Fixed-point type declarations with a null range

@item
Variables that are never assigned a value

@item
Variables that are referenced before being initialized

@item
Task entries with no corresponding accept statement

@item
Duplicate accepts for the same task entry in a select

@item
Objects that take too much storage

@item
Unchecked conversion between types of differing sizes

@item
Missing return statements along some execution paths in a function

@item
Incorrect (unrecognized) pragmas

@item
Incorrect external names

@item
Allocation from empty storage pool

@item
Potentially blocking operations in protected types

@item
Suspicious parenthesization of expressions

@item
Mismatching bounds in an aggregate

@item
Attempt to return local value by reference

@item
Unrecognized pragmas

@item
Premature instantiation of a generic body

@item
Attempt to pack aliased components

@item
Out of bounds array subscripts

@item
Wrong length on string assignment

@item
Violations of style rules if style checking is enabled

@item
Unused with clauses

@item
Bit_Order usage that does not have any effect

@item
Compile time biased rounding of floating-point constant

@item
Standard.Duration used to resolve universal fixed expression

@item
Dereference of possibly null value

@item
Declaration that is likely to cause storage error

@item
Internal GNAT unit with'ed by application unit

@item
Values known to be out of range at compile time

@item
Unreferenced labels and variables

@item
Address overlays that could clobber memory

@item
Unexpected initialization when address clause present

@item
Bad alignment for address clause

@item
Useless type conversions

@item
Redundant assignment statements

@item
Accidental hiding of name by child unit

@item
Unreachable code

@item
Access before elaboration detected at compile time

@item
A range in a @code{for} loop that is known to be null or might be null

@end itemize

@noindent
The following qualifiers are available to control the handling of
warning messages:

@table @code
@item /WARNINGS=OPTIONAL (activate all optional errors)
@cindex @option{/WARNINGS=OPTIONAL} (@code{GNAT COMPILE})
This qualifier activates most optional warning messages, see remaining list
in this section for details on optional warning messages that can be
individually controlled. The warnings that are not turned on by this
qualifier are @option{/WARNINGS=BIASED_ROUNDING} (biased rounding),
@option{/WARNINGS=IMPLICIT_DEREFERENCE} (implicit dereferencing),
and @option{/WARNINGS=HIDING} (hiding). All other optional warnings are
turned on.

@item /WARNINGS=NOOPTIONAL (suppress all optional errors)
@cindex @option{/WARNINGS=NOOPTIONAL} (@code{GNAT COMPILE})
This qualifier suppresses all optional warning messages, see remaining list
in this section for details on optional warning messages that can be
individually controlled.

@item /WARNINGS=BIASED_ROUNDING (activate warnings on biased rounding)
@cindex @option{/WARNINGS=BIASED_ROUNDING} (@code{GNAT COMPILE})
@cindex Rounding, biased
@cindex Biased rounding
If a static floating-point expression has a value that is exactly half
way between two adjacent machine numbers, then the rules of Ada
(Ada Reference Manual, section 4.9(38)) require that this rounding
be done away from zero, even if the normal unbiased rounding rules
at run time would require rounding towards zero. This warning message
alerts you to such instances where compile-time rounding and run-time
rounding are not equivalent. If it is important to get proper run-time
rounding, then you can force this by making one of the operands into
a variable. The default is that such warnings are not generated.
Note that @option{/WARNINGS=OPTIONAL} does not affect the setting of
this warning option.

@item /WARNINGS=NOBIASED_ROUNDING (suppress warnings on biased rounding)
@cindex @option{/WARNINGS=NOBIASED_ROUNDING} (@code{GNAT COMPILE})
This qualifier disables warnings on biased rounding.

@item /WARNINGS=CONDITIONALS (activate warnings on conditionals)
@cindex @option{/WARNINGS=CONDITIONALS} (@code{GNAT COMPILE})
@cindex Conditionals, constant
This qualifier activates warnings for conditional expressions used in
tests that are known to be True or False at compile time. The default
is that such warnings are not generated.
This warning can also be turned on using @option{/WARNINGS=OPTIONAL}.

@item /WARNINGS=NOCONDITIONALS (suppress warnings on conditionals)
@cindex @option{/WARNINGS=NOCONDITIONALS} (@code{GNAT COMPILE})
This qualifier suppresses warnings for conditional expressions used in
tests that are known to be True or False at compile time.

@item /WARNINGS=IMPLICIT_DEREFERENCE (activate warnings on implicit dereferencing)
@cindex @option{/WARNINGS=IMPLICIT_DEREFERENCE} (@code{GNAT COMPILE})
If this qualifier is set, then the use of a prefix of an access type
in an indexed component, slice, or selected component without an
explicit @code{.all} will generate a warning. With this warning
enabled, access checks occur only at points where an explicit
@code{.all} appears in the source code (assuming no warnings are
generated as a result of this qualifier). The default is that such
warnings are not generated.
Note that @option{/WARNINGS=OPTIONAL} does not affect the setting of
this warning option.

@item /WARNINGS=NOIMPLICIT_DEREFERENCE (suppress warnings on implicit dereferencing)
@cindex @option{/WARNINGS=NOIMPLICIT_DEREFERENCE} (@code{GNAT COMPILE})
@cindex Implicit dereferencing
@cindex Dereferencing, implicit
This qualifier suppresses warnings for implicit deferences in
indexed components, slices, and selected components.

@item /WARNINGS=ERROR (treat warnings as errors)
@cindex @option{/WARNINGS=ERROR} (@code{GNAT COMPILE})
@cindex Warnings, treat as error
This qualifier causes warning messages to be treated as errors.
The warning string still appears, but the warning messages are counted
as errors, and prevent the generation of an object file.

@item /WARNINGS=UNREFERENCED_FORMALS (activate warnings on unreferenced formals)
@cindex @option{/WARNINGS=UNREFERENCED_FORMALS} (@code{GNAT COMPILE})
@cindex Formals, unreferenced
This qualifier causes a warning to be generated if a formal parameter
is not referenced in the body of the subprogram. This warning can
also be turned on using @option{/WARNINGS=OPTIONAL} or @option{/WARNINGS=UNUSED}.

@item /WARNINGS=NOUNREFERENCED_FORMALS (suppress warnings on unreferenced formals)
@cindex @option{/WARNINGS=NOUNREFERENCED_FORMALS} (@code{GNAT COMPILE})
This qualifier suppresses warnings for unreferenced formal
parameters. Note that the
combination @option{/WARNINGS=UNUSED} followed by @option{/WARNINGS=NOUNREFERENCED_FORMALS} has the
effect of warning on unreferenced entities other than subprogram
formals.

@item /WARNINGS=HIDING (activate warnings on hiding)
@cindex @option{/WARNINGS=HIDING} (@code{GNAT COMPILE})
@cindex Hiding of Declarations
This qualifier activates warnings on hiding declarations.
A declaration is considered hiding
if it is for a non-overloadable entity, and it declares an entity with the
same name as some other entity that is directly or use-visible. The default
is that such warnings are not generated.
Note that @option{/WARNINGS=OPTIONAL} does not affect the setting of this warning option.

@item /WARNINGS=NOHIDING (suppress warnings on hiding)
@cindex @option{/WARNINGS=NOHIDING} (@code{GNAT COMPILE})
This qualifier suppresses warnings on hiding declarations.

@item /WARNINGS=IMPLEMENTATION (activate warnings on implementation units).
@cindex @option{/WARNINGS=IMPLEMENTATION} (@code{GNAT COMPILE})
This qualifier activates warnings for a @code{with} of an internal GNAT
implementation unit, defined as any unit from the @code{Ada},
@code{Interfaces}, @code{GNAT},
@code{DEC}, or @code{System}
hierarchies that is not
documented in either the Ada Reference Manual or the GNAT
Programmer's Reference Manual. Such units are intended only
for internal implementation purposes and should not be @code{with}'ed
by user programs. The default is that such warnings are generated
This warning can also be turned on using @option{/WARNINGS=OPTIONAL}.

@item /WARNINGS=NOIMPLEMENTATION (disable warnings on implementation units).
@cindex @option{/WARNINGS=NOIMPLEMENTATION} (@code{GNAT COMPILE})
This qualifier disables warnings for a @code{with} of an internal GNAT
implementation unit.

@item /WARNINGS=ELABORATION (activate warnings on elaboration pragmas)
@cindex @option{/WARNINGS=ELABORATION} (@code{GNAT COMPILE})
@cindex Elaboration, warnings
This qualifier activates warnings on missing pragma Elaborate_All statements.
See the section in this guide on elaboration checking for details on
when such pragma should be used. The default is that such warnings
are not generated.
This warning can also be turned on using @option{/WARNINGS=OPTIONAL}.

@item /WARNINGS=NOELABORATION (suppress warnings on elaboration pragmas)
@cindex @option{/WARNINGS=NOELABORATION} (@code{GNAT COMPILE})
This qualifier suppresses warnings on missing pragma Elaborate_All statements.
See the section in this guide on elaboration checking for details on
when such pragma should be used.

@item /WARNINGS=OVERLAYS (activate warnings on address clause overlays)
@cindex @option{/WARNINGS=OVERLAYS} (@code{GNAT COMPILE})
@cindex Address Clauses, warnings
This qualifier activates warnings for possibly unintended initialization
effects of defining address clauses that cause one variable to overlap
another. The default is that such warnings are generated.
This warning can also be turned on using @option{/WARNINGS=OPTIONAL}.

@item /WARNINGS=NOOVERLAYS (suppress warnings on address clause overlays)
@cindex @option{/WARNINGS=NOOVERLAYS} (@code{GNAT COMPILE})
This qualifier suppresses warnings on possibly unintended initialization
effects of defining address clauses that cause one variable to overlap
another.

@item -gnatwp (activate warnings on ineffective pragma Inlines)
@cindex @option{-gnatwp} (@code{GNAT COMPILE})
@cindex Inlining, warnings
This qualifier activates warnings for failure of front end inlining
(activated by @option{-gnatN}) to inline a particular call. There are
many reasons for not being able to inline a call, including most
commonly that the call is too complex to inline.
This warning can also be turned on using @option{/WARNINGS=OPTIONAL}.

@item -gnatwP (suppress warnings on ineffective pragma Inlines)
@cindex @option{-gnatwP} (@code{GNAT COMPILE})
This qualifier suppresses warnings on ineffective pragma Inlines. If the
inlining mechanism cannot inline a call, it will simply ignore the
request silently.

@item /WARNINGS=REDUNDANT (activate warnings on redundant constructs)
@cindex @option{/WARNINGS=REDUNDANT} (@code{GNAT COMPILE})
This qualifier activates warnings for redundant constructs. The following
is the current list of constructs regarded as redundant:
This warning can also be turned on using @option{/WARNINGS=OPTIONAL}.

@itemize @bullet
@item
Assignment of an item to itself.
@item
Type conversion that converts an expression to its own type.
@item
Use of the attribute @code{Base} where @code{typ'Base} is the same
as @code{typ}.
@item
Use of pragma @code{Pack} when all components are placed by a record
representation clause.
@end itemize

@item /WARNINGS=NOREDUNDANT (suppress warnings on redundant constructs)
@cindex @option{/WARNINGS=NOREDUNDANT} (@code{GNAT COMPILE})
This qualifier suppresses warnings for redundant constructs.

@item /WARNINGS=SUPPRESS (suppress all warnings)
@cindex @option{/WARNINGS=SUPPRESS} (@code{GNAT COMPILE})
This qualifier completely suppresses the
output of all warning messages from the GNAT front end.
Note that it does not suppress warnings from the @code{GNAT COMPILE} back end.
To suppress these back end warnings as well, use the qualifier @code{-w}
in addition to @option{/WARNINGS=SUPPRESS}.

@item /WARNINGS=UNUSED (activate warnings on unused entities)
@cindex @option{/WARNINGS=UNUSED} (@code{GNAT COMPILE})
This qualifier activates warnings to be generated for entities that
are defined but not referenced, and for units that are @code{with}'ed
and not
referenced. In the case of packages, a warning is also generated if
no entities in the package are referenced. This means that if the package
is referenced but the only references are in @code{use}
clauses or @code{renames}
declarations, a warning is still generated. A warning is also generated
for a generic package that is @code{with}'ed but never instantiated.
In the case where a package or subprogram body is compiled, and there
is a @code{with} on the corresponding spec
that is only referenced in the body,
a warning is also generated, noting that the
@code{with} can be moved to the body. The default is that
such warnings are not generated.
This qualifier also activates warnings on unreferenced formals
(it is includes the effect of @option{/WARNINGS=UNREFERENCED_FORMALS}).
This warning can also be turned on using @option{/WARNINGS=OPTIONAL}.

@item /WARNINGS=NOUNUSED (suppress warnings on unused entities)
@cindex @option{/WARNINGS=NOUNUSED} (@code{GNAT COMPILE})
This qualifier suppresses warnings for unused entities and packages.
It also turns off warnings on unreferenced formals (and thus includes
the effect of @option{/WARNINGS=NOUNREFERENCED_FORMALS}).

@noindent
A string of warning parameters can be used in the same parameter. For example:

@smallexample
-gnatwaLe
@end smallexample

@noindent
Would turn on all optional warnings except for elaboration pragma warnings,
and also specify that warnings should be treated as errors.

@item -w
@cindex @code{-w}
This qualifier suppresses warnings from the @code{GNAT COMPILE} backend. It may be
used in conjunction with @option{/WARNINGS=SUPPRESS} to ensure that all warnings
are suppressed during the entire compilation process.

@end table

@node Debugging and Assertion Control
@subsection Debugging and Assertion Control

@table @code
@item /CHECKS=ASSERTIONS
@cindex @option{/CHECKS=ASSERTIONS} (@code{GNAT COMPILE})
@findex Assert
@findex Debug
@cindex Assertions

@noindent
The pragmas @code{Assert} and @code{Debug} normally have no effect and
are ignored. This qualifier, where @samp{a} stands for assert, causes
@code{Assert} and @code{Debug} pragmas to be activated.

The pragmas have the form:

@smallexample
@group
@cartouche
   @b{pragma} Assert (@var{Boolean-expression} [,
                      @var{static-string-expression}])
   @b{pragma} Debug (@var{procedure call})
@end cartouche
@end group
@end smallexample

@noindent
The @code{Assert} pragma causes @var{Boolean-expression} to be tested.
If the result is @code{True}, the pragma has no effect (other than
possible side effects from evaluating the expression). If the result is
@code{False}, the exception @code{Assert_Failure} declared in the package
@code{System.Assertions} is
raised (passing @var{static-string-expression}, if present, as the
message associated with the exception). If no string expression is
given the default is a string giving the file name and line number
of the pragma.

The @code{Debug} pragma causes @var{procedure} to be called. Note that
@code{pragma Debug} may appear within a declaration sequence, allowing
debugging procedures to be called between declarations.

@item /DEBUG[=debug-level]
@itemx  /NODEBUG
Specifies how much debugging information is to be included in
the resulting object file where 'debug-level' is one of the following:
@table @code
@item   TRACEBACK (default)
Include both debugger symbol records and traceback
the object file.
@item   ALL
Include both debugger symbol records and traceback in
object file.
@item   NONE
Excludes both debugger symbol records and traceback
the object file. Same as /NODEBUG.
@item   SYMBOLS
Includes only debugger symbol records in the object
file. Note that this doesn't include traceback information.
@end table
@end table

@node Validity Checking
@subsection Validity Checking
@findex Validity Checking

@noindent
The Ada 95 Reference Manual has specific requirements for checking
for invalid values. In particular, RM 13.9.1 requires that the
evaluation of invalid values (for example from unchecked conversions),
not result in erroneous execution. In GNAT, the result of such an
evaluation in normal default mode is to either use the value
unmodified, or to raise Constraint_Error in those cases where use
of the unmodified value would cause erroneous execution. The cases
where unmodified values might lead to erroneous execution are case
statements (where a wild jump might result from an invalid value),
and subscripts on the left hand side (where memory corruption could
occur as a result of an invalid value).

The @option{-gnatVx} qualifier allows more control over the validity checking
mode. The @code{x} argument here is a string of letters which control which
validity checks are performed in addition to the default checks described
above.

@itemize @bullet
@item
@option{-gnatVc} Validity checks for copies

The right hand side of assignments, and the initializing values of
object declarations are validity checked.

@item
@option{/VALIDITY_CHECKING=RM} Default (RM) validity checks

Some validity checks are done by default following normal Ada semantics
(RM 13.9.1 (9-11)).
A check is done in case statements that the expression is within the range
of the subtype. If it is not, Constraint_Error is raised.
For assignments to array components, a check is done that the expression used
as index is within the range. If it is not, Constraint_Error is raised.
Both these validity checks may be turned off using qualifier @option{-gnatVD}.
They are turned on by default. If @option{-gnatVD} is specified, a subsequent
qualifier @option{/VALIDITY_CHECKING=RM} will leave the checks turned on.
Qualifier @option{-gnatVD} should be used only if you are sure that all such
expressions have valid values. If you use this qualifier and invalid values
are present, then the program is erroneous, and wild jumps or memory
overwriting may occur.

@item
@option{-gnatVi} Validity checks for @code{in} mode parameters

Arguments for parameters of mode @code{in} are validity checked in function
and procedure calls at the point of call.

@item
@option{-gnatVm} Validity checks for @code{in out} mode parameters

Arguments for parameters of mode @code{in out} are validity checked in
procedure calls at the point of call. The @code{'m'} here stands for
modify, since this concerns parameters that can be modified by the call.
Note that there is no specific option to test @code{out} parameters,
but any reference within the subprogram will be tested in the usual
manner, and if an invalid value is copied back, any reference to it
will be subject to validity checking.

@item
@option{-gnatVo} Validity checks for operator and attribute operands

Arguments for predefined operators and attributes are validity checked.
This includes all operators in package @code{Standard},
the shift operators defined as intrinsic in package @code{Interfaces}
and operands for attributes such as @code{Pos}.

@item
@option{-gnatVr} Validity checks for function returns

The expression in @code{return} statements in functions is validity
checked.

@item
@option{-gnatVs} Validity checks for subscripts

All subscripts expressions are checked for validity, whether they appear
on the right side or left side (in default mode only left side subscripts
are validity checked).

@item
@option{-gnatVt} Validity checks for tests

Expressions used as conditions in @code{if}, @code{while} or @code{exit}
statements are checked, as well as guard expressions in entry calls.

@item
@option{/VALIDITY_CHECKING=FULL} Validity checks for floating-point values

In the absence of this qualifier, validity checking occurs only for discrete
values. If @option{/VALIDITY_CHECKING=FULL} is specified, then validity checking also applies
for floating-point values, and NaN's and infinities are considered invalid,
as well as out of range values for constrained types. Note that this means
that standard @code{IEEE} infinity mode is not allowed. The exact contexts
in which floating-point values are checked depends on the setting of other
options. For example @option{-gnatVif} or @option{-gnatVfi} (the order does
not matter) specifies that floating-point parameters of mode @code{in} should
be validity checked.

@item
@option{-gnatVa} All validity checks

All the above validity checks are turned on. That is @option{-gnatVa} is
equivalent to @code{gnatVcdfimorst}.

@item
@option{-gnatVn} No validity checks

This qualifier turns off all validity checking, including the default checking
for case statements and left hand side subscripts. Note that the use of
the qualifier @option{/CHECKS=SUPPRESS_ALL} supresses all run-time checks, including
validity checks, and thus implies @option{-gnatVn}.

@end itemize

The @option{/VALIDITY_CHECKING} qualifier may be followed by a string of letters to turn on
a series of validity checking options. For example, @option{-gnatVcr} specifies
that in addition to the default validity checking, copies and function
return expressions be validity checked. In order to make it easier to specify
a set of options, the upper case letters @code{CDFIMORST} may be used to turn
off the corresponding lower case option, so for example @option{-gnatVaM} turns
on all validity checking options except for checking of @code{in out}
procedure arguments.

The specification of additional validity checking generates extra code (and
in the case of @option{-gnatva} the code expansion can be substantial. However,
these additional checks can be very useful in smoking out cases of
uninitialized variables, incorrect use of unchecked conversion, and other
errors leading to invalid values. The use of pragma @code{Initialize_Scalars}
is useful in conjunction with the extra validity checking, since this
ensures that wherever possible uninitialized variables have invalid values.

See also the pragma @code{Validity_Checks} which allows modification of
the validity checking mode at the program source level, and also allows for
temporary disabling of validity checks.

@node Style Checking
@subsection Style Checking
@findex Style checking

@noindent
The /STYLE=@var{(option,option,..)} qualifier causes the compiler to
enforce specified style rules. A limited set of style rules has been used
in writing the GNAT sources themselves. This qualifier allows user programs
to activate all or some of these checks. If the source program fails a
specified style check, an appropriate warning message is given, preceded by
the character sequence "(style)".
(OPTION,OPTION,..) is a sequence of keywords
indicating the particular style
checks to be performed. The following checks are defined:

@table @code
@item 1-9 (specify indentation level)
If a digit from 1-9 appears in the string after @option{/STYLE=} then proper
indentation is checked, with the digit indicating the indentation level
required. The general style of required indentation is as specified by
the examples in the Ada Reference Manual. Full line comments must be
aligned with the @code{--} starting on a column that is a multiple of
the alignment level.

@item ATTRIBUTE (check attribute casing)
If the word ATTRIBUTE appears in the string after @option{/STYLE=} then
attribute names, including the case of keywords such as @code{digits}
used as attributes names, must be written in mixed case, that is, the
initial letter and any letter following an underscore must be uppercase.
All other letters must be lowercase.

@item BLANKS (blanks not allowed at statement end)
If the word BLANKS appears in the string after @option{/STYLE=} then
trailing blanks are not allowed at the end of statements. The purpose of this
rule, together with h (no horizontal tabs), is to enforce a canonical format
for the use of blanks to separate source tokens.

@item COMMENTS (check comments)
If the word COMMENTS appears in the string after @option{/STYLE=} then
comments must meet the following set of rules:

@itemize @bullet

@item
The "--" that starts the column must either start in column one, or else
at least one blank must precede this sequence.

@item
Comments that follow other tokens on a line must have at least one blank
following the "--" at the start of the comment.

@item
Full line comments must have two blanks following the "--" that starts
the comment, with the following exceptions.

@item
A line consisting only of the "--" characters, possibly preceded by blanks
is permitted.

@item
A comment starting with "--x" where x is a special character is permitted.
This alows proper processing of the output generated by specialized tools
including @code{GNAT PREPROCESS} (where --! is used) and the SPARK annnotation
language (where --# is used). For the purposes of this rule, a special
character is defined as being in one of the ASCII ranges
16#21#..16#2F# or 16#3A#..16#3F#.

@item
A line consisting entirely of minus signs, possibly preceded by blanks, is
permitted. This allows the construction of box comments where lines of minus
signs are used to form the top and bottom of the box.

@item
If a comment starts and ends with "--" is permitted as long as at least
one blank follows the initial "--". Together with the preceding rule,
this allows the construction of box comments, as shown in the following
example:
@smallexample
---------------------------
-- This is a box comment --
-- with two text lines.  --
---------------------------
@end smallexample
@end itemize

@item END (check end/exit labels)
If the word END appears in the string after @option{/STYLE=} then
optional labels on @code{end} statements ending subprograms and on
@code{exit} statements exiting named loops, are required to be present.

@item VTABS (no form feeds or vertical tabs)
If the word VTABS appears in the string after @option{/STYLE=} then
neither form feeds nor vertical tab characters are not permitted
in the source text.

@item HTABS (no horizontal tabs)
If the word HTABS appears in the string after @option{/STYLE=} then
horizontal tab characters are not permitted in the source text.
Together with the b (no blanks at end of line) check, this
enforces a canonical form for the use of blanks to separate
source tokens.

@item IF_THEN (check if-then layout)
If the word IF_THEN appears in the string after @option{/STYLE=},
then the keyword @code{then} must appear either on the same
line as corresponding @code{if}, or on a line on its own, lined
up under the @code{if} with at least one non-blank line in between
containing all or part of the condition to be tested.

@item KEYWORD (check keyword casing)
If the word KEYWORD appears in the string after @option{/STYLE=} then
all keywords must be in lower case (with the exception of keywords
such as @code{digits} used as attribute names to which this check
does not apply).

@item LAYOUT (check layout)
If the word LAYOUT appears in the string after @option{/STYLE=} then
layout of statement and declaration constructs must follow the
recommendations in the Ada Reference Manual, as indicated by the
form of the syntax rules. For example an @code{else} keyword must
be lined up with the corresponding @code{if} keyword.

There are two respects in which the style rule enforced by this check
option are more liberal than those in the Ada Reference Manual. First
in the case of record declarations, it is permissible to put the
@code{record} keyword on the same line as the @code{type} keyword, and
then the @code{end} in @code{end record} must line up under @code{type}.
For example, either of the following two layouts is acceptable:

@smallexample
@group
@cartouche
@b{type} q @b{is record}
   a : integer;
   b : integer;
@b{end record};

@b{type} q @b{is}
   @b{record}
      a : integer;
      b : integer;
   @b{end record};
@end cartouche
@end group
@end smallexample

@noindent
Second, in the case of a block statement, a permitted alternative
is to put the block label on the same line as the @code{declare} or
@code{begin} keyword, and then line the @code{end} keyword up under
the block label. For example both the following are permitted:

@smallexample
@group
@cartouche
Block : @b{declare}
   A : Integer := 3;
@b{begin}
   Proc (A, A);
@b{end} Block;

Block :
   @b{declare}
      A : Integer := 3;
   @b{begin}
      Proc (A, A);
   @b{end} Block;
@end cartouche
@end group
@end smallexample

@noindent
The same alternative format is allowed for loops. For example, both of
the following are permitted:

@smallexample
@group
@cartouche
Clear : @b{while} J < 10 @b{loop}
   A (J) := 0;
@b{end loop} Clear;

Clear :
   @b{while} J < 10 @b{loop}
      A (J) := 0;
   @b{end loop} Clear;
@end cartouche
@end group
@end smallexample

@item LINE_LENGTH (check maximum line length)
If the word LINE_LENGTH appears in the string after @option{/STYLE=}
then the length of source lines must not exceed 79 characters, including
any trailing blanks. The value of 79 allows convenient display on an
80 character wide device or window, allowing for possible special
treatment of 80 character lines.

@item MAX_LENGTH=nnn (set maximum line length)
If the sequence MAX_LENGTH=nnn, where nnn is a decimal number, appears in
the string after @option{/STYLE=} then the length of lines must not exceed the
given value.

@item STANDARD_CASING (check casing of entities in Standard)
If the word STANDARD_CASING appears in the string
after @option{/STYLE=} then any identifier from Standard must be cased
to match the presentation in the Ada Reference Manual (for example,
@code{Integer} and @code{ASCII.NUL}).

@item ORDERED_SUBPROGRAMS (check order of subprogram bodies)
If the word ORDERED_SUBPROGRAMS appears in the string
after @option{/STYLE=} then all subprogram bodies in a given scope
(e.g. a package body) must be in alphabetical order. The ordering
rule uses normal Ada rules for comparing strings, ignoring casing
of letters, except that if there is a trailing numeric suffix, then
the value of this suffix is used in the ordering (e.g. Junk2 comes
before Junk10).

@item PRAGMA (check pragma casing)
If the word PRAGMA appears in the string after @option{/STYLE=} then
pragma names must be written in mixed case, that is, the
initial letter and any letter following an underscore must be uppercase.
All other letters must be lowercase.

@item REFERENCES (check references)
If the word REFERENCES appears in the string after @option{/STYLE=}
then all identifier references must be cased in the same way as the
corresponding declaration. No specific casing style is imposed on
identifiers. The only requirement is for consistency of references
with declarations.

@item SPECS (check separate specs)
If the word SPECS appears in the string after @option{/STYLE=} then
separate declarations ("specs") are required for subprograms (a
body is not allowed to serve as its own declaration). The only
exception is that parameterless library level procedures are
not required to have a separate declaration. This exception covers
the most frequent form of main program procedures.

@item TOKEN (check token spacing)
If the word TOKEN appears in the string after @option{/STYLE=} then
the following token spacing rules are enforced:

@itemize @bullet

@item
The keywords @code{abs} and @code{not} must be followed by a space.

@item
The token @code{=>} must be surrounded by spaces.

@item
The token @code{<>} must be preceded by a space or a left parenthesis.

@item
Binary operators other than @code{**} must be surrounded by spaces.
There is no restriction on the layout of the @code{**} binary operator.

@item
Colon must be surrounded by spaces.

@item
Colon-equal (assignment) must be surrounded by spaces.

@item
Comma must be the first non-blank character on the line, or be
immediately preceded by a non-blank character, and must be followed
by a space.

@item
If the token preceding a left paren ends with a letter or digit, then
a space must separate the two tokens.

@item
A right parenthesis must either be the first non-blank character on
a line, or it must be preceded by a non-blank character.

@item
A semicolon must not be preceded by a space, and must not be followed by
a non-blank character.

@item
A unary plus or minus may not be followed by a space.

@item
A vertical bar must be surrounded by spaces.
@end itemize

@noindent
In the above rules, appearing in column one is always permitted, that is,
counts as meeting either a requirement for a required preceding space,
or as meeting a requirement for no preceding space.

Appearing at the end of a line is also always permitted, that is, counts
as meeting either a requirement for a following space, or as meeting
a requirement for no following space.

@end table

@noindent
If any of these style rules is violated, a message is generated giving
details on the violation. The initial characters of such messages are
always "(style)". Note that these messages are treated as warning
messages, so they normally do not prevent the generation of an object
file. The @option{/WARNINGS=ERROR} qualifier can be used to treat warning messages,
including style messages, as fatal errors.

@noindent
The qualifier
/STYLE_CHECKS=ALL_BUILTIN
is equivalent to  all checking
options  enabled with
the exception of ORDERED_SUBPROGRAMS,
with an indentation level of 3. This is the standard
checking option that is used for the GNAT sources.

@node Run-Time Checks
@subsection Run-Time Checks
@cindex Division by zero
@cindex Access before elaboration
@cindex Checks, division by zero
@cindex Checks, access before elaboration

@noindent
If you compile with the default options, GNAT will insert many run-time
checks into the compiled code, including code that performs range
checking against constraints, but not arithmetic overflow checking for
integer operations (including division by zero) or checks for access
before elaboration on subprogram calls. All other run-time checks, as
required by the Ada 95 Reference Manual, are generated by default.
The following @code{GNAT COMPILE} qualifiers refine this default behavior:

@table @code
@item /CHECKS=SUPPRESS_ALL
@cindex @option{/CHECKS=SUPPRESS_ALL} (@code{GNAT COMPILE})
@cindex Suppressing checks
@cindex Checks, suppressing
@findex Suppress
Suppress all run-time checks as though @code{pragma Suppress (all_checks})
had been present in the source. Validity checks are also suppressed (in
other words @option{/CHECKS=SUPPRESS_ALL} also implies @option{-gnatVn}.
Use this qualifier to improve the performance
of the code at the expense of safety in the presence of invalid data or
program bugs.

@item /CHECKS=OVERFLOW
@cindex @option{/CHECKS=OVERFLOW} (@code{GNAT COMPILE})
@cindex Overflow checks
@cindex Check, overflow
Enables overflow checking for integer operations.
This causes GNAT to generate slower and larger executable
programs by adding code to check for overflow (resulting in raising
@code{Constraint_Error} as required by standard Ada
semantics). These overflow checks correspond to situations in which
the true value of the result of an operation may be outside the base
range of the result type. The following example shows the distinction:

@smallexample
X1 : Integer := Integer'Last;
X2 : Integer range 1 .. 5 := 5;
...
X1 := X1 + 1;   -- @option{/CHECKS=OVERFLOW} required to catch the Constraint_Error
X2 := X2 + 1;   -- range check, @option{/CHECKS=OVERFLOW} has no effect here
@end smallexample

@noindent
Here the first addition results in a value that is outside the base range
of Integer, and hence requires an overflow check for detection of the
constraint error. The second increment operation results in a violation
of the explicit range constraint, and such range checks are always
performed. Basically the compiler can assume that in the absence of
the @option{/CHECKS=OVERFLOW} qualifier that any value of type @code{xxx} is
in range of the base type of @code{xxx}.

@findex Machine_Overflows
Note that the @option{/CHECKS=OVERFLOW} qualifier does not affect the code generated
for any floating-point operations; it applies only to integer
semantics).
For floating-point, GNAT has the @code{Machine_Overflows}
attribute set to @code{False} and the normal mode of operation is to
generate IEEE NaN and infinite values on overflow or invalid operations
(such as dividing 0.0 by 0.0).

The reason that we distinguish overflow checking from other kinds of
range constraint checking is that a failure of an overflow check can
generate an incorrect value, but cannot cause erroneous behavior. This
is unlike the situation with a constraint check on an array subscript,
where failure to perform the check can result in random memory description,
or the range check on a case statement, where failure to perform the check
can cause a wild jump.

Note again that @option{/CHECKS=OVERFLOW} is off by default, so overflow checking is
not performed in default mode. This means that out of the box, with the
default settings, GNAT does not do all the checks expected from the
language description in the Ada Reference Manual. If you want all constraint
checks to be performed, as described in this Manual, then you must
explicitly use the /CHECKS=OVERFLOW qualifier either on the @code{GNAT MAKE} or
@code{GNAT COMPILE} command.

@item /CHECKS=ELABORATION
@cindex @option{/CHECKS=ELABORATION} (@code{GNAT COMPILE})
@cindex Elaboration checks
@cindex Check, elaboration
Enables dynamic checks for access-before-elaboration
on subprogram calls and generic instantiations.
For full details of the effect and use of this qualifier,
@xref{Compiling Using GNAT COMPILE}.
@end table

@findex Unsuppress
@noindent
The setting of these qualifiers only controls the default setting of the
checks. You may modify them using either @code{Suppress} (to remove
checks) or @code{Unsuppress} (to add back suppressed checks) pragmas in
the program source.

@node Stack Overflow Checking
@subsection Stack Overflow Checking
@cindex Stack Overflow Checking
@cindex -fstack-check

@noindent
For most operating systems, @code{GNAT COMPILE} does not perform stack overflow
checking by default. This means that if the main environment task or
some other task exceeds the available stack space, then unpredictable
behavior will occur.

To activate stack checking, compile all units with the GNAT COMPILE option
@code{-fstack-check}. For example:

@smallexample
GNAT COMPILE -fstack-check PACKAGE1.ADB
@end smallexample

@noindent
Units compiled with this option will generate extra instructions to check
that any use of the stack (for procedure calls or for declaring local
variables in declare blocks) do not exceed the available stack space.
If the space is exceeded, then a @code{Storage_Error} exception is raised.

For declared tasks, the stack size is always controlled by the size
given in an applicable @code{Storage_Size} pragma (or is set to
the default size if no pragma is used.

For the environment task, the stack size depends on
system defaults and is unknown to the compiler. The stack
may even dynamically grow on some systems, precluding the
normal Ada semantics for stack overflow. In the worst case,
unbounded stack usage, causes unbounded stack expansion
resulting in the system running out of virtual memory.

The stack checking may still work correctly if a fixed
size stack is allocated, but this cannot be guaranteed.
To ensure that a clean exception is signalled for stack
overflow, set the environment variable
@code{GNAT_STACK_LIMIT} to indicate the maximum
stack area that can be used, as in:
@cindex GNAT_STACK_LIMIT

@smallexample
SET GNAT_STACK_LIMIT 1600
@end smallexample

@noindent
The limit is given in kilobytes, so the above declaration would
set the stack limit of the environment task to 1.6 megabytes.
Note that the only purpose of this usage is to limit the amount
of stack used by the environment task. If it is necessary to
increase the amount of stack for the environment task, then this
is an operating systems issue, and must be addressed with the
appropriate operating systems commands.

@node Run-Time Control
@subsection Run-Time Control

@table @code
@item -gnatT nnn
@cindex @option{-gnatT} (@code{GNAT COMPILE})
@cindex Time Slicing

@noindent
The @code{gnatT} qualifier can be used to specify the time-slicing value
to be used for task switching between equal priority tasks. The value
@code{nnn} is given in microseconds as a decimal integer.

Setting the time-slicing value is only effective if the underlying thread
control system can accommodate time slicing. Check the documentation of
your operating system for details. Note that the time-slicing value can
also be set by use of pragma @code{Time_Slice} or by use of the
@code{t} qualifier in the GNAT BIND step. The pragma overrides a command
line argument if both are present, and the @code{t} qualifier for GNAT BIND
overrides both the pragma and the @code{GNAT COMPILE} command line qualifier.
@end table

@node Using GNAT COMPILE for Syntax Checking
@subsection Using @code{GNAT COMPILE} for Syntax Checking
@table @code
@item /SYNTAX_ONLY
@cindex @option{/SYNTAX_ONLY} (@code{GNAT COMPILE})

Run GNAT in syntax checking only mode. For
example, the command

@smallexample
$ GNAT COMPILE /SYNTAX_ONLY X.ADB
@end smallexample

@noindent
compiles file @file{X.ADB} in syntax-check-only mode. You can check a
series of files in a single command
.

You may use other qualifiers in conjunction with @option{/SYNTAX_ONLY}. In
particular, @option{/LIST} and @option{/REPORT_ERRORS=VERBOSE} are useful to control the
format of any generated error messages.

The output is simply the error messages, if any. No object file or ALI
file is generated by a syntax-only compilation. Also, no units other
than the one specified are accessed. For example, if a unit @code{X}
@code{with}'s a unit @code{Y}, compiling unit @code{X} in syntax
check only mode does not access the source file containing unit
@code{Y}.

@cindex Multiple units, syntax checking
Normally, GNAT allows only a single unit in a source file. However, this
restriction does not apply in syntax-check-only mode, and it is possible
to check a file containing multiple compilation units concatenated
together. This is primarily used by the @code{GNAT CHOP} utility
(@pxref{Renaming Files Using GNAT CHOP}).
@end table

@node Using GNAT COMPILE for Semantic Checking
@subsection Using @code{GNAT COMPILE} for Semantic Checking
@table @code
@item /NOLOAD
@cindex @option{/NOLOAD} (@code{GNAT COMPILE})

Causes the compiler to operate in semantic check mode,
with full checking for all illegalities specified in the
Ada 95 Reference Manual, but without generation of any object code
(no object file is generated).

Because dependent files must be accessed, you must follow the GNAT
semantic restrictions on file structuring to operate in this mode:

@itemize @bullet
@item
The needed source files must be accessible
(@pxref{Search Paths and the Run-Time Library (RTL)}).

@item
Each file must contain only one compilation unit.

@item
The file name and unit name must match (@pxref{File Naming Rules}).
@end itemize

The output consists of error messages as appropriate. No object file is
generated. An @file{ALI} file is generated for use in the context of
cross-reference tools, but this file is marked as not being suitable
for binding (since no object file is generated).
The checking corresponds exactly to the notion of
legality in the Ada 95 Reference Manual.

Any unit can be compiled in semantics-checking-only mode, including
units that would not normally be compiled (subunits,
and specifications where a separate body is present).
@end table

@node Compiling Ada 83 Programs
@subsection Compiling Ada 83 Programs
@table @code
@cindex Ada 83 compatibility
@item /83
@cindex @option{/83} (@code{GNAT COMPILE})
@cindex ACVC, Ada 83 tests

@noindent
Although GNAT is primarily an Ada 95 compiler, it accepts this qualifier to
specify that an Ada 83 program is to be compiled in Ada83 mode. If you specify
this qualifier, GNAT rejects most Ada 95 extensions and applies Ada 83 semantics
where this can be done easily.
It is not possible to guarantee this qualifier does a perfect
job; for example, some subtle tests, such as are
found in earlier ACVC tests (that have been removed from the ACVC suite for Ada
95), may not compile correctly. However, for most purposes, using
this qualifier should help to ensure that programs that compile correctly
under the @option{/83} qualifier can be ported easily to an Ada 83
compiler. This is the main use of the qualifier.

With few exceptions (most notably the need to use @code{<>} on
@cindex Generic formal parameters
unconstrained generic formal parameters, the use of the new Ada 95
keywords, and the use of packages
with optional bodies), it is not necessary to use the
@option{/83} qualifier when compiling Ada 83 programs, because, with rare
exceptions, Ada 95 is upwardly compatible with Ada 83. This
means that a correct Ada 83 program is usually also a correct Ada 95
program.

@end table

@node Character Set Control
@subsection Character Set Control
@table @code
@item /IDENTIFIER_CHARACTER_SET=@var{c}
@cindex @code{/IDENTIFIER_CHARACTER_SET} (@code{GNAT COMPILE})

@noindent
Normally GNAT recognizes the Latin-1 character set in source program
identifiers, as described in the Ada 95 Reference Manual.
This qualifier causes
GNAT to recognize alternate character sets in identifiers. @var{c} is a
single character or word indicating the character set, as follows:

@table @code
@item 1
Latin-1 identifiers

@item 2
Latin-2 letters allowed in identifiers

@item 3
Latin-3 letters allowed in identifiers

@item 4
Latin-4 letters allowed in identifiers

@item 5
Latin-5 (Cyrillic) letters allowed in identifiers

@item 9
Latin-9 letters allowed in identifiers

@item PC
IBM PC letters (code page 437) allowed in identifiers

@item PC850
IBM PC letters (code page 850) allowed in identifiers

@item FULL_UPPER
Full upper-half codes allowed in identifiers

@item NO_UPPER
No upper-half codes allowed in identifiers

@item WIDE
Wide-character codes (that is, codes greater than 255)
allowed in identifiers
@end table

@xref{Foreign Language Representation}, for full details on the
implementation of these character sets.

@item /WIDE_CHARACTER_ENCODING=@var{e}
@cindex @code{/WIDE_CHARACTER_ENCODING} (@code{GNAT COMPILE})
Specify the method of encoding for wide characters.
@var{e} is one of the following:

@table @code

@item HEX
Hex encoding (brackets coding also recognized)

@item UPPER
Upper half encoding (brackets encoding also recognized)

@item SHIFT_JIS
Shift/JIS encoding (brackets encoding also recognized)

@item EUC
EUC encoding (brackets encoding also recognized)

@item UTF8
UTF-8 encoding (brackets encoding also recognized)

@item BRACKETS
Brackets encoding only (default value)
@end table
For full details on the these encoding
methods see @xref{Wide Character Encodings}.
Note that brackets coding is always accepted, even if one of the other
options is specified, so for example @option{/WIDE_CHARACTER_ENCODING=UTF8} specifies that both
brackets and @code{UTF-8} encodings will be recognized. The units that are
with'ed directly or indirectly will be scanned using the specified
representation scheme, and so if one of the non-brackets scheme is
used, it must be used consistently throughout the program. However,
since brackets encoding is always recognized, it may be conveniently
used in standard libraries, allowing these libraries to be used with
any of the available coding schemes.
scheme. If no @option{/WIDE_CHARACTER_ENCODING=?} parameter is present, then the default
representation is Brackets encoding only.

Note that the wide character representation that is specified (explicitly
or by default) for the main program also acts as the default encoding used
for Wide_Text_IO files if not specifically overridden by a WCEM form
parameter.

@end table
@node File Naming Control
@subsection File Naming Control

@table @code
@item /FILE_NAME_MAX_LENGTH=@var{n}
@cindex @option{/FILE_NAME_MAX_LENGTH} (@code{GNAT COMPILE})
Activates file name "krunching". @var{n}, a decimal integer in the range
1-999, indicates the maximum allowable length of a file name (not
including the @file{.ADS} or @file{.ADB} extension). The default is not
to enable file name krunching.

For the source file naming rules, @xref{File Naming Rules}.
@end table

@node Subprogram Inlining Control
@subsection Subprogram Inlining Control

@table @code
@item /INLINE=PRAGMA
@cindex @option{/INLINE=PRAGMA} (@code{GNAT COMPILE})
GNAT recognizes and processes @code{Inline} pragmas. However, for the
inlining to actually occur, optimization must be enabled. To enable
inlining across unit boundaries, this is, inlining a call in one unit of
a subprogram declared in a @code{with}'ed unit, you must also specify
this qualifier.
In the absence of this qualifier, GNAT does not attempt
inlining across units and does not need to access the bodies of
subprograms for which @code{pragma Inline} is specified if they are not
in the current unit.

If you specify this qualifier the compiler will access these bodies,
creating an extra source dependency for the resulting object file, and
where possible, the call will be inlined.
For further details on when inlining is possible
see @xref{Inlining of Subprograms}.

@item -gnatN
@cindex @option{-gnatN} (@code{GNAT COMPILE})
The front end inlining activated by this qualifier is generally more extensive,
and quite often more effective than the standard @option{/INLINE=PRAGMA} inlining mode.
It will also generate additional dependencies.

@end table

@node Auxiliary Output Control
@subsection Auxiliary Output Control

@table @code
@item /TREE_OUTPUT
@cindex @option{/TREE_OUTPUT} (@code{GNAT COMPILE})
@cindex Writing internal trees
@cindex Internal trees, writing to file
Causes GNAT to write the internal tree for a unit to a file (with the
extension @file{.adt}.
This not normally required, but is used by separate analysis tools.
Typically
these tools do the necessary compilations automatically, so you should
not have to specify this qualifier in normal operation.

@item /UNITS_LIST
@cindex @option{/UNITS_LIST} (@code{GNAT COMPILE})
Print a list of units required by this compilation on @file{SYS$OUTPUT}.
The listing includes all units on which the unit being compiled depends
either directly or indirectly.

@end table

@node Debugging Control
@subsection Debugging Control

@table @code
@cindex Debugging options

@item /EXPAND_SOURCE
@cindex @option{/EXPAND_SOURCE} (@code{GNAT COMPILE})
This qualifier causes the compiler to generate auxiliary output containing
a pseudo-source listing of the generated expanded code. Like most Ada
compilers, GNAT works by first transforming the high level Ada code into
lower level constructs. For example, tasking operations are transformed
into calls to the tasking run-time routines. A unique capability of GNAT
is to list this expanded code in a form very close to normal Ada source.
This is very useful in understanding the implications of various Ada
usage on the efficiency of the generated code. There are many cases in
Ada (e.g. the use of controlled types), where simple Ada statements can
generate a lot of run-time code. By using @option{/EXPAND_SOURCE} you can identify
these cases, and consider whether it may be desirable to modify the coding
approach to improve efficiency.

The format of the output is very similar to standard Ada source, and is
easily understood by an Ada programmer. The following special syntactic
additions correspond to low level features used in the generated code that
do not have any exact analogies in pure Ada source form. The following
is a partial list of these special constructions. See the specification
of package @code{Sprint} in file @file{SPRINT.ADS} for a full list.

@table @code
@item new @var{xxx} [storage_pool = @var{yyy}]
Shows the storage pool being used for an allocator.

@item at end @var{procedure-name};
Shows the finalization (cleanup) procedure for a scope.

@item (if @var{expr} then @var{expr} else @var{expr})
Conditional expression equivalent to the @code{x?y:z} construction in C.

@item @var{target}^(@var{source})
A conversion with floating-point truncation instead of rounding.

@item @var{target}?(@var{source})
A conversion that bypasses normal Ada semantic checking. In particular
enumeration types and fixed-point types are treated simply as integers.

@item @var{target}?^(@var{source})
Combines the above two cases.

@item @var{x} #/ @var{y}
@itemx @var{x} #mod @var{y}
@itemx @var{x} #* @var{y}
@itemx @var{x} #rem @var{y}
A division or multiplication of fixed-point values which are treated as
integers without any kind of scaling.

@item free @var{expr} [storage_pool = @var{xxx}]
Shows the storage pool associated with a @code{free} statement.

@item freeze @var{typename} [@var{actions}]
Shows the point at which @var{typename} is frozen, with possible
associated actions to be performed at the freeze point.

@item reference @var{itype}
Reference (and hence definition) to internal type @var{itype}.

@item @var{function-name}! (@var{arg}, @var{arg}, @var{arg})
Intrinsic function call.

@item @var{labelname} : label
Declaration of label @var{labelname}.

@item @var{expr} && @var{expr} && @var{expr} ... && @var{expr}
A multiple concatenation (same effect as @var{expr} & @var{expr} &
@var{expr}, but handled more efficiently).

@item [constraint_error]
Raise the @code{Constraint_Error} exception.

@item @var{expression}'reference
A pointer to the result of evaluating @var{expression}.

@item @var{target-type}!(@var{source-expression})
An unchecked conversion of @var{source-expression} to @var{target-type}.

@item [@var{numerator}/@var{denominator}]
Used to represent internal real literals (that) have no exact
representation in base 2-16 (for example, the result of compile time
evaluation of the expression 1.0/27.0).

@item /XDEBUG
@cindex @option{/XDEBUG} (@code{GNAT COMPILE})
This qualifier is used in conjunction with @option{/EXPAND_SOURCE} to cause the expanded
source, as described above to be written to files with names
@file{XXX_DG}, where @file{xxx} is the normal file name,
for example, if the source file name is @file{HELLO.ADB},
then a file @file{HELLO.ADB_DG} will be written.
The debugging information generated
by the @code{GNAT COMPILE} @code{/DEBUG} qualifier will refer to the generated
@file{XXX_DG} file. This allows you to do source level debugging using
the generated code which is sometimes useful for complex code, for example
to find out exactly which part of a complex construction raised an
exception. This qualifier also suppress generation of cross-reference
information (see /XREF=SUPPRESS).

@item /COMPRESS_NAMES
@cindex @option{/CHECKS=ELABORATION} (@code{GNAT COMPILE})
In the generated debugging information, and also in the case of long external
names, the compiler uses a compression mechanism if the name is very long.
This compression method uses a checksum, and avoids trouble on some operating
systems which have difficulty with very long names. The @option{/COMPRESS_NAMES} qualifier
forces this compression approach to be used on all external names and names
in the debugging information tables. This reduces the size of the generated
executable, at the expense of making the naming scheme more complex. The
compression only affects the qualification of the name. Thus a name in
the source:

@smallexample
Very_Long_Package.Very_Long_Inner_Package.Var
@end smallexample

@noindent
would normally appear in these tables as:

@smallexample
very_long_package__very_long_inner_package__var
@end smallexample

@noindent
but if the @option{/COMPRESS_NAMES} qualifier is used, then the name appears as

@smallexample
XCb7e0c705__var
@end smallexample

@noindent
Here b7e0c705 is a compressed encoding of the qualification prefix.
The GNAT Ada aware version of GDB understands these encoded prefixes, so if this
debugger is used, the encoding is largely hidden from the user of the compiler.

@end table

@item /REPRESENTATION_INFO[0|1|2|3][s]
@cindex @option{/REPRESENTATION_INFO} (@code{GNAT COMPILE})
This qualifier controls output from the compiler of a listing showing
representation information for declared types and objects. For
@option{/REPRESENTATION_INFO=NONE}, no information is output (equivalent to omitting
the @option{/REPRESENTATION_INFO} qualifier). For @option{/REPRESENTATION_INFO=ARRAYS} (which is the default,
so @option{/REPRESENTATION_INFO} with no parameter has the same effect), size and alignment
information is listed for declared array and record types. For
@option{/REPRESENTATION_INFO=OBJECTS}, size and alignment information is listed for all
expression information for values that are computed at run time for
variant records. These symbolic expressions have a mostly obvious
format with #n being used to represent the value of the n'th
discriminant. See source files @file{REPINFO.ADS/adb} in the
@code{GNAT} sources for full detalis on the format of @option{/REPRESENTATION_INFO=SYMBOLIC}
output. If the qualifier is followed by an s (e.g. @option{-gnatR2s}), then
the output is to a file with the name @file{file_REP} where
file is the name of the corresponding source file.

@item /XREF=SUPPRESS
@cindex @option{/XREF=SUPPRESS} (@code{GNAT COMPILE})
Normally the compiler generates full cross-referencing information in
the @file{ALI} file. This information is used by a number of tools,
including @code{GNAT FIND} and @code{GNAT XREF}. The /XREF=SUPPRESS qualifier
suppresses this information. This saves some space and may slightly
speed up compilation, but means that these tools cannot be used.
@end table

@node Units to Sources Mapping Files
@subsection Units to Sources Mapping Files

@table @code

@item -gnatem@var{path}
@cindex @option{-gnatem} (@code{GNAT COMPILE})
A mapping file is a way to communicate to the compiler two mappings:
from unit names to file names (without any directory information) and from
file names to path names (with full directory information). These mappings
are used by the compiler to short-circuit the path search.

A mapping file is a sequence of sets of three lines. In each set,
the first line is the unit name, in lower case, with "%s" appended for
specifications and "%b" appended for bodies; the second line is the file
name; and the third line is the path name.

Example:
@smallexample
   main%b
   main.2.ADA
   /gnat/project1/sources/main.2.ADA
@end smallexample

When the qualifier @option{-gnatem} is specified, the compiler will create
in memory the two mappings from the specified file. If there is any problem
(non existent file, truncated file or duplicate entries), no mapping
will be created.

Several @option{-gnatem} qualifiers may be specified; however, only the last
one on the command line will be taken into account.

When using a project file, @code{GNAT MAKE} create a temporary mapping file
and communicates it to the compiler using this qualifier.

@end table

@node Search Paths and the Run-Time Library (RTL)
@section Search Paths and the Run-Time Library (RTL)

@noindent
With the GNAT source-based library system, the compiler must be able to
find source files for units that are needed by the unit being compiled.
Search paths are used to guide this process.

The compiler compiles one source file whose name must be given
explicitly on the command line. In other words, no searching is done
for this file. To find all other source files that are needed (the most
common being the specs of units), the compiler examines the following
directories, in the following order:

@enumerate
@item
The directory containing the source file of the main unit being compiled
(the file name on the command line).

@item
Each directory named by an @code{/SOURCE_SEARCH} qualifier given on the @code{GNAT COMPILE}
command line, in the order given.

@item
@findex ADA_INCLUDE_PATH
Each of the directories listed in the value of the
@code{ADA_INCLUDE_PATH} logical name.
Normally, define this value as a logical name containing a comma separated
list of directory names.

This variable can also be defined by means of an environment string
(an argument to the DEC C exec* set of functions).

Logical Name:
@smallexample
DEFINE ANOTHER_PATH FOO:[BAG]
DEFINE ADA_INCLUDE_PATH ANOTHER_PATH,FOO:[BAM],FOO:[BAR]
@end smallexample

By default, the path includes GNU:[LIB.OPENVMS7_x.2_8_x.DECLIB]
first, followed by the standard Ada 95
libraries in GNU:[LIB.OPENVMS7_x.2_8_x.ADAINCLUDE].
If this is not redefined, the user will obtain the DEC Ada83 IO packages
(Text_IO, Sequential_IO, etc)
instead of the Ada95 packages. Thus, in order to get the Ada 95
packages by default, ADA_INCLUDE_PATH must be redefined.
@item
The content of the "ada_source_path" file which is part of the GNAT
installation tree and is used to store standard libraries such as the
GNAT Run Time Library (RTL) source files.
@end enumerate

@noindent
Specifying the qualifier @code{/NOCURRENT_DIRECTORY}
inhibits the use of the directory
containing the source file named in the command line. You can still
have this directory on your search path, but in this case it must be
explicitly requested with a @code{/SOURCE_SEARCH} qualifier.

Specifying the qualifier @code{/NOSTD_INCLUDES}
inhibits the search of the default location for the GNAT Run Time
Library (RTL) source files.

The compiler outputs its object files and ALI files in the current
working directory.

@findex System.IO
The packages @code{Ada}, @code{System}, and @code{Interfaces} and their
children make up the GNAT RTL, together with the simple @code{System.IO}
package used in the "Hello World" example. The sources for these units
are needed by the compiler and are kept together in one directory. Not
all of the bodies are needed, but all of the sources are kept together
anyway. In a normal installation, you need not specify these directory
names when compiling or binding. Either the environment variables or
the built-in defaults cause these files to be found.

In addition to the language-defined hierarchies (System, Ada and
Interfaces), the GNAT distribution provides a fourth hierarchy,
consisting of child units of GNAT. This is a collection of generally
useful routines. See the GNAT Reference Manual for further details.

Besides simplifying access to the RTL, a major use of search paths is
in compiling sources from multiple directories. This can make
development environments much more flexible.

@node Order of Compilation Issues
@section Order of Compilation Issues

@noindent
If, in our earlier example, there was a spec for the @code{hello}
procedure, it would be contained in the file @file{HELLO.ADS}; yet this
file would not have to be explicitly compiled. This is the result of the
model we chose to implement library management. Some of the consequences
of this model are as follows:

@itemize @bullet
@item
There is no point in compiling specs (except for package
specs with no bodies) because these are compiled as needed by clients. If
you attempt a useless compilation, you will receive an error message.
It is also useless to compile subunits because they are compiled as needed
by the parent.

@item
There are no order of compilation requirements: performing a
compilation never obsoletes anything. The only way you can obsolete
something and require recompilations is to modify one of the
source files on which it depends.

@item
There is no library as such, apart from the ALI files
(@pxref{The Ada Library Information Files}, for information on the format of these
files). For now we find it convenient to create separate ALI files, but
eventually the information therein may be incorporated into the object
file directly.

@item
When you compile a unit, the source files for the specs of all units
that it @code{with}'s, all its subunits, and the bodies of any generics it
instantiates must be available (reachable by the search-paths mechanism
described above), or you will receive a fatal error message.
@end itemize

@node Examples
@section Examples

@noindent
The following are some typical Ada compilation command line examples:

@table @code
@item $ GNAT COMPILE XYZ.ADB
Compile body in file @file{XYZ.ADB} with all default options.

@item $ GNAT COMPILE /OPTIMIZE=ALL /CHECKS=ASSERTIONS XYZ-DEF.ADB

Compile the child unit package in file @file{XYZ-DEF.ADB} with extensive
optimizations, and pragma @code{Assert}/@code{Debug} statements
enabled.

@item $ GNAT COMPILE /NOLOAD ABC-DEF.ADB
Compile the subunit in file @file{ABC-DEF.ADB} in semantic-checking-only
mode.
@end table

@node Binding Using GNAT BIND
@chapter Binding Using @code{GNAT BIND}
@findex GNAT BIND

@menu
* Running GNAT BIND::
* Generating the Binder Program in C::
* Consistency-Checking Modes::
* Binder Error Message Control::
* Elaboration Control::
* Output Control::
* Binding with Non-Ada Main Programs::
* Binding Programs with No Main Subprogram::
* Summary of Binder Qualifiers::
* Command-Line Access::
* Search Paths for GNAT BIND::
* Examples of GNAT BIND Usage::
@end menu

@noindent
This chapter describes the GNAT binder, @code{GNAT BIND}, which is used
to bind compiled GNAT objects. The @code{GNAT BIND} program performs
four separate functions:

@enumerate
@item
Checks that a program is consistent, in accordance with the rules in
Chapter 10 of the Ada 95 Reference Manual. In particular, error
messages are generated if a program uses inconsistent versions of a
given unit.

@item
Checks that an acceptable order of elaboration exists for the program
and issues an error message if it cannot find an order of elaboration
that satisfies the rules in Chapter 10 of the Ada 95 Language Manual.

@item
Generates a main program incorporating the given elaboration order.
This program is a small Ada package (body and spec) that
must be subsequently compiled
using the GNAT compiler. The necessary compilation step is usually
performed automatically by @code{GNAT LINK}. The two most important
functions of this program
are to call the elaboration routines of units in an appropriate order
and to call the main program.

@item
Determines the set of object files required by the given main program.
This information is output in the forms of comments in the generated program,
to be read by the @code{GNAT LINK} utility used to link the Ada application.
@end enumerate

@node Running GNAT BIND
@section Running @code{GNAT BIND}

@noindent
The form of the @code{GNAT BIND} command is

@smallexample
$ GNAT BIND [@var{qualifiers}] @var{mainprog}[.ALI] [@var{qualifiers}]
@end smallexample

@noindent
where @var{mainprog}.ADB is the Ada file containing the main program
unit body. If no qualifiers are specified, @code{GNAT BIND} constructs an Ada
package in two files which names are
@file{B$@var{ada_main}.ADS}, and @file{B$@var{ada_main}.ADB}.
For example, if given the
parameter @samp{HELLO.ALI}, for a main program contained in file
@file{HELLO.ADB}, the binder output files would be @file{B~HELLO.ADS}
and @file{B~HELLO.ADB}.

When doing consistency checking, the binder takes into consideration
any source files it can locate. For example, if the binder determines
that the given main program requires the package @code{Pack}, whose
@file{.ALI}
file is @file{PACK.ALI} and whose corresponding source spec file is
@file{PACK.ADS}, it attempts to locate the source file @file{PACK.ADS}
(using the same search path conventions as previously described for the
@code{GNAT COMPILE} command). If it can locate this source file, it checks that
the time stamps
or source checksums of the source and its references to in @file{ali} files
match. In other words, any @file{ali} files that mentions this spec must have
resulted from compiling this version of the source file (or in the case
where the source checksums match, a version close enough that the
difference does not matter).

@cindex Source files, use by binder
The effect of this consistency checking, which includes source files, is
that the binder ensures that the program is consistent with the latest
version of the source files that can be located at bind time. Editing a
source file without compiling files that depend on the source file cause
error messages to be generated by the binder.

For example, suppose you have a main program @file{HELLO.ADB} and a
package @code{P}, from file @file{P.ADS} and you perform the following
steps:

@enumerate
@item
Enter @code{GNAT COMPILE HELLO.ADB} to compile the main program.

@item
Enter @code{GNAT COMPILE P.ADS} to compile package @code{P}.

@item
Edit file @file{P.ADS}.

@item
Enter @code{GNAT BIND hello}.
@end enumerate

At this point, the file @file{P.ALI} contains an out-of-date time stamp
because the file @file{P.ADS} has been edited. The attempt at binding
fails, and the binder generates the following error messages:

@smallexample
error: "HELLO.ADB" must be recompiled ("P.ADS" has been modified)
error: "P.ADS" has been modified and must be recompiled
@end smallexample

@noindent
Now both files must be recompiled as indicated, and then the bind can
succeed, generating a main program. You need not normally be concerned
with the contents of this file, but it is similar to the following which
is the binder file generated for a simple "hello world" program.

@smallexample
@iftex
@leftskip=0cm
@end iftex
--  The package is called Ada_Main unless this name is actually used
--  as a unit name in the partition, in which case some other unique
--  name is used.

with System;
package ada_main is

   Elab_Final_Code : Integer;
   pragma Import (C, Elab_Final_Code, "__gnat_inside_elab_final_code");

   --  The main program saves the parameters (argument count,
   --  argument values, environment pointer) in global variables
   --  for later access by other units including
   --  Ada.Command_Line.

   gnat_argc : Integer;
   gnat_argv : System.Address;
   gnat_envp : System.Address;

   --  The actual variables are stored in a library routine. This
   --  is useful for some shared library situations, where there
   --  are problems if variables are not in the library.

   pragma Import (C, gnat_argc);
   pragma Import (C, gnat_argv);
   pragma Import (C, gnat_envp);

   --  The exit status is similarly an external location

   gnat_exit_status : Integer;
   pragma Import (C, gnat_exit_status);

   GNAT_Version : constant String :=
                    "GNAT Version: 3.15w (20010315)";
   pragma Export (C, GNAT_Version, "__gnat_version");

   --  This is the generated adafinal routine that performs
   --  finalization at the end of execution. In the case where
   --  Ada is the main program, this main program makes a call
   --  to adafinal at program termination.

   procedure adafinal;
   pragma Export (C, adafinal, "adafinal");

   --  This is the generated adainit routine that performs
   --  initialization at the start of execution. In the case
   --  where Ada is the main program, this main program makes
   --  a call to adainit at program startup.

   procedure adainit;
   pragma Export (C, adainit, "adainit");

   --  This routine is called at the start of execution. It is
   --  a dummy routine that is used by the debugger to breakpoint
   --  at the start of execution.

   procedure Break_Start;
   pragma Import (C, Break_Start, "__gnat_break_start");

   --  This is the actual generated main program (it would be
   --  suppressed if the no main program qualifier were used). As
   --  required by standard system conventions, this program has
   --  the external name main.

   function main
     (argc : Integer;
      argv : System.Address;
      envp : System.Address)
      return Integer;
   pragma Export (C, main, "main");

   --  The following set of constants give the version
   --  identification values for every unit in the bound
   --  partition. This identification is computed from all
   --  dependent semantic units, and corresponds to the
   --  string that would be returned by use of the
   --  Body_Version or Version attributes.

   type Version_32 is mod 2 ** 32;
   u00001 : constant Version_32 := 16#7880BEB3#;
   u00002 : constant Version_32 := 16#0D24CBD0#;
   u00003 : constant Version_32 := 16#3283DBEB#;
   u00004 : constant Version_32 := 16#2359F9ED#;
   u00005 : constant Version_32 := 16#664FB847#;
   u00006 : constant Version_32 := 16#68E803DF#;
   u00007 : constant Version_32 := 16#5572E604#;
   u00008 : constant Version_32 := 16#46B173D8#;
   u00009 : constant Version_32 := 16#156A40CF#;
   u00010 : constant Version_32 := 16#033DABE0#;
   u00011 : constant Version_32 := 16#6AB38FEA#;
   u00012 : constant Version_32 := 16#22B6217D#;
   u00013 : constant Version_32 := 16#68A22947#;
   u00014 : constant Version_32 := 16#18CC4A56#;
   u00015 : constant Version_32 := 16#08258E1B#;
   u00016 : constant Version_32 := 16#367D5222#;
   u00017 : constant Version_32 := 16#20C9ECA4#;
   u00018 : constant Version_32 := 16#50D32CB6#;
   u00019 : constant Version_32 := 16#39A8BB77#;
   u00020 : constant Version_32 := 16#5CF8FA2B#;
   u00021 : constant Version_32 := 16#2F1EB794#;
   u00022 : constant Version_32 := 16#31AB6444#;
   u00023 : constant Version_32 := 16#1574B6E9#;
   u00024 : constant Version_32 := 16#5109C189#;
   u00025 : constant Version_32 := 16#56D770CD#;
   u00026 : constant Version_32 := 16#02F9DE3D#;
   u00027 : constant Version_32 := 16#08AB6B2C#;
   u00028 : constant Version_32 := 16#3FA37670#;
   u00029 : constant Version_32 := 16#476457A0#;
   u00030 : constant Version_32 := 16#731E1B6E#;
   u00031 : constant Version_32 := 16#23C2E789#;
   u00032 : constant Version_32 := 16#0F1BD6A1#;
   u00033 : constant Version_32 := 16#7C25DE96#;
   u00034 : constant Version_32 := 16#39ADFFA2#;
   u00035 : constant Version_32 := 16#571DE3E7#;
   u00036 : constant Version_32 := 16#5EB646AB#;
   u00037 : constant Version_32 := 16#4249379B#;
   u00038 : constant Version_32 := 16#0357E00A#;
   u00039 : constant Version_32 := 16#3784FB72#;
   u00040 : constant Version_32 := 16#2E723019#;
   u00041 : constant Version_32 := 16#623358EA#;
   u00042 : constant Version_32 := 16#107F9465#;
   u00043 : constant Version_32 := 16#6843F68A#;
   u00044 : constant Version_32 := 16#63305874#;
   u00045 : constant Version_32 := 16#31E56CE1#;
   u00046 : constant Version_32 := 16#02917970#;
   u00047 : constant Version_32 := 16#6CCBA70E#;
   u00048 : constant Version_32 := 16#41CD4204#;
   u00049 : constant Version_32 := 16#572E3F58#;
   u00050 : constant Version_32 := 16#20729FF5#;
   u00051 : constant Version_32 := 16#1D4F93E8#;
   u00052 : constant Version_32 := 16#30B2EC3D#;
   u00053 : constant Version_32 := 16#34054F96#;
   u00054 : constant Version_32 := 16#5A199860#;
   u00055 : constant Version_32 := 16#0E7F912B#;
   u00056 : constant Version_32 := 16#5760634A#;
   u00057 : constant Version_32 := 16#5D851835#;

   --  The following Export pragmas export the version numbers
   --  with symbolic names ending in B (for body) or S
   --  (for spec) so that they can be located in a link. The
   --  information provided here is sufficient to track down
   --  the exact versions of units used in a given build.

   pragma Export (C, u00001, "helloB");
   pragma Export (C, u00002, "system__standard_libraryB");
   pragma Export (C, u00003, "system__standard_libraryS");
   pragma Export (C, u00004, "adaS");
   pragma Export (C, u00005, "ada__text_ioB");
   pragma Export (C, u00006, "ada__text_ioS");
   pragma Export (C, u00007, "ada__exceptionsB");
   pragma Export (C, u00008, "ada__exceptionsS");
   pragma Export (C, u00009, "gnatS");
   pragma Export (C, u00010, "gnat__heap_sort_aB");
   pragma Export (C, u00011, "gnat__heap_sort_aS");
   pragma Export (C, u00012, "systemS");
   pragma Export (C, u00013, "system__exception_tableB");
   pragma Export (C, u00014, "system__exception_tableS");
   pragma Export (C, u00015, "gnat__htableB");
   pragma Export (C, u00016, "gnat__htableS");
   pragma Export (C, u00017, "system__exceptionsS");
   pragma Export (C, u00018, "system__machine_state_operationsB");
   pragma Export (C, u00019, "system__machine_state_operationsS");
   pragma Export (C, u00020, "system__machine_codeS");
   pragma Export (C, u00021, "system__storage_elementsB");
   pragma Export (C, u00022, "system__storage_elementsS");
   pragma Export (C, u00023, "system__secondary_stackB");
   pragma Export (C, u00024, "system__secondary_stackS");
   pragma Export (C, u00025, "system__parametersB");
   pragma Export (C, u00026, "system__parametersS");
   pragma Export (C, u00027, "system__soft_linksB");
   pragma Export (C, u00028, "system__soft_linksS");
   pragma Export (C, u00029, "system__stack_checkingB");
   pragma Export (C, u00030, "system__stack_checkingS");
   pragma Export (C, u00031, "system__tracebackB");
   pragma Export (C, u00032, "system__tracebackS");
   pragma Export (C, u00033, "ada__streamsS");
   pragma Export (C, u00034, "ada__tagsB");
   pragma Export (C, u00035, "ada__tagsS");
   pragma Export (C, u00036, "system__string_opsB");
   pragma Export (C, u00037, "system__string_opsS");
   pragma Export (C, u00038, "interfacesS");
   pragma Export (C, u00039, "interfaces__c_streamsB");
   pragma Export (C, u00040, "interfaces__c_streamsS");
   pragma Export (C, u00041, "system__file_ioB");
   pragma Export (C, u00042, "system__file_ioS");
   pragma Export (C, u00043, "ada__finalizationB");
   pragma Export (C, u00044, "ada__finalizationS");
   pragma Export (C, u00045, "system__finalization_rootB");
   pragma Export (C, u00046, "system__finalization_rootS");
   pragma Export (C, u00047, "system__finalization_implementationB");
   pragma Export (C, u00048, "system__finalization_implementationS");
   pragma Export (C, u00049, "system__string_ops_concat_3B");
   pragma Export (C, u00050, "system__string_ops_concat_3S");
   pragma Export (C, u00051, "system__stream_attributesB");
   pragma Export (C, u00052, "system__stream_attributesS");
   pragma Export (C, u00053, "ada__io_exceptionsS");
   pragma Export (C, u00054, "system__unsigned_typesS");
   pragma Export (C, u00055, "system__file_control_blockS");
   pragma Export (C, u00056, "ada__finalization__list_controllerB");
   pragma Export (C, u00057, "ada__finalization__list_controllerS");

   -- BEGIN ELABORATION ORDER
   -- ada (spec)
   -- gnat (spec)
   -- gnat.heap_sort_a (spec)
   -- gnat.heap_sort_a (body)
   -- gnat.htable (spec)
   -- gnat.htable (body)
   -- interfaces (spec)
   -- system (spec)
   -- system.machine_code (spec)
   -- system.parameters (spec)
   -- system.parameters (body)
   -- interfaces.c_streams (spec)
   -- interfaces.c_streams (body)
   -- system.standard_library (spec)
   -- ada.exceptions (spec)
   -- system.exception_table (spec)
   -- system.exception_table (body)
   -- ada.io_exceptions (spec)
   -- system.exceptions (spec)
   -- system.storage_elements (spec)
   -- system.storage_elements (body)
   -- system.machine_state_operations (spec)
   -- system.machine_state_operations (body)
   -- system.secondary_stack (spec)
   -- system.stack_checking (spec)
   -- system.soft_links (spec)
   -- system.soft_links (body)
   -- system.stack_checking (body)
   -- system.secondary_stack (body)
   -- system.standard_library (body)
   -- system.string_ops (spec)
   -- system.string_ops (body)
   -- ada.tags (spec)
   -- ada.tags (body)
   -- ada.streams (spec)
   -- system.finalization_root (spec)
   -- system.finalization_root (body)
   -- system.string_ops_concat_3 (spec)
   -- system.string_ops_concat_3 (body)
   -- system.traceback (spec)
   -- system.traceback (body)
   -- ada.exceptions (body)
   -- system.unsigned_types (spec)
   -- system.stream_attributes (spec)
   -- system.stream_attributes (body)
   -- system.finalization_implementation (spec)
   -- system.finalization_implementation (body)
   -- ada.finalization (spec)
   -- ada.finalization (body)
   -- ada.finalization.list_controller (spec)
   -- ada.finalization.list_controller (body)
   -- system.file_control_block (spec)
   -- system.file_io (spec)
   -- system.file_io (body)
   -- ada.text_io (spec)
   -- ada.text_io (body)
   -- hello (body)
   -- END ELABORATION ORDER

end ada_main;

--  The following source file name pragmas allow the generated file
--  names to be unique for different main programs. They are needed
--  since the package name will always be Ada_Main.

pragma Source_File_Name (ada_main, Spec_File_Name => "B~HELLO.ADS");
pragma Source_File_Name (ada_main, Body_File_Name => "B~HELLO.ADB");

--  Generated package body for Ada_Main starts here

package body ada_main is

   --  The actual finalization is performed by calling the
   --  library routine in System.Standard_Library.Adafinal

   procedure Do_Finalize;
   pragma Import (C, Do_Finalize, "system__standard_library__adafinal");

   -------------
   -- adainit --
   -------------

@findex adainit
   procedure adainit is

      --  These booleans are set to True once the associated unit has
      --  been elaborated. It is also used to avoid elaborating the
      --  same unit twice.

      E040 : Boolean; pragma Import (Ada, E040, "interfaces__c_streams_E");
      E008 : Boolean; pragma Import (Ada, E008, "ada__exceptions_E");
      E014 : Boolean; pragma Import (Ada, E014, "system__exception_table_E");
      E053 : Boolean; pragma Import (Ada, E053, "ada__io_exceptions_E");
      E017 : Boolean; pragma Import (Ada, E017, "system__exceptions_E");
      E024 : Boolean; pragma Import (Ada, E024, "system__secondary_stack_E");
      E030 : Boolean; pragma Import (Ada, E030, "system__stack_checking_E");
      E028 : Boolean; pragma Import (Ada, E028, "system__soft_links_E");
      E035 : Boolean; pragma Import (Ada, E035, "ada__tags_E");
      E033 : Boolean; pragma Import (Ada, E033, "ada__streams_E");
      E046 : Boolean; pragma Import (Ada, E046, "system__finalization_root_E");
      E048 : Boolean; pragma Import (Ada, E048, "system__finalization_implementation_E");
      E044 : Boolean; pragma Import (Ada, E044, "ada__finalization_E");
      E057 : Boolean; pragma Import (Ada, E057, "ada__finalization__list_controller_E");
      E055 : Boolean; pragma Import (Ada, E055, "system__file_control_block_E");
      E042 : Boolean; pragma Import (Ada, E042, "system__file_io_E");
      E006 : Boolean; pragma Import (Ada, E006, "ada__text_io_E");

      --  Set_Globals is a library routine that stores away the
      --  value of the indicated set of global values in global
      --  variables within the library.

      procedure Set_Globals
        (Main_Priority            : Integer;
         Time_Slice_Value         : Integer;
         WC_Encoding              : Character;
         Locking_Policy           : Character;
         Queuing_Policy           : Character;
         Task_Dispatching_Policy  : Character;
         Adafinal                 : System.Address;
         Unreserve_All_Interrupts : Integer;
         Exception_Tracebacks     : Integer);
@findex __gnat_set_globals
      pragma Import (C, Set_Globals, "__gnat_set_globals");

      --  SDP_Table_Build is a library routine used to build the
      --  exception tables. See unit Ada.Exceptions in files
      --  A-EXCEPT.ADS/adb for full details of how zero cost
      --  exception handling works. This procedure, the call to
      --  it, and the two following tables are all omitted if the
      --  build is in longjmp/setjump exception mode.

@findex SDP_Table_Build
@findex Zero Cost Exceptions
      procedure SDP_Table_Build
        (SDP_Addresses   : System.Address;
         SDP_Count       : Natural;
         Elab_Addresses  : System.Address;
         Elab_Addr_Count : Natural);
      pragma Import (C, SDP_Table_Build, "__gnat_SDP_Table_Build");

      --  Table of Unit_Exception_Table addresses. Used for zero
      --  cost exception handling to build the top level table.

      ST : aliased constant array (1 .. 23) of System.Address := (
        Hello'UET_Address,
        Ada.Text_Io'UET_Address,
        Ada.Exceptions'UET_Address,
        Gnat.Heap_Sort_A'UET_Address,
        System.Exception_Table'UET_Address,
        System.Machine_State_Operations'UET_Address,
        System.Secondary_Stack'UET_Address,
        System.Parameters'UET_Address,
        System.Soft_Links'UET_Address,
        System.Stack_Checking'UET_Address,
        System.Traceback'UET_Address,
        Ada.Streams'UET_Address,
        Ada.Tags'UET_Address,
        System.String_Ops'UET_Address,
        Interfaces.C_Streams'UET_Address,
        System.File_Io'UET_Address,
        Ada.Finalization'UET_Address,
        System.Finalization_Root'UET_Address,
        System.Finalization_Implementation'UET_Address,
        System.String_Ops_Concat_3'UET_Address,
        System.Stream_Attributes'UET_Address,
        System.File_Control_Block'UET_Address,
        Ada.Finalization.List_Controller'UET_Address);

      --  Table of addresses of elaboration routines. Used for
      --  zero cost exception handling to make sure these
      --  addresses are included in the top level procedure
      --  address table.

      EA : aliased constant array (1 .. 23) of System.Address := (
        adainit'Code_Address,
        Do_Finalize'Code_Address,
        Ada.Exceptions'Elab_Spec'Address,
        System.Exceptions'Elab_Spec'Address,
        Interfaces.C_Streams'Elab_Spec'Address,
        System.Exception_Table'Elab_Body'Address,
        Ada.Io_Exceptions'Elab_Spec'Address,
        System.Stack_Checking'Elab_Spec'Address,
        System.Soft_Links'Elab_Body'Address,
        System.Secondary_Stack'Elab_Body'Address,
        Ada.Tags'Elab_Spec'Address,
        Ada.Tags'Elab_Body'Address,
        Ada.Streams'Elab_Spec'Address,
        System.Finalization_Root'Elab_Spec'Address,
        Ada.Exceptions'Elab_Body'Address,
        System.Finalization_Implementation'Elab_Spec'Address,
        System.Finalization_Implementation'Elab_Body'Address,
        Ada.Finalization'Elab_Spec'Address,
        Ada.Finalization.List_Controller'Elab_Spec'Address,
        System.File_Control_Block'Elab_Spec'Address,
        System.File_Io'Elab_Body'Address,
        Ada.Text_Io'Elab_Spec'Address,
        Ada.Text_Io'Elab_Body'Address);

   --  Start of processing for adainit

   begin

      --  Call SDP_Table_Build to build the top level procedure
      --  table for zero cost exception handling (omitted in
      --  longjmp/setjump mode).

      SDP_Table_Build (ST'Address, 23, EA'Address, 23);

      --  Call Set_Globals to record various information for
      --  this partition.  The values are derived by the binder
      --  from information stored in the ali files by the compiler.

@findex __gnat_set_globals
      Set_Globals
        (Main_Priority            => -1,
         --  Priority of main program, -1 if no pragma Priority used

         Time_Slice_Value         => -1,
         --  Time slice from Time_Slice pragma, -1 if none used

         WC_Encoding              => 'b',
         --  Wide_Character encoding used, default is brackets

         Locking_Policy           => ' ',
         --  Locking_Policy used, default of space means not
         --  specified, otherwise it is the first character of
         --  the policy name.

         Queuing_Policy           => ' ',
         --  Queuing_Policy used, default of space means not
         --  specified, otherwise it is the first character of
         --  the policy name.

         Task_Dispatching_Policy  => ' ',
         --  Task_Dispatching_Policy used, default of space means
         --  not specified, otherwise first character of the
         --  policy name.

         Adafinal                 => System.Null_Address,
         --  Address of Adafinal routine, not used anymore

         Unreserve_All_Interrupts => 0,
         --  Set true if pragma Unreserve_All_Interrupts was used

         Exception_Tracebacks     => 0);
         --  Indicates if exception tracebacks are enabled

      Elab_Final_Code := 1;

      --  Now we have the elaboration calls for all units in the partition.
      --  The Elab_Spec and Elab_Body attributes generate references to the
      --  implicit elaboration procedures generated by the compiler for
      --  each unit that requires elaboration.

      if not E040 then
         Interfaces.C_Streams'Elab_Spec;
      end if;
      E040 := True;
      if not E008 then
         Ada.Exceptions'Elab_Spec;
      end if;
      if not E014 then
         System.Exception_Table'Elab_Body;
         E014 := True;
      end if;
      if not E053 then
         Ada.Io_Exceptions'Elab_Spec;
         E053 := True;
      end if;
      if not E017 then
         System.Exceptions'Elab_Spec;
         E017 := True;
      end if;
      if not E030 then
         System.Stack_Checking'Elab_Spec;
      end if;
      if not E028 then
         System.Soft_Links'Elab_Body;
         E028 := True;
      end if;
      E030 := True;
      if not E024 then
         System.Secondary_Stack'Elab_Body;
         E024 := True;
      end if;
      if not E035 then
         Ada.Tags'Elab_Spec;
      end if;
      if not E035 then
         Ada.Tags'Elab_Body;
         E035 := True;
      end if;
      if not E033 then
         Ada.Streams'Elab_Spec;
         E033 := True;
      end if;
      if not E046 then
         System.Finalization_Root'Elab_Spec;
      end if;
      E046 := True;
      if not E008 then
         Ada.Exceptions'Elab_Body;
         E008 := True;
      end if;
      if not E048 then
         System.Finalization_Implementation'Elab_Spec;
      end if;
      if not E048 then
         System.Finalization_Implementation'Elab_Body;
         E048 := True;
      end if;
      if not E044 then
         Ada.Finalization'Elab_Spec;
      end if;
      E044 := True;
      if not E057 then
         Ada.Finalization.List_Controller'Elab_Spec;
      end if;
      E057 := True;
      if not E055 then
         System.File_Control_Block'Elab_Spec;
         E055 := True;
      end if;
      if not E042 then
         System.File_Io'Elab_Body;
         E042 := True;
      end if;
      if not E006 then
         Ada.Text_Io'Elab_Spec;
      end if;
      if not E006 then
         Ada.Text_Io'Elab_Body;
         E006 := True;
      end if;

      Elab_Final_Code := 0;
   end adainit;

   --------------
   -- adafinal --
   --------------

@findex adafinal
   procedure adafinal is
   begin
      Do_Finalize;
   end adafinal;

   ----------
   -- main --
   ----------

   --  main is actually a function, as in the ANSI C standard,
   --  defined to return the exit status. The three parameters
   --  are the argument count, argument values and environment
   --  pointer.

@findex Main Program
   function main
     (argc : Integer;
      argv : System.Address;
      envp : System.Address)
      return Integer
   is
      --  The initialize routine performs low level system
      --  initialization using a standard library routine which
      --  sets up signal handling and performs any other
      --  required setup. The routine can be found in file
      --  A-INIT.C.

@findex __gnat_initialize
      procedure initialize;
      pragma Import (C, initialize, "__gnat_initialize");

      --  The finalize routine performs low level system
      --  finalization using a standard library routine. The
      --  routine is found in file A-FINAL.C and in the standard
      --  distribution is a dummy routine that does nothing, so
      --  really this is a hook for special user finalization.

@findex __gnat_finalize
      procedure finalize;
      pragma Import (C, finalize, "__gnat_finalize");

      --  We get to the main program of the partition by using
      --  pragma Import because if we try to with the unit and
      --  call it Ada style, then not only do we waste time
      --  recompiling it, but also, we don't really know the right
      --  qualifiers (e.g. identifier character set) to be used
      --  to compile it.

      procedure Ada_Main_Program;
      pragma Import (Ada, Ada_Main_Program, "_ada_hello");

   --  Start of processing for main

   begin
      --  Save global variables

      gnat_argc := argc;
      gnat_argv := argv;
      gnat_envp := envp;

      --  Call low level system initialization

      Initialize;

      --  Call our generated Ada initialization routine

      adainit;

      --  This is the point at which we want the debugger to get
      --  control

      Break_Start;

      --  Now we call the main program of the partition

      Ada_Main_Program;

      --  Perform Ada finalization

      adafinal;

      --  Perform low level system finalization

      Finalize;

      --  Return the proper exit status
      return (gnat_exit_status);
   end;

--  This section is entirely comments, so it has no effect on the
--  compilation of the Ada_Main package. It provides the list of
--  object files and linker options, as well as some standard
--  libraries needed for the link. The GNAT LINK utility parses
--  this B~HELLO.ADB file to read these comment lines to generate
--  the appropriate command line arguments for the call to the
--  system linker. The BEGIN/END lines are used for sentinels for
--  this parsing operation.

--  The exact file names will of course depend on the environment,
--  host/target and location of files on the host system.

@findex Object file list
-- BEGIN Object file/option list
   --   ./HELLO.OBJ
   --   -L./
   --   -L/usr/local/gnat/lib/gcc-lib/i686-pc-linux-gnu/2.8.1/adalib/
   --   /usr/local/gnat/lib/gcc-lib/i686-pc-linux-gnu/2.8.1/adalib/libgnat.a
-- END Object file/option list

end ada_main;

@end smallexample

@noindent
The Ada code in the above example is exactly what is generated by the
binder. We have added comments to more clearly indicate the function
of each part of the generated @code{Ada_Main} package.

The code is standard Ada in all respects, and can be processed by any
tools that handle Ada. In particular, it is possible to use the debugger
in Ada mode to debug the generated Ada_Main package. For example, suppose
that for reasons that you do not understand, your program is blowing up
during elaboration of the body of @code{Ada.Text_IO}. To chase this bug
down, you can place a breakpoint on the call:

@smallexample
Ada.Text_Io'Elab_Body;
@end smallexample

@noindent
and trace the elaboration routine for this package to find out where
the problem might be (more usually of course you would be debugging
elaboration code in your own application).

@node Generating the Binder Program in C
@section Generating the Binder Program in C
@noindent
In most normal usage, the default mode of @code{GNAT BIND} which is to
generate the main package in Ada, as described in the previous section.
In particular, this means that any Ada programmer can read and understand
the generated main program. It can also be debugged just like any other
Ada code provided the @code{-g} qualifier is used for @code{GNAT BIND}
and @code{GNAT LINK}.

However for some purposes it may be convenient to generate the main
program in C rather than Ada. This may for example be helpful when you
are generating a mixed language program with the main program in C. The
GNAT compiler itself is an example. The use of the @code{-C} qualifier
for both @code{GNAT BIND} and @code{GNAT LINK} will cause the program to
be generated in C (and compiled using the gnu C compiler). The
following shows the C code generated for the same "Hello World"
program:

@smallexample

#ifdef __STDC__
#define PARAMS(paramlist) paramlist
#else
#define PARAMS(paramlist) ()
#endif

extern void __gnat_set_globals
 PARAMS ((int, int, int, int, int, int,
          void (*) PARAMS ((void)), int, int));
extern void adafinal PARAMS ((void));
extern void adainit PARAMS ((void));
extern void system__standard_library__adafinal PARAMS ((void));
extern int main PARAMS ((int, char **, char **));
extern void exit PARAMS ((int));
extern void __gnat_break_start PARAMS ((void));
extern void _ada_hello PARAMS ((void));
extern void __gnat_initialize PARAMS ((void));
extern void __gnat_finalize PARAMS ((void));

extern void ada__exceptions___elabs PARAMS ((void));
extern void system__exceptions___elabs PARAMS ((void));
extern void interfaces__c_streams___elabs PARAMS ((void));
extern void system__exception_table___elabb PARAMS ((void));
extern void ada__io_exceptions___elabs PARAMS ((void));
extern void system__stack_checking___elabs PARAMS ((void));
extern void system__soft_links___elabb PARAMS ((void));
extern void system__secondary_stack___elabb PARAMS ((void));
extern void ada__tags___elabs PARAMS ((void));
extern void ada__tags___elabb PARAMS ((void));
extern void ada__streams___elabs PARAMS ((void));
extern void system__finalization_root___elabs PARAMS ((void));
extern void ada__exceptions___elabb PARAMS ((void));
extern void system__finalization_implementation___elabs PARAMS ((void));
extern void system__finalization_implementation___elabb PARAMS ((void));
extern void ada__finalization___elabs PARAMS ((void));
extern void ada__finalization__list_controller___elabs PARAMS ((void));
extern void system__file_control_block___elabs PARAMS ((void));
extern void system__file_io___elabb PARAMS ((void));
extern void ada__text_io___elabs PARAMS ((void));
extern void ada__text_io___elabb PARAMS ((void));

extern int __gnat_inside_elab_final_code;

extern int gnat_argc;
extern char **gnat_argv;
extern char **gnat_envp;
extern int gnat_exit_status;

char __gnat_version[] = "GNAT Version: 3.15w (20010315)";
void adafinal () @{
   system__standard_library__adafinal ();
@}

void adainit ()
@{
   extern char ada__exceptions_E;
   extern char system__exceptions_E;
   extern char interfaces__c_streams_E;
   extern char system__exception_table_E;
   extern char ada__io_exceptions_E;
   extern char system__secondary_stack_E;
   extern char system__stack_checking_E;
   extern char system__soft_links_E;
   extern char ada__tags_E;
   extern char ada__streams_E;
   extern char system__finalization_root_E;
   extern char system__finalization_implementation_E;
   extern char ada__finalization_E;
   extern char ada__finalization__list_controller_E;
   extern char system__file_control_block_E;
   extern char system__file_io_E;
   extern char ada__text_io_E;

   extern void *__gnat_hello__SDP;
   extern void *__gnat_ada__text_io__SDP;
   extern void *__gnat_ada__exceptions__SDP;
   extern void *__gnat_gnat__heap_sort_a__SDP;
   extern void *__gnat_system__exception_table__SDP;
   extern void *__gnat_system__machine_state_operations__SDP;
   extern void *__gnat_system__secondary_stack__SDP;
   extern void *__gnat_system__parameters__SDP;
   extern void *__gnat_system__soft_links__SDP;
   extern void *__gnat_system__stack_checking__SDP;
   extern void *__gnat_system__traceback__SDP;
   extern void *__gnat_ada__streams__SDP;
   extern void *__gnat_ada__tags__SDP;
   extern void *__gnat_system__string_ops__SDP;
   extern void *__gnat_interfaces__c_streams__SDP;
   extern void *__gnat_system__file_io__SDP;
   extern void *__gnat_ada__finalization__SDP;
   extern void *__gnat_system__finalization_root__SDP;
   extern void *__gnat_system__finalization_implementation__SDP;
   extern void *__gnat_system__string_ops_concat_3__SDP;
   extern void *__gnat_system__stream_attributes__SDP;
   extern void *__gnat_system__file_control_block__SDP;
   extern void *__gnat_ada__finalization__list_controller__SDP;

   void **st[23] = @{
     &__gnat_hello__SDP,
     &__gnat_ada__text_io__SDP,
     &__gnat_ada__exceptions__SDP,
     &__gnat_gnat__heap_sort_a__SDP,
     &__gnat_system__exception_table__SDP,
     &__gnat_system__machine_state_operations__SDP,
     &__gnat_system__secondary_stack__SDP,
     &__gnat_system__parameters__SDP,
     &__gnat_system__soft_links__SDP,
     &__gnat_system__stack_checking__SDP,
     &__gnat_system__traceback__SDP,
     &__gnat_ada__streams__SDP,
     &__gnat_ada__tags__SDP,
     &__gnat_system__string_ops__SDP,
     &__gnat_interfaces__c_streams__SDP,
     &__gnat_system__file_io__SDP,
     &__gnat_ada__finalization__SDP,
     &__gnat_system__finalization_root__SDP,
     &__gnat_system__finalization_implementation__SDP,
     &__gnat_system__string_ops_concat_3__SDP,
     &__gnat_system__stream_attributes__SDP,
     &__gnat_system__file_control_block__SDP,
     &__gnat_ada__finalization__list_controller__SDP@};

   extern void ada__exceptions___elabs ();
   extern void system__exceptions___elabs ();
   extern void interfaces__c_streams___elabs ();
   extern void system__exception_table___elabb ();
   extern void ada__io_exceptions___elabs ();
   extern void system__stack_checking___elabs ();
   extern void system__soft_links___elabb ();
   extern void system__secondary_stack___elabb ();
   extern void ada__tags___elabs ();
   extern void ada__tags___elabb ();
   extern void ada__streams___elabs ();
   extern void system__finalization_root___elabs ();
   extern void ada__exceptions___elabb ();
   extern void system__finalization_implementation___elabs ();
   extern void system__finalization_implementation___elabb ();
   extern void ada__finalization___elabs ();
   extern void ada__finalization__list_controller___elabs ();
   extern void system__file_control_block___elabs ();
   extern void system__file_io___elabb ();
   extern void ada__text_io___elabs ();
   extern void ada__text_io___elabb ();

   void (*ea[23]) () = @{
     adainit,
     system__standard_library__adafinal,
     ada__exceptions___elabs,
     system__exceptions___elabs,
     interfaces__c_streams___elabs,
     system__exception_table___elabb,
     ada__io_exceptions___elabs,
     system__stack_checking___elabs,
     system__soft_links___elabb,
     system__secondary_stack___elabb,
     ada__tags___elabs,
     ada__tags___elabb,
     ada__streams___elabs,
     system__finalization_root___elabs,
     ada__exceptions___elabb,
     system__finalization_implementation___elabs,
     system__finalization_implementation___elabb,
     ada__finalization___elabs,
     ada__finalization__list_controller___elabs,
     system__file_control_block___elabs,
     system__file_io___elabb,
     ada__text_io___elabs,
     ada__text_io___elabb@};

   __gnat_SDP_Table_Build (&st, 23, ea, 23);
   __gnat_set_globals (
      -1,      /* Main_Priority              */
      -1,      /* Time_Slice_Value           */
      'b',     /* WC_Encoding                */
      ' ',     /* Locking_Policy             */
      ' ',     /* Queuing_Policy             */
      ' ',     /* Tasking_Dispatching_Policy */
      0,       /* Finalization routine address, not used anymore */
      0,       /* Unreserve_All_Interrupts */
      0);      /* Exception_Tracebacks */

   __gnat_inside_elab_final_code = 1;

   if (ada__exceptions_E == 0) @{
      ada__exceptions___elabs ();
   @}
   if (system__exceptions_E == 0) @{
      system__exceptions___elabs ();
      system__exceptions_E++;
   @}
   if (interfaces__c_streams_E == 0) @{
      interfaces__c_streams___elabs ();
   @}
   interfaces__c_streams_E = 1;
   if (system__exception_table_E == 0) @{
      system__exception_table___elabb ();
      system__exception_table_E++;
   @}
   if (ada__io_exceptions_E == 0) @{
      ada__io_exceptions___elabs ();
      ada__io_exceptions_E++;
   @}
   if (system__stack_checking_E == 0) @{
      system__stack_checking___elabs ();
   @}
   if (system__soft_links_E == 0) @{
      system__soft_links___elabb ();
      system__soft_links_E++;
   @}
   system__stack_checking_E = 1;
   if (system__secondary_stack_E == 0) @{
      system__secondary_stack___elabb ();
      system__secondary_stack_E++;
   @}
   if (ada__tags_E == 0) @{
      ada__tags___elabs ();
   @}
   if (ada__tags_E == 0) @{
      ada__tags___elabb ();
      ada__tags_E++;
   @}
   if (ada__streams_E == 0) @{
      ada__streams___elabs ();
      ada__streams_E++;
   @}
   if (system__finalization_root_E == 0) @{
      system__finalization_root___elabs ();
   @}
   system__finalization_root_E = 1;
   if (ada__exceptions_E == 0) @{
      ada__exceptions___elabb ();
      ada__exceptions_E++;
   @}
   if (system__finalization_implementation_E == 0) @{
      system__finalization_implementation___elabs ();
   @}
   if (system__finalization_implementation_E == 0) @{
      system__finalization_implementation___elabb ();
      system__finalization_implementation_E++;
   @}
   if (ada__finalization_E == 0) @{
      ada__finalization___elabs ();
   @}
   ada__finalization_E = 1;
   if (ada__finalization__list_controller_E == 0) @{
      ada__finalization__list_controller___elabs ();
   @}
   ada__finalization__list_controller_E = 1;
   if (system__file_control_block_E == 0) @{
      system__file_control_block___elabs ();
      system__file_control_block_E++;
   @}
   if (system__file_io_E == 0) @{
      system__file_io___elabb ();
      system__file_io_E++;
   @}
   if (ada__text_io_E == 0) @{
      ada__text_io___elabs ();
   @}
   if (ada__text_io_E == 0) @{
      ada__text_io___elabb ();
      ada__text_io_E++;
   @}

   __gnat_inside_elab_final_code = 0;
@}
int main (argc, argv, envp)
    int argc;
    char **argv;
    char **envp;
@{
   gnat_argc = argc;
   gnat_argv = argv;
   gnat_envp = envp;

   __gnat_initialize ();
   adainit ();
   __gnat_break_start ();

   _ada_hello ();

   system__standard_library__adafinal ();
   __gnat_finalize ();
   exit (gnat_exit_status);
@}
unsigned helloB = 0x7880BEB3;
unsigned system__standard_libraryB = 0x0D24CBD0;
unsigned system__standard_libraryS = 0x3283DBEB;
unsigned adaS = 0x2359F9ED;
unsigned ada__text_ioB = 0x47C85FC4;
unsigned ada__text_ioS = 0x496FE45C;
unsigned ada__exceptionsB = 0x74F50187;
unsigned ada__exceptionsS = 0x6736945B;
unsigned gnatS = 0x156A40CF;
unsigned gnat__heap_sort_aB = 0x033DABE0;
unsigned gnat__heap_sort_aS = 0x6AB38FEA;
unsigned systemS = 0x0331C6FE;
unsigned system__exceptionsS = 0x20C9ECA4;
unsigned system__exception_tableB = 0x68A22947;
unsigned system__exception_tableS = 0x394BADD5;
unsigned gnat__htableB = 0x08258E1B;
unsigned gnat__htableS = 0x367D5222;
unsigned system__machine_state_operationsB = 0x4F3B7492;
unsigned system__machine_state_operationsS = 0x182F5CF4;
unsigned system__storage_elementsB = 0x2F1EB794;
unsigned system__storage_elementsS = 0x102C83C7;
unsigned system__secondary_stackB = 0x1574B6E9;
unsigned system__secondary_stackS = 0x708E260A;
unsigned system__parametersB = 0x56D770CD;
unsigned system__parametersS = 0x237E39BE;
unsigned system__soft_linksB = 0x08AB6B2C;
unsigned system__soft_linksS = 0x1E2491F3;
unsigned system__stack_checkingB = 0x476457A0;
unsigned system__stack_checkingS = 0x5299FCED;
unsigned system__tracebackB = 0x2971EBDE;
unsigned system__tracebackS = 0x2E9C3122;
unsigned ada__streamsS = 0x7C25DE96;
unsigned ada__tagsB = 0x39ADFFA2;
unsigned ada__tagsS = 0x769A0464;
unsigned system__string_opsB = 0x5EB646AB;
unsigned system__string_opsS = 0x63CED018;
unsigned interfacesS = 0x0357E00A;
unsigned interfaces__c_streamsB = 0x3784FB72;
unsigned interfaces__c_streamsS = 0x2E723019;
unsigned system__file_ioB = 0x623358EA;
unsigned system__file_ioS = 0x31F873E6;
unsigned ada__finalizationB = 0x6843F68A;
unsigned ada__finalizationS = 0x63305874;
unsigned system__finalization_rootB = 0x31E56CE1;
unsigned system__finalization_rootS = 0x23169EF3;
unsigned system__finalization_implementationB = 0x6CCBA70E;
unsigned system__finalization_implementationS = 0x604AA587;
unsigned system__string_ops_concat_3B = 0x572E3F58;
unsigned system__string_ops_concat_3S = 0x01F57876;
unsigned system__stream_attributesB = 0x1D4F93E8;
unsigned system__stream_attributesS = 0x30B2EC3D;
unsigned ada__io_exceptionsS = 0x34054F96;
unsigned system__unsigned_typesS = 0x7B9E7FE3;
unsigned system__file_control_blockS = 0x2FF876A8;
unsigned ada__finalization__list_controllerB = 0x5760634A;
unsigned ada__finalization__list_controllerS = 0x5D851835;

/* BEGIN ELABORATION ORDER
ada (spec)
gnat (spec)
gnat.heap_sort_a (spec)
gnat.htable (spec)
gnat.htable (body)
interfaces (spec)
system (spec)
system.parameters (spec)
system.standard_library (spec)
ada.exceptions (spec)
system.exceptions (spec)
system.parameters (body)
gnat.heap_sort_a (body)
interfaces.c_streams (spec)
interfaces.c_streams (body)
system.exception_table (spec)
system.exception_table (body)
ada.io_exceptions (spec)
system.storage_elements (spec)
system.storage_elements (body)
system.machine_state_operations (spec)
system.machine_state_operations (body)
system.secondary_stack (spec)
system.stack_checking (spec)
system.soft_links (spec)
system.soft_links (body)
system.stack_checking (body)
system.secondary_stack (body)
system.standard_library (body)
system.string_ops (spec)
system.string_ops (body)
ada.tags (spec)
ada.tags (body)
ada.streams (spec)
system.finalization_root (spec)
system.finalization_root (body)
system.string_ops_concat_3 (spec)
system.string_ops_concat_3 (body)
system.traceback (spec)
system.traceback (body)
ada.exceptions (body)
system.unsigned_types (spec)
system.stream_attributes (spec)
system.stream_attributes (body)
system.finalization_implementation (spec)
system.finalization_implementation (body)
ada.finalization (spec)
ada.finalization (body)
ada.finalization.list_controller (spec)
ada.finalization.list_controller (body)
system.file_control_block (spec)
system.file_io (spec)
system.file_io (body)
ada.text_io (spec)
ada.text_io (body)
hello (body)
   END ELABORATION ORDER */

/* BEGIN Object file/option list
./HELLO.OBJ
-L./
-L/usr/local/gnat/lib/gcc-lib/alpha-dec-osf5.1/2.8.1/adalib/
/usr/local/gnat/lib/gcc-lib/alpha-dec-osf5.1/2.8.1/adalib/libgnat.a
-lexc
   END Object file/option list */

@end smallexample

@noindent
Here again, the C code is exactly what is generated by the binder. The
functions of the various parts of this code correspond in an obvious
manner with the commented Ada code shown in the example in the previous
section.

@node Consistency-Checking Modes
@section Consistency-Checking Modes

@noindent
As described in the previous section, by default @code{GNAT BIND} checks
that object files are consistent with one another and are consistent
with any source files it can locate. The following qualifiers control binder
access to sources.

@table @code
@item /READ_SOURCES=ALL
@cindex @code{/READ_SOURCES=ALL} (@code{GNAT BIND})
Require source files to be present. In this mode, the binder must be
able to locate all source files that are referenced, in order to check
their consistency. In normal mode, if a source file cannot be located it
is simply ignored. If you specify this qualifier, a missing source
file is an error.

@item /READ_SOURCES=NONE
@cindex @code{/READ_SOURCES=NONE} (@code{GNAT BIND})
Exclude source files. In this mode, the binder only checks that ALI
files are consistent with one another. Source files are not accessed.
The binder runs faster in this mode, and there is still a guarantee that
the resulting program is self-consistent.
If a source file has been edited since it was last compiled, and you
specify this qualifier, the binder will not detect that the object
file is out of date with respect to the source file. Note that this is the
mode that is automatically used by @code{GNAT MAKE} because in this
case the checking against sources has already been performed by
@code{GNAT MAKE} in the course of compilation (i.e. before binding).

@item /READ_SOURCES=AVAILABLE
This is the default mode in which source files are checked if they are
available, and ignored if they are not available.
@end table

@node Binder Error Message Control
@section Binder Error Message Control

@noindent
The following qualifiers provide control over the generation of error
messages from the binder:

@table @code
@item /REPORT_ERRORS=VERBOSE
@cindex @code{/REPORT_ERRORS=VERBOSE} (@code{GNAT BIND})
Verbose mode. In the normal mode, brief error messages are generated to
@file{SYS$ERROR}. If this qualifier is present, a header is written
to @file{SYS$OUTPUT} and any error messages are directed to @file{SYS$OUTPUT}.
All that is written to @file{SYS$ERROR} is a brief summary message.

@item /REPORT_ERRORS=BRIEF
@cindex @code{/REPORT_ERRORS=BRIEF} (@code{GNAT BIND})
Generate brief error messages to @file{SYS$ERROR} even if verbose mode is
specified. This is relevant only when used with the
@code{/REPORT_ERRORS=VERBOSE} qualifier.


@item /WARNINGS=SUPPRESS
@cindex @code{/WARNINGS=SUPPRESS} (@code{GNAT BIND})
@cindex Warnings
Suppress all warning messages.

@item /WARNINGS=ERROR
@cindex @code{/WARNINGS=ERROR} (@code{GNAT BIND})
Treat any warning messages as fatal errors.

@item /WARNINGS=NORMAL
Standard mode with warnings generated, but warnings do not get treated
as errors.

@item /NOTIME_STAMP_CHECK
@cindex @code{/NOTIME_STAMP_CHECK} (@code{GNAT BIND})
@cindex Time stamp checks, in binder
@cindex Binder consistency checks
@cindex Consistency checks, in binder
The binder performs a number of consistency checks including:

@itemize @bullet
@item
Check that time stamps of a given source unit are consistent
@item
Check that checksums of a given source unit are consistent
@item
Check that consistent versions of @code{GNAT} were used for compilation
@item
Check consistency of configuration pragmas as required
@end itemize

@noindent
Normally failure of such checks, in accordance with the consistency
requirements of the Ada Reference Manual, causes error messages to be
generated which abort the binder and prevent the output of a binder
file and subsequent link to obtain an executable.

The @code{/NOTIME_STAMP_CHECK} qualifier converts these error messages
into warnings, so that
binding and linking can continue to completion even in the presence of such
errors. The result may be a failed link (due to missing symbols), or a
non-functional executable which has undefined semantics.
@emph{This means that
@code{/NOTIME_STAMP_CHECK} should be used only in unusual situations,
with extreme care.}
@end table

@node Elaboration Control
@section Elaboration Control

@noindent
The following qualifiers provide additional control over the elaboration
order. For full details see @xref{Elaboration Order Handling in GNAT}.

@table @code
@item /PESSIMISTIC_ELABORATION
@cindex @code{/PESSIMISTIC_ELABORATION} (@code{GNAT BIND})
Normally the binder attempts to choose an elaboration order that is
likely to minimize the likelihood of an elaboration order error resulting
in raising a @code{Program_Error} exception. This qualifier reverses the
action of the binder, and requests that it deliberately choose an order
that is likely to maximize the likelihood of an elaboration error.
This is useful in ensuring portability and avoiding dependence on
accidental fortuitous elaboration ordering.

Normally it only makes sense to use the @code{-p} qualifier if dynamic
elaboration checking is used (@option{/CHECKS=ELABORATION} qualifier used for compilation).
This is because in the default static elaboration mode, all necessary
@code{Elaborate_All} pragmas are implicitly inserted. These implicit
pragmas are still respected by the binder in @code{-p} mode, so a
safe elaboration order is assured.
@end table

@node Output Control
@section Output Control

@noindent
The following qualifiers allow additional control over the output
generated by the binder.

@table @code

@item /BIND_FILE=ADA
@cindex @code{/BIND_FILE=ADA} (@code{GNAT BIND})
Generate binder program in Ada (default). The binder program is named
@file{B$@var{mainprog}.ADB} by default. This can be changed with
@code{-o} @code{GNAT BIND} option.

@item /NOOUTPUT
@cindex @code{/NOOUTPUT} (@code{GNAT BIND})
Check only. Do not generate the binder output file. In this mode the
binder performs all error checks but does not generate an output file.

@item /BIND_FILE=C
@cindex @code{/BIND_FILE=C} (@code{GNAT BIND})
Generate binder program in C. The binder program is named
@file{B_@var{mainprog}.C}. This can be changed with @code{-o} @code{GNAT BIND}
option.

@item /ELABORATION_DEPENDENCIES
@cindex @code{/ELABORATION_DEPENDENCIES} (@code{GNAT BIND})
Output complete list of elaboration-order dependencies, showing the
reason for each dependency. This output can be rather extensive but may
be useful in diagnosing problems with elaboration order. The output is
written to @file{SYS$OUTPUT}.

@item /HELP
@cindex @code{/HELP} (@code{GNAT BIND})
Output usage information. The output is written to @file{SYS$OUTPUT}.

@item /LINKER_OPTION_LIST
@cindex @code{/LINKER_OPTION_LIST} (@code{GNAT BIND})
Output linker options to @file{SYS$OUTPUT}. Includes library search paths,
contents of pragmas Ident and Linker_Options, and libraries added
by @code{GNAT BIND}.

@item /ORDER_OF_ELABORATION
@cindex @code{/ORDER_OF_ELABORATION} (@code{GNAT BIND})
Output chosen elaboration order. The output is written to @file{SYS$OUTPUT}.

@item /OBJECT_LIST
@cindex @code{/OBJECT_LIST} (@code{GNAT BIND})
Output full names of all the object files that must be linked to provide
the Ada component of the program. The output is written to @file{SYS$OUTPUT}.
This list includes the files explicitly supplied and referenced by the user
as well as implicitly referenced run-time unit files. The latter are
omitted if the corresponding units reside in shared libraries. The
directory names for the run-time units depend on the system configuration.

@item /OUTPUT=@var{file}
@cindex @code{/OUTPUT} (@code{GNAT BIND})
Set name of output file to @var{file} instead of the normal
@file{B$@var{mainprog}.ADB} default. Note that @var{file} denote the Ada
binder generated body filename. In C mode you would normally give
@var{file} an extension of @file{.C} because it will be a C source program.
Note that if this option is used, then linking must be done manually.
It is not possible to use GNAT LINK in this case, since it cannot locate
the binder file.

@item /RESTRICTION_LIST
@cindex @code{/RESTRICTION_LIST} (@code{GNAT BIND})
Generate list of @code{pragma Rerstrictions} that could be applied to
the current unit. This is useful for code audit purposes, and also may
be used to improve code generation in some cases.

@end table

@node Binding with Non-Ada Main Programs
@section Binding with Non-Ada Main Programs

@noindent
In our description so far we have assumed that the main
program is in Ada, and that the task of the binder is to generate a
corresponding function @code{main} that invokes this Ada main
program. GNAT also supports the building of executable programs where
the main program is not in Ada, but some of the called routines are
written in Ada and compiled using GNAT (@pxref{Mixed Language Programming}).
The following qualifier is used in this situation:

@table @code
@item /NOMAIN
@cindex @code{/NOMAIN} (@code{GNAT BIND})
No main program. The main program is not in Ada.
@end table

@noindent
In this case, most of the functions of the binder are still required,
but instead of generating a main program, the binder generates a file
containing the following callable routines:

@table @code
@item adainit
@findex adainit
You must call this routine to initialize the Ada part of the program by
calling the necessary elaboration routines. A call to @code{adainit} is
required before the first call to an Ada subprogram.

Note that it is assumed that the basic execution environment must be setup
to be appropriate for Ada execution at the point where the first Ada
subprogram is called. In particular, if the Ada code will do any
floating-point operations, then the FPU must be setup in an appropriate
manner. For the case of the x86, for example, full precision mode is
required. The procedure GNAT.Float_Control.Reset may be used to ensure
that the FPU is in the right state.

@item adafinal
@findex adafinal
You must call this routine to perform any library-level finalization
required by the Ada subprograms. A call to @code{adafinal} is required
after the last call to an Ada subprogram, and before the program
terminates.
@end table

@noindent
If the @code{/NOMAIN} qualifier
@cindex Binder, multiple input files
is given, more than one ALI file may appear on
the command line for @code{GNAT BIND}. The normal @dfn{closure}
calculation is performed for each of the specified units. Calculating
the closure means finding out the set of units involved by tracing
@code{with} references. The reason it is necessary to be able to
specify more than one ALI file is that a given program may invoke two or
more quite separate groups of Ada units.

The binder takes the name of its output file from the last specified ALI
file, unless overridden by the use of the @code{/OUTPUT=file}.
The output is an Ada unit in source form that can
be compiled with GNAT unless the -C qualifier is used in which case the
output is a C source file, which must be compiled using the C compiler.
This compilation occurs automatically as part of the @code{GNAT LINK}
processing.

Currently the GNAT run time requires a FPU using 80 bits mode
precision. Under targets where this is not the default it is required to
call GNAT.Float_Control.Reset before using floating point numbers (this
include float computation, float input and output) in the Ada code. A
side effect is that this could be the wrong mode for the foreign code
where floating point computation could be broken after this call.

@node Binding Programs with No Main Subprogram
@section Binding Programs with No Main Subprogram

@noindent
It is possible to have an Ada program which does not have a main
subprogram. This program will call the elaboration routines of all the
packages, then the finalization routines.

The following qualifier is used to bind programs organized in this manner:

@table @code
@item /ZERO_MAIN
@cindex @code{/ZERO_MAIN} (@code{GNAT BIND})
Normally the binder checks that the unit name given on the command line
corresponds to a suitable main subprogram. When this qualifier is used,
a list of ALI files can be given, and the execution of the program
consists of elaboration of these units in an appropriate order.
@end table

@node Summary of Binder Qualifiers
@section Summary of Binder Qualifiers

@noindent
The following are the qualifiers available with @code{GNAT BIND}:

@table @code
@item /OBJECT_SEARCH
Specify directory to be searched for ALI files.

@item /SOURCE_SEARCH
Specify directory to be searched for source file.

@item /BIND_FILE=ADA
Generate binder program in Ada (default)

@item /REPORT_ERRORS=BRIEF
Generate brief messages to @file{SYS$ERROR} even if verbose mode set.

@item /NOOUTPUT
Check only, no generation of binder output file.

@item /BIND_FILE=C
Generate binder program in C

@item /ELABORATION_DEPENDENCIES
Output complete list of elaboration-order dependencies.

@item -E
Store tracebacks in exception occurrences when the target supports it.
This is the default with the zero cost exception mechanism.
This option is currently supported on the following targets:
all x86 ports, Solaris, Windows, HP-UX, AIX, PowerPC VxWorks and Alpha VxWorks.
See also the packages @code{GNAT.Traceback} and
@code{GNAT.Traceback.Symbolic} for more information.
Note that on x86 ports, you must not use @code{-fomit-frame-pointer}
@code{GNAT COMPILE} option.

@item -h
Output usage (help) information

@item /SEARCH
Specify directory to be searched for source and ALI files.

@item /NOCURRENT_DIRECTORY
Do not look for sources in the current directory where @code{GNAT BIND} was
invoked, and do not look for ALI files in the directory containing the
ALI file named in the @code{GNAT BIND} command line.

@item /ORDER_OF_ELABORATION
Output chosen elaboration order.

@item -Lxxx
Binds the units for library building. In this case the adainit and
adafinal procedures (See @pxref{Binding with Non-Ada Main Programs})
are renamed to xxxinit and xxxfinal. Implies -n.

@item -Mxyz
Rename generated main program from main to xyz

@item /ERROR_LIMIT=@var{n}
Limit number of detected errors to @var{n} (1-999).

@item /NOMAIN
No main program.

@item /NOSTD_INCLUDES
Do not look for sources in the system default directory.

@item /NOSTD_LIBRARIES
Do not look for library files in the system default directory.

@item /RUNTIME_SYSTEM=@var{rts-path}
@cindex @code{/RUNTIME_SYSTEM} (@code{GNAT BIND})
Specifies the default location of the runtime library. Same meaning as the
equivalent @code{GNAT MAKE} flag (see @ref{Qualifiers for GNAT MAKE}).

@item /OUTPUT=@var{file}
Name the output file @var{file} (default is @file{B$@var{xxx}.ADB}).
Note that if this option is used, then linking must be done manually,
GNAT LINK cannot be used.

@item /OBJECT_LIST
Output object list.

@item -p
Pessimistic (worst-case) elaboration order

@item /READ_SOURCES=ALL
Require all source files to be present.


@item /NOTIME_STAMP_CHECK
Tolerate time stamp and other consistency errors

@item -T@var{n}
Set the time slice value to n microseconds. A value of zero means no time
slicing and also indicates to the tasking run time to match as close as
possible to the annex D requirements of the RM.

@item /REPORT_ERRORS=VERBOSE
Verbose mode. Write error messages, header, summary output to
@file{SYS$OUTPUT}.


@item /WARNINGS=NORMAL
Normal warnings mode. Warnings are issued but ignored

@item /WARNINGS=SUPPRESS
All warning messages are suppressed

@item /WARNINGS=ERROR
Warning messages are treated as fatal errors

@item /READ_SOURCES=NONE
Exclude source files (check object consistency only).

@item /READ_SOURCES=AVAILABLE
Default mode, in which sources are checked for consistency only if
they are available.

@item /ZERO_MAIN
No main subprogram.

@end table


@node Command-Line Access
@section Command-Line Access

@noindent
The package @code{Ada.Command_Line} provides access to the command-line
arguments and program name. In order for this interface to operate
correctly, the two variables

@smallexample
@group
@cartouche
int gnat_argc;
char **gnat_argv;
@end cartouche
@end group
@end smallexample

@noindent
@findex gnat_argv
@findex gnat_argc
are declared in one of the GNAT library routines. These variables must
be set from the actual @code{argc} and @code{argv} values passed to the
main program. With no @code{/NOMAIN} present, @code{GNAT BIND}
generates the C main program to automatically set these variables.
If the @code{/NOMAIN} qualifier is used, there is no automatic way to
set these variables. If they are not set, the procedures in
@code{Ada.Command_Line} will not be available, and any attempt to use
them will raise @code{Constraint_Error}. If command line access is
required, your main program must set @code{gnat_argc} and
@code{gnat_argv} from the @code{argc} and @code{argv} values passed to
it.

@node Search Paths for GNAT BIND
@section Search Paths for @code{GNAT BIND}

@noindent
The binder takes the name of an ALI file as its argument and needs to
locate source files as well as other ALI files to verify object consistency.

For source files, it follows exactly the same search rules as @code{GNAT COMPILE}
(@pxref{Search Paths and the Run-Time Library (RTL)}). For ALI files the
directories searched are:

@enumerate
@item
The directory containing the ALI file named in the command line, unless
the qualifier @code{/NOCURRENT_DIRECTORY} is specified.

@item
All directories specified by @code{/SEARCH}
qualifiers on the @code{GNAT BIND}
command line, in the order given.

@item
@findex ADA_OBJECTS_PATH
Each of the directories listed in the value of the
@code{ADA_OBJECTS_PATH} logical name.
Normally, define this value as a logical name containing a comma separated
list of directory names.

This variable can also be defined by means of an environment string
(an argument to the DEC C exec* set of functions).

Logical Name:
@smallexample
DEFINE ANOTHER_PATH FOO:[BAG]
DEFINE ADA_OBJECTS_PATH ANOTHER_PATH,FOO:[BAM],FOO:[BAR]
@end smallexample

By default, the path includes GNU:[LIB.OPENVMS7_x.2_8_x.DECLIB]
first, followed by the standard Ada 95
libraries in GNU:[LIB.OPENVMS7_x.2_8_x.ADALIB].
If this is not redefined, the user will obtain the DEC Ada83 IO packages
(Text_IO, Sequential_IO, etc)
instead of the Ada95 packages. Thus, in order to get the Ada 95
packages by default, ADA_OBJECTS_PATH must be redefined.

@item
The content of the "ada_object_path" file which is part of the GNAT
installation tree and is used to store standard libraries such as the
GNAT Run Time Library (RTL) unless the qualifier @code{/NOSTD_LIBRARIES} is
specified.
@end enumerate

@noindent
In the binder the qualifier @code{/SEARCH}
is used to specify both source and
library file paths. Use @code{/SOURCE_SEARCH}
instead if you want to specify
source paths only, and @code{/LIBRARY_SEARCH}
if you want to specify library paths
only. This means that for the binder
@code{/SEARCH=}@var{dir} is equivalent to
@code{/SOURCE_SEARCH=}@var{dir}
@code{/OBJECT_SEARCH=}@var{dir}.
The binder generates the bind file (a C language source file) in the
current working directory.

@findex Ada
@findex System
@findex Interfaces
@findex GNAT
The packages @code{Ada}, @code{System}, and @code{Interfaces} and their
children make up the GNAT Run-Time Library, together with the package
GNAT and its children, which contain a set of useful additional
library functions provided by GNAT. The sources for these units are
needed by the compiler and are kept together in one directory. The ALI
files and object files generated by compiling the RTL are needed by the
binder and the linker and are kept together in one directory, typically
different from the directory containing the sources. In a normal
installation, you need not specify these directory names when compiling
or binding. Either the environment variables or the built-in defaults
cause these files to be found.

Besides simplifying access to the RTL, a major use of search paths is
in compiling sources from multiple directories. This can make
development environments much more flexible.

@node Examples of GNAT BIND Usage
@section Examples of @code{GNAT BIND} Usage

@noindent
This section contains a number of examples of using the GNAT binding
utility @code{GNAT BIND}.

@table @code
@item GNAT BIND hello
The main program @code{Hello} (source program in @file{HELLO.ADB}) is
bound using the standard qualifier settings. The generated main program is
@file{B~HELLO.ADB}. This is the normal, default use of the binder.

@item GNAT BIND HELLO.ALI /OUTPUT=Mainprog.ADB
The main program @code{Hello} (source program in @file{HELLO.ADB}) is
bound using the standard qualifier settings. The generated main program is
@file{MAINPROG.ADB} with the associated spec in
@file{MAINPROG.ADS}. Note that you must specify the body here not the
spec, in the case where the output is in Ada. Note that if this option
is used, then linking must be done manually, since GNAT LINK will not
be able to find the generated file.

@item GNAT BIND MAIN.ALI /BIND_FILE=C /OUTPUT=Mainprog.C /READ_SOURCES=NONE
The main program @code{Main} (source program in
@file{MAIN.ADB}) is bound, excluding source files from the
consistency checking, generating
the file @file{MAINPROG.C}.


@item GNAT BIND /NOMAIN math dbase /BIND_FILE=C /OUTPUT=ADA-CONTROL.C
The main program is in a language other than Ada, but calls to
subprograms in packages @code{Math} and @code{Dbase} appear. This call
to @code{GNAT BIND} generates the file @file{ADA-CONTROL.C} containing
the @code{adainit} and @code{adafinal} routines to be called before and
after accessing the Ada units.
@end table

@node Linking Using GNAT LINK
@chapter Linking Using @code{GNAT LINK}
@findex GNAT LINK

@noindent
This chapter discusses @code{GNAT LINK}, a utility program used to link
Ada programs and build an executable file. This is a simple program
that invokes the Unix linker (via the @code{GNAT COMPILE}
command) with a correct list of object files and library references.
@code{GNAT LINK} automatically determines the list of files and
references for the Ada part of a program. It uses the binder file
generated by the binder to determine this list.

@menu
* Running GNAT LINK::
* Qualifiers for GNAT LINK::
* Setting Stack Size from GNAT LINK::
* Setting Heap Size from GNAT LINK::
@end menu

@node Running GNAT LINK
@section Running @code{GNAT LINK}

@noindent
The form of the @code{GNAT LINK} command is

@smallexample
$ GNAT LINK [@var{qualifiers}] @var{mainprog}[.ALI] [@var{non-Ada objects}]
      [@var{linker options}]
@end smallexample

@noindent
@file{@var{mainprog}.ALI} references the ALI file of the main program.
The @file{.ALI} extension of this file can be omitted. From this
reference, @code{GNAT LINK} locates the corresponding binder file
@file{B$@var{mainprog}.ADB} and, using the information in this file along
with the list of non-Ada objects and linker options, constructs a Unix
linker command file to create the executable.

The arguments following @file{@var{mainprog}.ALI} are passed to the
linker uninterpreted. They typically include the names of object files
for units written in other languages than Ada and any library references
required to resolve references in any of these foreign language units,
or in @code{pragma Import} statements in any Ada units.

@var{linker options} is an optional list of linker specific
qualifiers. The default linker called by GNAT LINK is @var{GNAT COMPILE} which in
turn calls the appropriate system linker usually called
@var{ld}. Standard options for the linker such as @code{-lmy_lib} or
@code{-Ldir} can be added as is. For options that are not recognized by
@var{GNAT COMPILE} as linker options, the @var{GNAT COMPILE} qualifiers @code{-Xlinker} or
@code{-Wl,} shall be used. Refer to the GCC documentation for
details. Here is an example showing how to generate a linker map
assuming that the underlying linker is GNU ld:

@smallexample
$ GNAT LINK my_prog -Wl,-Map,MAPFILE
@end smallexample

Using @var{linker options} it is possible to set the program stack and
heap size. See @pxref{Setting Stack Size from GNAT LINK} and
@pxref{Setting Heap Size from GNAT LINK}.

@code{GNAT LINK} determines the list of objects required by the Ada
program and prepends them to the list of objects passed to the linker.
@code{GNAT LINK} also gathers any arguments set by the use of
@code{pragma Linker_Options} and adds them to the list of arguments
presented to the linker.

@code{GNAT LINK} accepts the following types of extra files on the command
line: objects (.OBJ), libraries (.OLB), shareable images (.EXE), and
options files (.OPT). These are recognized and handled according to their
extension.

@node Qualifiers for GNAT LINK
@section Qualifiers for @code{GNAT LINK}

@noindent
The following qualifiers are available with the @code{GNAT LINK} utility:

@table @code

@item /BIND_FILE=ADA
@cindex @code{/BIND_FILE=ADA} (@code{GNAT LINK})
The binder has generated code in Ada. This is the default.

@item /BIND_FILE=C
@cindex @code{/BIND_FILE=C} (@code{GNAT LINK})
If instead of generating a file in Ada, the binder has generated one in
C, then the linker needs to know about it. Use this qualifier to signal
to @code{GNAT LINK} that the binder has generated C code rather than
Ada code.

@item -f
@cindex Command line length
@cindex @code{-f} (@code{GNAT LINK})
On some targets, the command line length is limited, and @code{GNAT LINK}
will generate a separate file for the linker if the list of object files
is too long. The @code{-f} flag forces this file to be generated even if
the limit is not exceeded. This is useful in some cases to deal with
special situations where the command line length is exceeded.

@item /DEBUG
@cindex Debugging information, including
@cindex @code{/DEBUG} (@code{GNAT LINK})
The option to include debugging information causes the Ada bind file (in
other words, @file{B$@var{mainprog}.ADB}) to be compiled with
@code{/DEBUG}.
In addition, the binder does not delete the @file{B$@var{mainprog}.ADB},
@file{B$@var{mainprog}.OBJ} and @file{B$@var{mainprog}.ALI} files.
Without @code{/DEBUG}, the binder removes these files by
default. The same procedure apply if a C bind file was generated using
@code{/BIND_FILE=C} @code{GNAT BIND} option, in this case the filenames are
@file{B_@var{mainprog}.C} and @file{B_@var{mainprog}.OBJ}.


@item /VERBOSE
@cindex @code{/VERBOSE} (@code{GNAT LINK})
Causes additional information to be output, including a full list of the
included object files. This qualifier option is most useful when you want
to see what set of object files are being used in the link step.


@item /EXECUTABLE=@var{exec-name}
@cindex @code{/EXECUTABLE} (@code{GNAT LINK})
@var{exec-name} specifies an alternate name for the generated
executable program. If this qualifier is omitted, the executable has the same
name as the main unit. For example, @code{GNAT LINK TRY.ALI} creates
an executable called @file{TRY.EXE}.


@item /DEBUG=TRACEBACK
@cindex @code{/DEBUG=TRACEBACK} (@code{GNAT LINK})
This qualifier causes sufficient information to be included in the
executable file to allow a traceback, but does not include the full
symbol information needed by the debugger.

@item /IDENTIFICATION="<string>"
"<string>" specifies the string to be stored in the image file identification
field in the image header. It overrides any pragma Ident specified string.

@item /NOINHIBIT-EXEC
Generate the executable file even if there are linker warnings.

@item /NOSTART_FILES
Don't link in the object file containing the "main" transfer address.
Used when linking with a foreign language main program compiled with a
Digital compiler.

@item /STATIC
Prefer linking with object libraries over shareable images, even without
/DEBUG.

@end table

@node Setting Stack Size from GNAT LINK
@section Setting Stack Size from @code{GNAT LINK}

@noindent
It is possible to specify the program stack size from @code{GNAT LINK}.
Assuming that the underlying linker is GNU ld there is two ways to do so:

@itemize @bullet

@item using @code{-Xlinker} linker option

@smallexample
$ GNAT LINK hello -Xlinker --stack=0x10000,0x1000
@end smallexample

This set the stack reserve size to 0x10000 bytes and the stack commit
size to 0x1000 bytes.

@item using @code{-Wl} linker option

@smallexample
$ GNAT LINK hello -Wl,--stack=0x1000000
@end smallexample

This set the stack reserve size to 0x1000000 bytes. Note that with
@code{-Wl} option it is not possible to set the stack commit size
because the coma is a separator for this option.

@end itemize

@node Setting Heap Size from GNAT LINK
@section Setting Heap Size from @code{GNAT LINK}

@noindent
It is possible to specify the program heap size from @code{GNAT LINK}.
Assuming that the underlying linker is GNU ld there is two ways to do so:

@itemize @bullet

@item using @code{-Xlinker} linker option

@smallexample
$ GNAT LINK hello -Xlinker --heap=0x10000,0x1000
@end smallexample

This set the heap reserve size to 0x10000 bytes and the heap commit
size to 0x1000 bytes.

@item using @code{-Wl} linker option

@smallexample
$ GNAT LINK hello -Wl,--heap=0x1000000
@end smallexample

This set the heap reserve size to 0x1000000 bytes. Note that with
@code{-Wl} option it is not possible to set the heap commit size
because the coma is a separator for this option.

@end itemize

@node The GNAT Make Program GNAT MAKE
@chapter The GNAT Make Program @code{GNAT MAKE}
@findex GNAT MAKE

@menu
* Running GNAT MAKE::
* Qualifiers for GNAT MAKE::
* Mode Qualifiers for GNAT MAKE::
* Notes on the Command Line::
* How GNAT MAKE Works::
* Examples of GNAT MAKE Usage::
@end menu
@noindent
A typical development cycle when working on an Ada program consists of
the following steps:

@enumerate
@item
Edit some sources to fix bugs.

@item
Add enhancements.

@item
Compile all sources affected.

@item
Rebind and relink.

@item
Test.
@end enumerate

@noindent
The third step can be tricky, because not only do the modified files
@cindex Dependency rules
have to be compiled, but any files depending on these files must also be
recompiled. The dependency rules in Ada can be quite complex, especially
in the presence of overloading, @code{use} clauses, generics and inlined
subprograms.

@code{GNAT MAKE} automatically takes care of the third and fourth steps
of this process. It determines which sources need to be compiled,
compiles them, and binds and links the resulting object files.

Unlike some other Ada make programs, the dependencies are always
accurately recomputed from the new sources. The source based approach of
the GNAT compilation model makes this possible. This means that if
changes to the source program cause corresponding changes in
dependencies, they will always be tracked exactly correctly by
@code{GNAT MAKE}.

@node Running GNAT MAKE
@section Running @code{GNAT MAKE}

@noindent
The usual form of the @code{GNAT MAKE} command is

@smallexample
$ GNAT MAKE [@var{qualifiers}] @var{file_name} [@var{file_names}] [@var{mode_qualifiers}]
@end smallexample

@noindent
The only required argument is one @var{file_name}, which specifies
a compilation unit that is a main program. Several @var{file_names} can be
specified: this will result in several executables being built.
If @code{qualifiers} are present, they can be placed before the first
@var{file_name}, between @var{file_names} or after the last @var{file_name}.
If @var{mode_qualifiers} are present, they must always be placed after
the last @var{file_name} and all @code{qualifiers}.

If you are using standard file extensions (.ADB and .ADS), then the
extension may be omitted from the @var{file_name} arguments. However, if
you are using non-standard extensions, then it is required that the
extension be given. A relative or absolute directory path can be
specified in a @var{file_name}, in which case, the input source file will
be searched for in the specified directory only. Otherwise, the input
source file will first be searched in the directory where
@code{GNAT MAKE} was invoked and if it is not found, it will be search on
the source path of the compiler as described in
@ref{Search Paths and the Run-Time Library (RTL)}.

When several @var{file_names} are specified, if an executable needs to be
rebuilt and relinked, all subsequent executables will be rebuilt and
relinked, even if this would not be absolutely necessary.

All @code{GNAT MAKE} output (except when you specify
@code{/DEPENDENCIES_LIST}) is to
@file{SYS$ERROR}. The output produced by the
@code{/DEPENDENCIES_LIST} qualifier is send to
@file{SYS$OUTPUT}.

@node Qualifiers for GNAT MAKE
@section Qualifiers for @code{GNAT MAKE}

@noindent
You may specify any of the following qualifiers to @code{GNAT MAKE}:

@table @code

@item /ALL_FILES
@cindex @code{/ALL_FILES} (@code{GNAT MAKE})
Consider all files in the make process, even the GNAT internal system
files (for example, the predefined Ada library files), as well as any
locked files. Locked files are files whose ALI file is write-protected.
By default,
@code{GNAT MAKE} does not check these files,
because the assumption is that the GNAT internal files are properly up
to date, and also that any write protected ALI files have been properly
installed. Note that if there is an installation problem, such that one
of these files is not up to date, it will be properly caught by the
binder.
You may have to specify this qualifier if you are working on GNAT
itself. @code{/ALL_FILES} is also useful in conjunction with
@code{/FORCE_COMPILE}
if you need to recompile an entire application,
including run-time files, using special configuration pragma settings,
such as a non-standard @code{Float_Representation} pragma.
By default
@code{GNAT MAKE /ALL_FILES} compiles all GNAT
internal files with
the @code{/CHECKS=SUPPRESS_ALL /STYLE_CHECKS=GNAT} qualifier.

@item /ACTIONS=BIND
@cindex @code{/ACTIONS=BIND} (@code{GNAT MAKE})
Bind only. Can be combined with @code{/ACTIONS=COMPILE} to do compilation
and binding, but no link. Can be combined with @code{/ACTIONS=LINK}
to do binding and linking. When not combined with @code{/ACTIONS=COMPILE}
all the units in the closure of the main program must have been previously
compiled and must be up to date. The root unit specified by @var{file_name}
may be given without extension, with the source extension or, if no GNAT
Project File is specified, with the ALI file extension.

@item /ACTIONS=COMPILE
@cindex @code{/ACTIONS=COMPILE} (@code{GNAT MAKE})
Compile only. Do not perform binding, except when @code{/ACTIONS=BIND}
is also specified. Do not perform linking, except if both
@code{/ACTIONS=BIND} and
 @code{/ACTIONS=LINK} are also specified.
If the root unit specified by @var{file_name} is not a main unit, this is the
default. Otherwise @code{GNAT MAKE} will attempt binding and linking
unless all objects are up to date and the executable is more recent than
the objects.

@item /MAPPING
@cindex @code{/MAPPING} (@code{GNAT MAKE})
Use a mapping file. A mapping file is a way to communicate to the compiler
two mappings: from unit names to file names (without any directory information)
and from file names to path names (with full directory information).
These mappings are used by the compiler to short-circuit the path search.
When @code{GNAT MAKE} is invoked with this qualifier, it will create a mapping
file, initially populated by the project manager, if @code{-P} is used,
otherwise initially empty. Each invocation of the compiler will add the newly
accessed sources to the mapping file. This will improve the source search
during the next invocation of the compiler.

@item /FORCE_COMPILE
@cindex @code{/FORCE_COMPILE} (@code{GNAT MAKE})
Force recompilations. Recompile all sources, even though some object
files may be up to date, but don't recompile predefined or GNAT internal
files or locked files (files with a write-protected ALI file),
unless the @code{/ALL_FILES} qualifier is also specified.

@item
@item /IN_PLACE
@cindex @code{/IN_PLACE} (@code{GNAT MAKE})
In normal mode, @code{GNAT MAKE} compiles all object files and ALI files
into the current directory. If the @code{/IN_PLACE} qualifier is used,
then instead object files and ALI files that already exist are overwritten
in place. This means that once a large project is organized into separate
directories in the desired manner, then @code{GNAT MAKE} will automatically
maintain and update this organization. If no ALI files are found on the
Ada object path (@ref{Search Paths and the Run-Time Library (RTL)}),
the new object and ALI files are created in the
directory containing the source being compiled. If another organization
is desired, where objects and sources are kept in different directories,
a useful technique is to create dummy ALI files in the desired directories.
When detecting such a dummy file, @code{GNAT MAKE} will be forced to recompile
the corresponding source file, and it will be put the resulting object
and ALI files in the directory where it found the dummy file.

@item /PROCESSES=@var{n}
@cindex @code{/PROCESSES} (@code{GNAT MAKE})
@cindex Parallel make
Use @var{n} processes to carry out the (re)compilations. On a
multiprocessor machine compilations will occur in parallel. In the
event of compilation errors, messages from various compilations might
get interspersed (but @code{GNAT MAKE} will give you the full ordered
list of failing compiles at the end). If this is problematic, rerun
the make process with n set to 1 to get a clean list of messages.

@item /CONTINUE_ON_ERROR
@cindex @code{/CONTINUE_ON_ERROR} (@code{GNAT MAKE})
Keep going. Continue as much as possible after a compilation error. To
ease the programmer's task in case of compilation errors, the list of
sources for which the compile fails is given when @code{GNAT MAKE}
terminates.

If @code{GNAT MAKE} is invoked with several @file{file_names} and with this
qualifier, if there are compilation errors when building an executable,
@code{GNAT MAKE} will not attempt to build the following executables.

@item /ACTIONS=LINK
@cindex @code{/ACTIONS=LINK} (@code{GNAT MAKE})
Link only. Can be combined with @code{/ACTIONS=BIND} to binding
and linking. Linking will not be performed if combined with
@code{/ACTIONS=COMPILE}
but not with @code{/ACTIONS=BIND}.
When not combined with @code{/ACTIONS=BIND}
all the units in the closure of the main program must have been previously
compiled and must be up to date, and the main program need to have been bound.
The root unit specified by @var{file_name}
may be given without extension, with the source extension or, if no GNAT
Project File is specified, with the ALI file extension.

@item /MINIMAL_RECOMPILATION
@cindex @code{/MINIMAL_RECOMPILATION} (@code{GNAT MAKE})
Specifies that the minimum necessary amount of recompilations
be performed. In this mode @code{GNAT MAKE} ignores time
stamp differences when the only
modifications to a source file consist in adding/removing comments,
empty lines, spaces or tabs. This means that if you have changed the
comments in a source file or have simply reformatted it, using this
qualifier will tell GNAT MAKE not to recompile files that depend on it
(provided other sources on which these files depend have undergone no
semantic modifications). Note that the debugging information may be
out of date with respect to the sources if the @code{-m} qualifier causes
a compilation to be switched, so the use of this qualifier represents a
trade-off between compilation time and accurate debugging information.

@item /DEPENDENCIES_LIST
@cindex Dependencies, producing list
@cindex @code{/DEPENDENCIES_LIST} (@code{GNAT MAKE})
Check if all objects are up to date. If they are, output the object
dependences to @file{SYS$OUTPUT} in a form that can be directly exploited in
a @file{Makefile}. By default, each source file is prefixed with its
(relative or absolute) directory name. This name is whatever you
specified in the various @code{/SOURCE_SEARCH}
and @code{/SEARCH} qualifiers. If you use
@code{GNAT MAKE /DEPENDENCIES_LIST}
@code{/QUIET}
(see below), only the source file names,
without relative paths, are output. If you just specify the
@code{/DEPENDENCIES_LIST}
qualifier, dependencies of the GNAT internal system files are omitted. This
is typically what you want. If you also specify
the @code{/ALL_FILES} qualifier,
dependencies of the GNAT internal files are also listed. Note that
dependencies of the objects in external Ada libraries (see qualifier
@code{/SKIP_MISSING=}@var{dir} in the following list) are never reported.

@item /DO_OBJECT_CHECK
@cindex @code{/DO_OBJECT_CHECK} (@code{GNAT MAKE})
Don't compile, bind, or link. Checks if all objects are up to date.
If they are not, the full name of the first file that needs to be
recompiled is printed.
Repeated use of this option, followed by compiling the indicated source
file, will eventually result in recompiling all required units.

@item /EXECUTABLE=@var{exec_name}
@cindex @code{/EXECUTABLE} (@code{GNAT MAKE})
Output executable name. The name of the final executable program will be
@var{exec_name}. If the @code{/EXECUTABLE} qualifier is omitted the default
name for the executable will be the name of the input file in appropriate form
for an executable file on the host system.

This qualifier cannot be used when invoking @code{GNAT MAKE} with several
@file{file_names}.

@item /QUIET
@cindex @code{/QUIET} (@code{GNAT MAKE})
Quiet. When this flag is not set, the commands carried out by
@code{GNAT MAKE} are displayed.

@item /SWITCH_CHECK/
@cindex @code{/SWITCH_CHECK} (@code{GNAT MAKE})
Recompile if compiler qualifiers have changed since last compilation.
All compiler qualifiers but -I and -o are taken into account in the
following way:
orders between different ``first letter'' qualifiers are ignored, but
orders between same qualifiers are taken into account. For example,
@code{-O /OPTIMIZE=ALL} is different than @code{/OPTIMIZE=ALL -O}, but @code{-g -O} is equivalent
to @code{-O -g}.

@item /UNIQUE
@cindex @code{/UNIQUE} (@code{GNAT MAKE})
Unique. Recompile at most the main file. It implies -c. Combined with
-f, it is equivalent to calling the compiler directly.

@item /REASONS
@cindex @code{/REASONS} (@code{GNAT MAKE})
Verbose. Displays the reason for all recompilations @code{GNAT MAKE}
decides are necessary.

@item /NOMAIN
@cindex @code{/NOMAIN} (@code{GNAT MAKE})
No main subprogram. Bind and link the program even if the unit name
given on the command line is a package name. The resulting executable
will execute the elaboration routines of the package and its closure,
then the finalization routines.

@item @code{GNAT COMPILE} @asis{qualifiers}
Any qualifier that cannot be recognized as a qualifier for @code{GNAT MAKE}
but is recognizable as a valid qualifier for @code{GNAT COMPILE} is
automatically treated as a compiler qualifier, and passed on to all
compilations that are carried out.
@end table

@noindent
Source and library search path qualifiers:

@table @code
@item /SOURCE_SEARCH=@var{dir}
@cindex @code{/SOURCE_SEARCH} (@code{GNAT MAKE})
When looking for source files also look in directory @var{dir}.
The order in which source files search is undertaken is
described in @ref{Search Paths and the Run-Time Library (RTL)}.

@item /SKIP_MISSING=@var{dir}
@cindex @code{/SKIP_MISSING} (@code{GNAT MAKE})
Consider @var{dir} as being an externally provided Ada library.
Instructs @code{GNAT MAKE} to skip compilation units whose @file{.ALI}
files have been located in directory @var{dir}. This allows you to have
missing bodies for the units in @var{dir} and to ignore out of date bodies
for the same units. You still need to specify
the location of the specs for these units by using the qualifiers
@code{/SOURCE_SEARCH=@var{dir}}
or @code{/SEARCH=@var{dir}}.
Note: this qualifier is provided for compatibility with previous versions
of @code{GNAT MAKE}. The easier method of causing standard libraries
to be excluded from consideration is to write-protect the corresponding
ALI files.

@item /OBJECT_SEARCH=@var{dir}
@cindex @code{/OBJECT_SEARCH} (@code{GNAT MAKE})
When searching for library and object files, look in directory
@var{dir}. The order in which library files are searched is described in
@ref{Search Paths for GNAT BIND}.

@item /CONDITIONAL_SOURCE_SEARCH=@var{dir}
@cindex Search paths, for @code{GNAT MAKE}
@cindex @code{/CONDITIONAL_SOURCE_SEARCH} (@code{GNAT MAKE})
Equivalent to @code{/SKIP_MISSING=@var{dir}
/SOURCE_SEARCH=@var{dir}}.

@item /SEARCH=@var{dir}
@cindex @code{/SEARCH} (@code{GNAT MAKE})
Equivalent to @code{/OBJECT_SEARCH=@var{dir}
/SOURCE_SEARCH=@var{dir}}.

@item /NOCURRENT_DIRECTORY
@cindex @code{/NOCURRENT_DIRECTORY} (@code{GNAT MAKE})
@cindex Source files, suppressing search
Do not look for source files in the directory containing the source
file named in the command line.
Do not look for ALI or object files in the directory
where @code{GNAT MAKE} was invoked.

@item /LIBRARY_SEARCH=@var{dir}
@cindex @code{/LIBRARY_SEARCH} (@code{GNAT MAKE})
@cindex Linker libraries
Add directory @var{dir} to the list of directories in which the linker
will search for libraries. This is equivalent to
@code{/LINKER_QUALIFIERS /LIBRARY_SEARCH=}@var{dir}.

@item /NOSTD_INCLUDES
@cindex @code{/NOSTD_INCLUDES} (@code{GNAT MAKE})
Do not look for source files in the system default directory.

@item /NOSTD_LIBRARIES
@cindex @code{/NOSTD_LIBRARIES} (@code{GNAT MAKE})
Do not look for library files in the system default directory.

@item /RUNTIME_SYSTEM=@var{rts-path}
@cindex @code{/RUNTIME_SYSTEM} (@code{GNAT MAKE})
Specifies the default location of the runtime library. We look for the runtime
in the following directories, and stop as soon as a valid runtime is found
("adainclude" or "ada_source_path", and "adalib" or "ada_object_path" present):

@itemize @bullet
@item <current directory>/$rts_path

@item <default-search-dir>/$rts_path

@item <default-search-dir>/rts-$rts_path
@end itemize

@noindent
The selected path is handled like a normal RTS path.

@end table

@node Mode Qualifiers for GNAT MAKE
@section Mode Qualifiers for @code{GNAT MAKE}

@noindent
The mode qualifiers (referred to as @code{mode_qualifiers}) allow the
inclusion of qualifiers that are to be passed to the compiler itself, the
binder or the linker. The effect of a mode qualifier is to cause all
subsequent qualifiers up to the end of the qualifier list, or up to the next
mode qualifier, to be interpreted as qualifiers to be passed on to the
designated component of GNAT.

@table @code
@item /COMPILER_QUALIFIERS @var{qualifiers}
@cindex @code{/COMPILER_QUALIFIERS} (@code{GNAT MAKE})
Compiler qualifiers. Here @var{qualifiers} is a list of qualifiers
that are valid qualifiers for @code{GNAT COMPILE}. They will be passed on to
all compile steps performed by @code{GNAT MAKE}.

@item /BINDER_QUALIFIERS @var{qualifiers}
@cindex @code{/BINDER_QUALIFIERS} (@code{GNAT MAKE})
Binder qualifiers. Here @var{qualifiers} is a list of qualifiers
that are valid qualifiers for @code{GNAT COMPILE}. They will be passed on to
all bind steps performed by @code{GNAT MAKE}.

@item /LINKER_QUALIFIERS @var{qualifiers}
@cindex @code{/LINKER_QUALIFIERS} (@code{GNAT MAKE})
Linker qualifiers. Here @var{qualifiers} is a list of qualifiers
that are valid qualifiers for @code{GNAT COMPILE}. They will be passed on to
all link steps performed by @code{GNAT MAKE}.
@end table

@node Notes on the Command Line
@section Notes on the Command Line

@noindent
This section contains some additional useful notes on the operation
of the @code{GNAT MAKE} command.

@itemize @bullet
@item
@cindex Recompilation, by @code{GNAT MAKE}
If @code{GNAT MAKE} finds no ALI files, it recompiles the main program
and all other units required by the main program.
This means that @code{GNAT MAKE}
can be used for the initial compile, as well as during subsequent steps of
the development cycle.

@item
If you enter @code{GNAT MAKE @var{file}.ADB}, where @file{@var{file}.ADB}
is a subunit or body of a generic unit, @code{GNAT MAKE} recompiles
@file{@var{file}.ADB} (because it finds no ALI) and stops, issuing a
warning.

@item
In @code{GNAT MAKE} the qualifier @code{/SEARCH}
is used to specify both source and
library file paths. Use @code{/SOURCE_SEARCH}
instead if you just want to specify
source paths only and @code{/OBJECT_SEARCH}
if you want to specify library paths
only.

@item
@code{GNAT MAKE} examines both an ALI file and its corresponding object file
for consistency. If an ALI is more recent than its corresponding object,
or if the object file is missing, the corresponding source will be recompiled.
Note that @code{GNAT MAKE} expects an ALI and the corresponding object file
to be in the same directory.

@item
@code{GNAT MAKE} will ignore any files whose ALI file is write-protected.
This may conveniently be used to exclude standard libraries from
consideration and in particular it means that the use of the
@code{/FORCE_COMPILE} qualifier will not recompile these files
unless @code{/ALL_FILES} is also specified.

@item
@code{GNAT MAKE} has been designed to make the use of Ada libraries
particularly convenient. Assume you have an Ada library organized
as follows: @var{[OBJ_DIR]} contains the objects and ALI files for
of your Ada compilation units,
whereas @var{[INCLUDE_DIR]} contains the
specs of these units, but no bodies. Then to compile a unit
stored in @code{MAIN.ADB}, which uses this Ada library you would just type

@smallexample
$ GNAT MAKE /SOURCE_SEARCH=@var{[INCLUDE_DIR]}
           /SKIP_MISSING=@var{[OBJ_DIR]} main
@end smallexample

@item
Using @code{GNAT MAKE} along with the
@code{/MINIMAL_RECOMPILATION}
qualifier provides a mechanism for avoiding unnecessary rcompilations. Using
this qualifier,
you can update the comments/format of your
source files without having to recompile everything. Note, however, that
adding or deleting lines in a source files may render its debugging
info obsolete. If the file in question is a spec, the impact is rather
limited, as that debugging info will only be useful during the
elaboration phase of your program. For bodies the impact can be more
significant. In all events, your debugger will warn you if a source file
is more recent than the corresponding object, and alert you to the fact
that the debugging information may be out of date.
@end itemize

@node How GNAT MAKE Works
@section How @code{GNAT MAKE} Works

@noindent
Generally @code{GNAT MAKE} automatically performs all necessary
recompilations and you don't need to worry about how it works. However,
it may be useful to have some basic understanding of the @code{GNAT MAKE}
approach and in particular to understand how it uses the results of
previous compilations without incorrectly depending on them.

First a definition: an object file is considered @dfn{up to date} if the
corresponding ALI file exists and its time stamp predates that of the
object file and if all the source files listed in the
dependency section of this ALI file have time stamps matching those in
the ALI file. This means that neither the source file itself nor any
files that it depends on have been modified, and hence there is no need
to recompile this file.

@code{GNAT MAKE} works by first checking if the specified main unit is up
to date. If so, no compilations are required for the main unit. If not,
@code{GNAT MAKE} compiles the main program to build a new ALI file that
reflects the latest sources. Then the ALI file of the main unit is
examined to find all the source files on which the main program depends,
and @code{GNAT MAKE} recursively applies the above procedure on all these files.

This process ensures that @code{GNAT MAKE} only trusts the dependencies
in an existing ALI file if they are known to be correct. Otherwise it
always recompiles to determine a new, guaranteed accurate set of
dependencies. As a result the program is compiled "upside down" from what may
be more familiar as the required order of compilation in some other Ada
systems. In particular, clients are compiled before the units on which
they depend. The ability of GNAT to compile in any order is critical in
allowing an order of compilation to be chosen that guarantees that
@code{GNAT MAKE} will recompute a correct set of new dependencies if
necessary.

When invoking @code{GNAT MAKE} with several @var{file_names}, if a unit is
imported by several of the executables, it will be recompiled at most once.

@node Examples of GNAT MAKE Usage
@section Examples of @code{GNAT MAKE} Usage

@table @code
@item GNAT MAKE HELLO.ADB
Compile all files necessary to bind and link the main program
@file{HELLO.ADB} (containing unit @code{Hello}) and bind and link the
resulting object files to generate an executable file @file{HELLO.EXE}.

@item GNAT MAKE main1 main2 main3
Compile all files necessary to bind and link the main programs
@file{MAIN1.ADB} (containing unit @code{Main1}), @file{MAIN2.ADB}
(containing unit @code{Main2}) and @file{MAIN3.ADB}
(containing unit @code{Main3}) and bind and link the resulting object files
to generate three executable files @file{MAIN1.EXE},
@file{MAIN2.EXE}
and @file{MAIN3.EXE}.


@item GNAT MAKE Main_Unit /QUIET /COMPILER_QUALIFIERS /OPTIMIZE=ALL /BINDER_QUALIFIERS /ORDER_OF_ELABORATION
Compile all files necessary to bind and link the main program unit
@code{Main_Unit} (from file @file{MAIN_UNIT.ADB}). All compilations will
be done with optimization level 2 and the order of elaboration will be
listed by the binder. @code{GNAT MAKE} will operate in quiet mode, not
displaying commands it is executing.
@end table

@node Renaming Files Using GNAT CHOP
@chapter Renaming Files Using @code{GNAT CHOP}
@findex GNAT CHOP

@noindent
This chapter discusses how to handle files with multiple units by using
the @code{GNAT CHOP} utility. This utility is also useful in renaming
files to meet the standard GNAT default file naming conventions.

@menu
* Handling Files with Multiple Units::
* Operating GNAT CHOP in Compilation Mode::
* Command Line for GNAT CHOP::
* Qualifiers for GNAT CHOP::
* Examples of GNAT CHOP Usage::
@end menu

@node Handling Files with Multiple Units
@section Handling Files with Multiple Units

@noindent
The basic compilation model of GNAT requires that a file submitted to the
compiler have only one unit and there be a strict correspondence
between the file name and the unit name.

The @code{GNAT CHOP} utility allows both of these rules to be relaxed,
allowing GNAT to process files which contain multiple compilation units
and files with arbitrary file names. @code{GNAT CHOP}
reads the specified file and generates one or more output files,
containing one unit per file. The unit and the file name correspond,
as required by GNAT.

If you want to permanently restructure a set of "foreign" files so that
they match the GNAT rules, and do the remaining development using the
GNAT structure, you can simply use @code{GNAT CHOP} once, generate the
new set of files and work with them from that point on.

Alternatively, if you want to keep your files in the "foreign" format,
perhaps to maintain compatibility with some other Ada compilation
system, you can set up a procedure where you use @code{GNAT CHOP} each
time you compile, regarding the source files that it writes as temporary
files that you throw away.

@node Operating GNAT CHOP in Compilation Mode
@section Operating GNAT CHOP in Compilation Mode

@noindent
The basic function of @code{GNAT CHOP} is to take a file with multiple units
and split it into separate files. The boundary between files is reasonably
clear, except for the issue of comments and pragmas. In default mode, the
rule is that any pragmas between units belong to the previous unit, except
that configuration pragmas always belong to the following unit. Any comments
belong to the following unit. These rules
almost always result in the right choice of
the split point without needing to mark it explicitly and most users will
find this default to be what they want. In this default mode it is incorrect to
submit a file containing only configuration pragmas, or one that ends in
configuration pragmas, to @code{GNAT CHOP}.

However, using a special option to activate "compilation mode",
@code{GNAT CHOP}
can perform another function, which is to provide exactly the semantics
required by the RM for handling of configuration pragmas in a compilation.
In the absence of configuration pragmas (at the main file level), this
option has no effect, but it causes such configuration pragmas to be handled
in a quite different manner.

First, in compilation mode, if @code{GNAT CHOP} is given a file that consists of
only configuration pragmas, then this file is appended to the
@file{GNAT.ADC} file in the current directory. This behavior provides
the required behavior described in the RM for the actions to be taken
on submitting such a file to the compiler, namely that these pragmas
should apply to all subsequent compilations in the same compilation
environment. Using GNAT, the current directory, possibly containing a
@file{GNAT.ADC} file is the representation
of a compilation environment. For more information on the
@file{GNAT.ADC} file, see the section on handling of configuration
pragmas @pxref{Handling of Configuration Pragmas}.

Second, in compilation mode, if @code{GNAT CHOP}
is given a file that starts with
configuration pragmas, and contains one or more units, then these
configuration pragmas are prepended to each of the chopped files. This
behavior provides the required behavior described in the RM for the
actions to be taken on compiling such a file, namely that the pragmas
apply to all units in the compilation, but not to subsequently compiled
units.

Finally, if configuration pragmas appear between units, they are appended
to the previous unit. This results in the previous unit being illegal,
since the compiler does not accept configuration pragmas that follow
a unit. This provides the required RM behavior that forbids configuration
pragmas other than those preceding the first compilation unit of a
compilation.

For most purposes, @code{GNAT CHOP} will be used in default mode. The
compilation mode described above is used only if you need exactly
accurate behavior with respect to compilations, and you have files
that contain multiple units and configuration pragmas. In this
circumstance the use of @code{GNAT CHOP} with the compilation mode
qualifier provides the required behavior, and is for example the mode
in which GNAT processes the ACVC tests.

@node Command Line for GNAT CHOP
@section Command Line for @code{GNAT CHOP}

@noindent
The @code{GNAT CHOP} command has the form:

@smallexample
$ GNAT CHOP qualifiers @var{file name} [@var{file name} @var{file name} ...]
      [@var{directory}]
@end smallexample

@noindent
The only required argument is the file name of the file to be chopped.
There are no restrictions on the form of this file name. The file itself
contains one or more Ada units, in normal GNAT format, concatenated
together. As shown, more than one file may be presented to be chopped.

When run in default mode, @code{GNAT CHOP} generates one output file in
the current directory for each unit in each of the files.

@var{directory}, if specified, gives the name of the directory to which
the output files will be written. If it is not specified, all files are
written to the current directory.

For example, given a
file called @file{hellofiles} containing

@smallexample
@group
@cartouche
@b{procedure} hello;

@b{with} Text_IO; @b{use} Text_IO;
@b{procedure} hello @b{is}
@b{begin}
   Put_Line ("Hello");
@b{end} hello;
@end cartouche
@end group
@end smallexample

@noindent
the command

@smallexample
$ GNAT CHOP HELLOFILES.
@end smallexample

@noindent
generates two files in the current directory, one called
@file{HELLO.ADS} containing the single line that is the procedure spec,
and the other called @file{HELLO.ADB} containing the remaining text. The
original file is not affected. The generated files can be compiled in
the normal manner.

@node Qualifiers for GNAT CHOP
@section Qualifiers for @code{GNAT CHOP}

@noindent
@code{GNAT CHOP} recognizes the following qualifiers:

@table @code

@item /COMPILATION
@cindex @code{/COMPILATION} (@code{GNAT CHOP})
Causes @code{GNAT CHOP} to operate in compilation mode, in which
configuration pragmas are handled according to strict RM rules. See
previous section for a full description of this mode.


@item /HELP
Causes @code{GNAT CHOP} to generate a brief help summary to the standard
output file showing usage information.

@item /FILE_NAME_MAX_LENGTH=@var{mm}
@cindex @code{/FILE_NAME_MAX_LENGTH} (@code{GNAT CHOP})
Limit generated file names to the specified number @code{mm}
of characters.
This is useful if the
resulting set of files is required to be interoperable with systems
which limit the length of file names.
If no value is given, or
if no @code{/FILE_NAME_MAX_LENGTH} qualifier is given,
a default of 39, suitable for OpenVMS Alpha
Systems, is assumed

@item /PRESERVE
@cindex @code{/PRESERVE} (@code{GNAT CHOP})
Causes the file creation time stamp of the input file to be
preserved and used for the time stamp of the output file(s). This may be
useful for preserving coherency of time stamps in an enviroment where
@code{GNAT CHOP} is used as part of a standard build process.

@item /QUIET
@cindex @code{/QUIET} (@code{GNAT CHOP})
Causes output of informational messages indicating the set of generated
files to be suppressed. Warnings and error messages are unaffected.

@item /REFERENCE
@cindex @code{/REFERENCE} (@code{GNAT CHOP})
@findex Source_Reference
Generate @code{Source_Reference} pragmas. Use this qualifier if the output
files are regarded as temporary and development is to be done in terms
of the original unchopped file. This qualifier causes
@code{Source_Reference} pragmas to be inserted into each of the
generated files to refers back to the original file name and line number.
The result is that all error messages refer back to the original
unchopped file.
In addition, the debugging information placed into the object file (when
the @code{/DEBUG} qualifier of @code{GNAT COMPILE} or @code{GNAT MAKE} is specified) also
refers back to this original file so that tools like profilers and
debuggers will give information in terms of the original unchopped file.

If the original file to be chopped itself contains
a @code{Source_Reference}
pragma referencing a third file, then GNAT CHOP respects
this pragma, and the generated @code{Source_Reference} pragmas
in the chopped file refer to the original file, with appropriate
line numbers. This is particularly useful when @code{GNAT CHOP}
is used in conjunction with @code{GNAT PREPROCESS} to compile files that
contain preprocessing statements and multiple units.

@item /VERBOSE
@cindex @code{/VERBOSE} (@code{GNAT CHOP})
Causes @code{GNAT CHOP} to operate in verbose mode. The version
number and copyright notice are output, as well as exact copies of
the GNAT1 commands spawned to obtain the chop control information.

@item /OVERWRITE
@cindex @code{/OVERWRITE} (@code{GNAT CHOP})
Overwrite existing file names. Normally @code{GNAT CHOP} regards it as a
fatal error if there is already a file with the same name as a
file it would otherwise output, in other words if the files to be
chopped contain duplicated units. This qualifier bypasses this
check, and causes all but the last instance of such duplicated
units to be skipped.

@end table

@node Examples of GNAT CHOP Usage
@section Examples of @code{GNAT CHOP} Usage

@table @code
@item GNAT CHOP /OVERWRITE HELLO_S.ADA [ICHBIAH.FILES]

Chops the source file @file{HELLO_S.ADA}. The output files will be
placed in the directory @file{[ICHBIAH.FILES]},
overwriting any
files with matching names in that directory (no files in the current
directory are modified).

@item GNAT CHOP ARCHIVE.
Chops the source file @file{ARCHIVE.}
into the current directory. One
useful application of @code{GNAT CHOP} is in sending sets of sources
around, for example in email messages. The required sources are simply
concatenated (for example, using a VMS @code{APPEND/NEW}
command), and then
@code{GNAT CHOP} is used at the other end to reconstitute the original
file names.

@item GNAT CHOP file1 file2 file3 direc
Chops all units in files @file{file1}, @file{file2}, @file{file3}, placing
the resulting files in the directory @file{direc}. Note that if any units
occur more than once anywhere within this set of files, an error message
is generated, and no files are written. To override this check, use the
@code{/OVERWRITE} qualifier,
in which case the last occurrence in the last file will
be the one that is output, and earlier duplicate occurrences for a given
unit will be skipped.
@end table

@node Configuration Pragmas
@chapter Configuration Pragmas
@cindex Configuration pragmas
@cindex Pragmas, configuration

@noindent
In Ada 95, configuration pragmas include those pragmas described as
such in the Ada 95 Reference Manual, as well as
implementation-dependent pragmas that are configuration pragmas. See the
individual descriptions of pragmas in the GNAT Reference Manual for
details on these additional GNAT-specific configuration pragmas. Most
notably, the pragma @code{Source_File_Name}, which allows
specifying non-default names for source files, is a configuration
pragma. The following is a complete list of configuration pragmas
recognized by @code{GNAT}:

@smallexample
   Ada_83
   Ada_95
   C_Pass_By_Copy
   Component_Alignment
   Discard_Names
   Elaboration_Checks
   Eliminate
   Extend_System
   Extensions_Allowed
   External_Name_Casing
   Float_Representation
   Initialize_Scalars
   License
   Locking_Policy
   Long_Float
   No_Run_Time
   Normalize_Scalars
   Polling
   Propagate_Exceptions
   Queuing_Policy
   Ravenscar
   Restricted_Run_Time
   Restrictions
   Reviewable
   Source_File_Name
   Style_Checks
   Suppress
   Task_Dispatching_Policy
   Unsuppress
   Use_VADS_Size
   Warnings
   Validity_Checks
@end smallexample

@menu
* Handling of Configuration Pragmas::
* The Configuration Pragmas Files::
@end menu

@node Handling of Configuration Pragmas
@section Handling of Configuration Pragmas

Configuration pragmas may either appear at the start of a compilation
unit, in which case they apply only to that unit, or they may apply to
all compilations performed in a given compilation environment.

GNAT also provides the @code{GNAT CHOP} utility to provide an automatic
way to handle configuration pragmas following the semantics for
compilations (that is, files with multiple units), described in the RM.
See section @pxref{Operating GNAT CHOP in Compilation Mode} for details.
However, for most purposes, it will be more convenient to edit the
@file{GNAT.ADC} file that contains configuration pragmas directly,
as described in the following section.

@node The Configuration Pragmas Files
@section The Configuration Pragmas Files
@cindex @file{GNAT.ADC}

@noindent
In GNAT a compilation environment is defined by the current
directory at the time that a compile command is given. This current
directory is searched for a file whose name is @file{GNAT.ADC}. If
this file is present, it is expected to contain one or more
configuration pragmas that will be applied to the current compilation.
However, if the qualifier @option{-gnatA} is used, @file{GNAT.ADC} is not
considered.

Configuration pragmas may be entered into the @file{GNAT.ADC} file
either by running @code{GNAT CHOP} on a source file that consists only of
configuration pragmas, or more conveniently  by
direct editing of the @file{GNAT.ADC} file, which is a standard format
source file.

In addition to @file{GNAT.ADC}, one additional file containing configuration
pragmas may be applied to the current compilation using the qualifier
@option{-gnatec}@var{path}. @var{path} must designate an existing file that
contains only configuration pragmas. These configuration pragmas are
in addition to those found in @file{GNAT.ADC} (provided @file{GNAT.ADC}
is present and qualifier @option{-gnatA} is not used).

It is allowed to specify several qualifiers @option{-gnatec}, however only
the last one on the command line will be taken into account.

Of special interest to GNAT OpenVMS Alpha is the following configuration pragma:

@smallexample
@cartouche
@b{pragma} Extend_System (Aux_DEC);
@end cartouche
@end smallexample

@noindent
In the presence of this pragma, GNAT adds to the definition of the
predefined package SYSTEM all the additional types and subprograms that are
defined in DEC Ada. See @pxref{Compatibility with DEC Ada} for details.

@node Handling Arbitrary File Naming Conventions Using gnatname
@chapter Handling Arbitrary File Naming Conventions Using @code{gnatname}
@cindex Arbitrary File Naming Conventions

@menu
* Arbitrary File Naming Conventions::
* Running gnatname::
* Qualifiers for gnatname::
* Examples of gnatname Usage::
@end menu

@node Arbitrary File Naming Conventions
@section Arbitrary File Naming Conventions

@noindent
The GNAT compiler must be able to know the source file name of a compilation unit.
When using the standard GNAT default file naming conventions (@code{.ADS} for specs,
@code{.ADB} for bodies), the GNAT compiler does not need additional information.

@noindent
When the source file names do not follow the standard GNAT default file naming
conventions, the GNAT compiler must be given additional information through
a configuration pragmas file (see @ref{Configuration Pragmas}) or a project file.
When the non standard file naming conventions are well-defined, a small number of
pragmas @code{Source_File_Name} specifying a naming pattern
(see @ref{Alternative File Naming Schemes}) may be sufficient. However,
if the file naming conventions are irregular or arbitrary, a number
of pragma @code{Source_File_Name} for individual compilation units must be defined.
To help maintain the correspondence between compilation unit names and
source file names within the compiler,
GNAT provides a tool @code{gnatname} to generate the required pragmas for a
set of files.

@node Running gnatname
@section Running @code{gnatname}

@noindent
The usual form of the @code{gnatname} command is

@smallexample
$ gnatname [@var{qualifiers}] @var{naming_pattern} [@var{naming_patterns}]
@end smallexample

@noindent
All of the arguments are optional. If invoked without any argument,
@code{gnatname} will display its usage.

@noindent
When used with at least one naming pattern, @code{gnatname} will attempt to
find all the compilation units in files that follow at least one of the
naming patterns. To find these compilation units,
@code{gnatname} will use the GNAT compiler in syntax-check-only mode on all
regular files.

@noindent
One or several Naming Patterns may be given as arguments to @code{gnatname}.
Each Naming Pattern is enclosed between double quotes.
A Naming Pattern is a regular expression similar to the wildcard patterns
used in file names by the Unix shells or the DOS prompt.

@noindent
Examples of Naming Patterns are

@smallexample
   "*.[12].ADA"
   "*.ad[sb]*"
   "body_*"    "spec_*"
@end smallexample

@noindent
For a more complete description of the syntax of Naming Patterns, see the second kind
of regular expressions described in @file{G-REGEXP.ADS} (the "Glob" regular
expressions).

@noindent
When invoked with no qualifiers, @code{gnatname} will create a configuration
pragmas file @file{GNAT.ADC} in the current working directory, with pragmas
@code{Source_File_Name} for each file that contains a valid Ada unit.

@node Qualifiers for gnatname
@section Qualifiers for @code{gnatname}

@noindent
Qualifiers for @code{gnatname} must precede any specified Naming Pattern.

@noindent
You may specify any of the following qualifiers to @code{gnatname}:

@table @code

@item -c@file{file}
@cindex @code{-c} (@code{gnatname})
Create a configuration pragmas file @file{file} (instead of the default
@file{GNAT.ADC}). There may be zero, one or more space between @code{-c} and
@file{file}. @file{file} may include directory information. @file{file} must be
writeable. There may be only one qualifier @code{-c}. When a qualifier @code{-c} is
specified, no qualifier @code{-P} may be specified (see below).

@item -d@file{dir}
@cindex @code{-d} (@code{gnatname})
Look for source files in directory @file{dir}. There may be zero, one or more spaces
between @code{-d} and @file{dir}. When a qualifier @code{-d} is specified,
the current working directory will not be searched for source files, unless it
is explictly
specified with a @code{-d} or @code{-D} qualifier. Several qualifiers @code{-d} may be
specified. If @file{dir} is a relative path, it is relative to the directory of
the configuration pragmas file specified with qualifier @code{-c}, or to the directory
of the project file specified with qualifier @code{-P} or, if neither qualifier @code{-c}
nor qualifier @code{-P} are specified, it is relative to the current working
directory. The directory
specified with qualifier @code{-c} must exist and be readable.

@item -D@file{file}
@cindex @code{-D} (@code{gnatname})
Look for source files in all directories listed in text file @file{file}. There may be
zero, one or more spaces between @code{-d} and @file{dir}. @file{file}
must be an existing, readable text file. Each non empty line in @file{file} must be
a directory. Specifying qualifier @code{-D} is equivalent to specifying as many qualifiers
@code{-d} as there are non empty lines in @file{file}.

@item -h
@cindex @code{-h} (@code{gnatname})
Output usage (help) information. The output is written to @file{SYS$OUTPUT}.

@item -P@file{proj}
@cindex @code{-P} (@code{gnatname})
Create or update project file @file{proj}. There may be zero, one or more space
between @code{-P} and @file{proj}. @file{proj} may include directory information.
@file{proj} must be writeable. There may be only one qualifier @code{-P}.
When a qualifier @code{-P} is specified, no qualifier @code{-c} may be specified.

@item -v
@cindex @code{-v} (@code{gnatname})
Verbose mode. Output detailed explanation of behavior to @file{SYS$OUTPUT}. This includes
name of the file written, the name of the directories to search and, for each file
in those directories whose name matches at least one of the Naming Patterns, an
indication of whether the file contains a unit, and if so the name of the unit.

@item -v -v
Very Verbose mode. In addition to the output produced in verbose mode, for each file
in the searched directories whose name matches none of the Naming Patterns, an
indication is given that there is no match.

@item -x@file{pattern}
Excluded patterns. Using this qualifier, it is possible to exclude some files
that would match the name patterns. For example,
@code{"gnatname -x "*_NT.ADA" "*.ADA"} will look for Ada units in all files
with the @file{.ADA} extension, except those whose names end with
@file{_NT.ADA}.

@end table

@node Examples of gnatname Usage
@section Examples of @code{gnatname} Usage

@smallexample
$ gnatname -c /home/me/NAMES.ADC -d sources "[a-z]*.ADA*"
@end smallexample

In this example, the directory @file{/home/me} must already exist and be
writeable. In addition, the directory @file{/home/me/sources} (specified by
@code{-d sources}) must exist and be readable. Note the optional spaces after
@code{-c} and @code{-d}.

@smallexample
$ gnatname -P/home/me/proj -x "*_NT_BODY.ADA" -dsources -dsources/plus -Dcommon_dirs.txt "body_*" "spec_*"
@end smallexample

Note that several qualifiers @code{-d} may be used, even in conjunction with one
or several qualifiers @code{-D}. Several Naming Patterns and one excluded pattern
are used in this example.


@c *****************************************
@c * G N A T  P r o j e c t  M a n a g e r *
@c *****************************************
@node GNAT Project Manager
@chapter GNAT Project Manager

@menu
* Introduction::
* Examples of Project Files::
* Project File Syntax::
* Objects and Sources in Project Files::
* Importing Projects::
* Project Extension::
* External References in Project Files::
* Packages in Project Files::
* Variables from Imported Projects::
* Naming Schemes::
* Library Projects::
* Qualifiers Related to Project Files::
* Tools Supporting Project Files::
* An Extended Example::
* Project File Complete Syntax::
@end menu


@c ****************
@c * Introduction *
@c ****************

@node Introduction
@section Introduction

@noindent
This chapter describes GNAT's @emph{Project Manager}, a facility that
lets you configure various properties for a collection of source files.  In
particular, you can specify:
@itemize @bullet
@item
The directory or set of directories containing the source files, and/or the
names of the specific source files themselves
@item
The directory in which the compiler's output
(@file{ALI} files, object files, tree files) will be placed
@item
The directory in which the executable programs will be placed
@item
Qualifier settings for any of the project-enabled tools (@command{GNAT MAKE},
compiler, binder, linker, @code{GNAT LIST}, @code{GNAT XREF}, @code{GNAT FIND});
you can apply these settings either globally or to individual units
@item
The source files containing the main subprogram(s) to be built
@item
The source programming language(s) (currently Ada and/or C)
@item
Source file naming conventions; you can specify these either globally or for
individual units
@end itemize

@menu
* Project Files::
@end menu

@node Project Files
@subsection Project Files

@noindent
A @dfn{project} is a specific set of values for these properties.  You can
define a project's settings in a @dfn{project file}, a text file with an
Ada-like syntax; a property value is either a string or a list of strings.
Properties that are not explicitly set receive default values.  A project
file may interrogate the values of @dfn{external variables} (user-defined
command-line qualifiers or environment variables), and it may specify property
settings conditionally, based on the value of such variables.

In simple cases, a project's source files depend only on other source files
in the same project, or on the predefined libraries.  ("Dependence" is in
the technical sense; for example, one Ada unit "with"ing another.)  However,
the Project Manager also allows much more sophisticated arrangements,
with the source files in one project depending on source files in other
projects:
@itemize @bullet
@item
One project can @emph{import} other projects containing needed source files.
@item
You can organize GNAT projects in a hierarchy: a @emph{child} project
can extend a @emph{parent} project, inheriting the parent's source files and
optionally overriding any of them with alternative versions
@end itemize

@noindent
More generally, the Project Manager lets you structure large development
efforts into hierarchical subsystems, with build decisions deferred to the
subsystem level and thus different compilation environments (qualifier settings)
used for different subsystems.

The Project Manager is invoked through the @option{-P@emph{projectfile}}
qualifier to @command{GNAT MAKE} or to the @command{gnat} front driver.
If you want to define (on the command line) an external variable that is
queried by the project file, additionally use the
@option{-X@emph{vbl}=@emph{value}} qualifier.
The Project Manager parses and interprets the project file, and drives the
invoked tool based on the project settings.

The Project Manager supports a wide range of development strategies,
for systems of all sizes.  Some typical practices that are easily handled:
@itemize @bullet
@item
Using a common set of source files, but generating object files in different
directories via different qualifier settings
@item
Using a mostly-shared set of source files, but with different versions of
some unit or units
@end itemize

@noindent
The destination of an executable can be controlled inside a project file
using the @option{-o} qualifier. In the absence of such a qualifier either inside
the project file or on the command line, any executable files generated by
@command{GNAT MAKE} will be placed in the directory @code{Exec_Dir} specified
in the project file. If no @code{Exec_Dir} is specified, they will be placed
in the object directory of the project.

You can use project files to achieve some of the effects of a source
versioning system (for example, defining separate projects for
the different sets of sources that comprise different releases) but the
Project Manager is independent of any source configuration management tools
that might be used by the developers.

The next section introduces the main features of GNAT's project facility
through a sequence of examples; subsequent sections will present the syntax
and semantics in more detail.


@c *****************************
@c * Examples of Project Files *
@c *****************************

@node Examples of Project Files
@section Examples of Project Files
@noindent
This section illustrates some of the typical uses of project files and
explains their basic structure and behavior.

@menu
* Common Sources with Different Qualifiers and Different Output Directories::
* Using External Variables::
* Importing Other Projects::
* Extending a Project::
@end menu

@node Common Sources with Different Qualifiers and Different Output Directories
@subsection Common Sources with Different Qualifiers and Different Output Directories

@menu
* Source Files::
* Specifying the Object Directory::
* Specifying the Exec Directory::
* Project File Packages::
* Specifying Qualifier Settings::
* Main Subprograms::
* Source File Naming Conventions::
* Source Language(s)::
@end menu

@noindent
Assume that the Ada source files @file{PACK.ADS}, @file{PACK.ADB}, and
@file{PROC.ADB} are in the @file{/common} directory.  The file
@file{PROC.ADB} contains an Ada main subprogram @code{Proc} that "with"s
package @code{Pack}.  We want to compile these source files under two sets
of qualifiers:
@itemize @bullet
@item
When debugging, we want to pass the @option{-g} qualifier to @command{GNAT MAKE},
and the @option{/CHECKS=ASSERTIONS}, @option{/CHECKS=OVERFLOW}, and @option{/CHECKS=ELABORATION} qualifiers to the
compiler; the compiler's output is to appear in @file{/common/debug}
@item
When preparing a release version, we want to pass the @option{/OPTIMIZE=ALL} qualifier to
the compiler; the compiler's output is to appear in @file{/common/release}
@end itemize

@noindent
The GNAT project files shown below, respectively @file{debug.gpr} and
@file{release.gpr} in the @file{/common} directory, achieve these effects.

Diagrammatically:
@smallexample
@group
/common
  debug.gpr
  release.gpr
  PACK.ADS
  PACK.ADB
  PROC.ADB
@end group
@group
/common/debug @{-g, /CHECKS=ASSERTIONS, /CHECKS=OVERFLOW, /CHECKS=ELABORATION@}
  PROC.ALI, PROC.OBJ
  PACK.ALI, PACK.OBJ
@end group
@group
/common/release @{/OPTIMIZE=ALL@}
  PROC.ALI, PROC.OBJ
  PACK.ALI, PACK.OBJ
@end group
@end smallexample
Here are the project files:
@smallexample
@group
project Debug is
  for Object_Dir use "debug";
  for Main use ("proc");

  package Builder is
    for Default_Qualifiers ("Ada") use ("-g");
  end Builder;
@end group

@group
  package Compiler is
    for Default_Qualifiers ("Ada")
       use ("-fstack-check", "/CHECKS=ASSERTIONS", "/CHECKS=OVERFLOW", "/CHECKS=ELABORATION");
  end Compiler;
end Debug;
@end group
@end smallexample

@smallexample
@group
project Release is
  for Object_Dir use "release";
  for Exec_Dir use ".";
  for Main use ("proc");

  package Compiler is
    for Default_Qualifiers ("Ada") use ("/OPTIMIZE=ALL");
  end Compiler;
end Release;
@end group
@end smallexample

@noindent
The name of the project defined by @file{debug.gpr} is @code{"Debug"} (case
insensitive), and analogously the project defined by @file{release.gpr} is
@code{"Release"}.  For consistency the file should have the same name as the
project, and the project file's extension should be @code{"gpr"}. These
conventions are not required, but a warning is issued if they are not followed.

If the current directory is @file{/temp}, then the command
@smallexample
GNAT MAKE -P/common/debug.gpr
@end smallexample

@noindent
generates object and ALI files in @file{/common/debug}, and the @code{proc}
executable also in @file{/common/debug}, using the qualifier settings defined in
the project file.

Likewise, the command
@smallexample
GNAT MAKE -P/common/release.gpr
@end smallexample

@noindent
generates object and ALI files in @file{/common/release}, and the @code{proc}
executable in @file{/common}, using the qualifier settings from the project file.

@node Source Files
@unnumberedsubsubsec Source Files

@noindent
If a project file does not explicitly specify a set of source directories or
a set of source files, then by default the project's source files are the
Ada source files in the project file directory.  Thus @file{PACK.ADS},
@file{PACK.ADB}, and @file{PROC.ADB} are the source files for both projects.

@node Specifying the Object Directory
@unnumberedsubsubsec Specifying the Object Directory

@noindent
Several project properties are modeled by Ada-style @emph{attributes};
you define the property by supplying the equivalent of an Ada attribute
definition clause in the project file.
A project's object directory is such a property; the corresponding
attribute is @code{Object_Dir}, and its value is a string expression.  A
directory may be specified either as absolute or as relative; in the latter
case, it is relative to the project file directory.  Thus the compiler's
output is directed to @file{/common/debug} (for the @code{Debug} project)
and to @file{/common/release} (for the @code{Release} project).  If
@code{Object_Dir} is not specified, then the default is the project file
directory.

@node Specifying the Exec Directory
@unnumberedsubsubsec Specifying the Exec Directory

@noindent
A project's exec directory is another property; the corresponding
attribute is @code{Exec_Dir}, and its value is also a string expression,
either specified as relative or absolute. If @code{Exec_Dir} is not specified,
then the default is the object directory (which may also be the project file
directory if attribute @code{Object_Dir} is not specified). Thus the executable
is placed in @file{/common/debug} for the @code{Debug} project (attribute
@code{Exec_Dir} not specified) and in @file{/common} for the @code{Release}
project.

@node Project File Packages
@unnumberedsubsubsec Project File Packages

@noindent
A GNAT tool integrated with the Project Manager is modeled by a
corresponding package in the project file.
The @code{Debug} project defines the packages @code{Builder}
(for @command{GNAT MAKE}) and @code{Compiler};
the @code{Release} project defines only the @code{Compiler} package.

The Ada package syntax is not to be taken literally.  Although packages in
project files bear a surface resemblance to packages in Ada source code, the
notation is simply a way to convey a grouping of properties for a named
entity.  Indeed, the package names permitted in project files are restricted
to a predefined set, corresponding to the project-aware tools, and the contents
of packages are limited to a small set of constructs.
The packages in the example above contain attribute definitions.


@node Specifying Qualifier Settings
@unnumberedsubsubsec Specifying Qualifier Settings

@noindent
Qualifier settings for a project-aware tool can be specified through attributes
in the package corresponding to the tool.
The example above illustrates one of the relevant attributes,
@code{Default_Qualifiers}, defined in the packages in both project files.
Unlike simple attributes like @code{Source_Dirs}, @code{Default_Qualifiers} is
known as an @emph{associative array}.  When you define this attribute, you must
supply an "index" (a literal string), and the effect of the attribute
definition is to set the value of the "array" at the specified "index".
For the @code{Default_Qualifiers} attribute, the index is a programming
language (in our case, Ada) , and the value specified (after @code{use})
must be a list of string expressions.

The attributes permitted in project files are restricted to a predefined set.
Some may appear at project level, others in packages.
For any attribute that is an associate array, the index must always be a
literal string, but the restrictions on this string (e.g., a file name or a
language name) depend on the individual attribute.
Also depending on the attribute, its specified value will need to be either a
string or a string list.

In the @code{Debug} project, we set the qualifiers for two tools,
@command{GNAT MAKE} and the compiler, and thus we include corresponding
packages, with each package defining the @code{Default_Qualifiers} attribute
with index @code{"Ada"}.
Note that the package corresponding to
@command{GNAT MAKE} is named @code{Builder}.  The @code{Release} project is
similar, but with just the @code{Compiler} package.

In project @code{Debug} above the qualifiers starting with @option{-gnat} that
are specified in package @code{Compiler} could have been placed in package
@code{Builder}, since @command{GNAT MAKE} transmits all such qualifiers to the
compiler.

@node Main Subprograms
@unnumberedsubsubsec Main Subprograms

@noindent
One of the properties of a project is its list of main subprograms (actually
a list of names of source files containing main subprograms, with the file
extension optional.  This property is captured in the @code{Main} attribute,
whose value is a list of strings.  If a project defines the @code{Main}
attribute, then you do not need to identify the main subprogram(s) when
invoking @command{GNAT MAKE} (see @ref{GNAT MAKE and Project Files}).

@node Source File Naming Conventions
@unnumberedsubsubsec Source File Naming Conventions

@noindent
Since the project files do not specify any source file naming conventions,
the GNAT defaults are used.  The mechanism for defining source file naming
conventions -- a package named @code{Naming} -- will be described below
(@pxref{Naming Schemes}).

@node Source Language(s)
@unnumberedsubsubsec Source Language(s)

@noindent
Since the project files do not specify a @code{Languages} attribute, by
default the GNAT tools assume that the language of the project file is Ada.
More generally, a project can comprise source files
in Ada, C, and/or other languages.

@node Using External Variables
@subsection Using External Variables

@noindent
Instead of supplying different project files for debug and release, we can
define a single project file that queries an external variable (set either
on the command line or via an environment variable) in order to
conditionally define the appropriate settings.  Again, assume that the
source files @file{PACK.ADS}, @file{PACK.ADB}, and @file{PROC.ADB} are
located in directory @file{/common}.  The following project file,
@file{build.gpr}, queries the external variable named @code{STYLE} and
defines an object directory and qualifier settings based on whether the value
is @code{"deb"} (debug) or @code{"rel"} (release), where the default is
@code{"deb"}.

@smallexample
@group
project Build is
  for Main use ("proc");

  type Style_Type is ("deb", "rel");
  Style : Style_Type := external ("STYLE", "deb");

  case Style is
    when "deb" =>
      for Object_Dir use "debug";

    when "rel" =>
      for Object_Dir use "release";
      for Exec_Dir use ".";
  end case;
@end group

@group
  package Builder is

    case Style is
      when "deb" =>
        for Default_Qualifiers ("Ada") use ("-g");
    end case;

  end Builder;
@end group

@group
  package Compiler is

    case Style is
      when "deb" =>
        for Default_Qualifiers ("Ada") use ("/CHECKS=ASSERTIONS", "/CHECKS=OVERFLOW", "/CHECKS=ELABORATION");

      when "rel" =>
        for Default_Qualifiers ("Ada") use ("/OPTIMIZE=ALL");
    end case;

  end Compiler;

end Build;
@end group
@end smallexample

@noindent
@code{Style_Type} is an example of a @emph{string type}, which is the project
file analog of an Ada enumeration type but containing string literals rather
than identifiers.  @code{Style} is declared as a variable of this type.

The form @code{external("STYLE", "deb")} is known as an
@emph{external reference}; its first argument is the name of an
@emph{external variable}, and the second argument is a default value to be
used if the external variable doesn't exist.  You can define an external
variable on the command line via the @option{-X} qualifier, or you can use an
environment variable as an external variable.

Each @code{case} construct is expanded by the Project Manager based on the
value of @code{Style}. Thus the command
@smallexample
GNAT MAKE -P/common/build.gpr -XSTYLE=deb
@end smallexample

@noindent
is equivalent to the @command{GNAT MAKE} invocation using the project file
@file{debug.gpr} in the earlier example.  So is the command
@smallexample
GNAT MAKE -P/common/build.gpr
@end smallexample

@noindent
since @code{"deb"} is the default for @code{STYLE}.

Analogously,
@smallexample
GNAT MAKE -P/common/build.gpr -XSTYLE=rel
@end smallexample

@noindent
is equivalent to the @command{GNAT MAKE} invocation using the project file
@file{release.gpr} in the earlier example.


@node Importing Other Projects
@subsection Importing Other Projects

@noindent
A compilation unit in a source file in one project may depend on compilation
units in source files in other projects.  To obtain this behavior, the
dependent project must @emph{import} the projects containing the needed source
files.  This effect is embodied in syntax similar to an Ada @code{with} clause,
but the "with"ed entities are strings denoting project files.

As an example, suppose that the two projects @code{GUI_Proj} and
@code{Comm_Proj} are defined in the project files @file{gui_proj.gpr} and
@file{comm_proj.gpr} in directories @file{/gui} and @file{/comm},
respectively.  Assume that the source files for @code{GUI_Proj} are
@file{GUI.ADS} and @file{GUI.ADB}, and that the source files for
@code{Comm_Proj} are @file{COMM.ADS} and @file{COMM.ADB}, with each set of
files located in its respective project file directory.  Diagrammatically:

@smallexample
@group
/gui
  gui_proj.gpr
  GUI.ADS
  GUI.ADB
@end group

@group
/comm
  comm_proj.gpr
  COMM.ADS
  COMM.ADB
@end group
@end smallexample

@noindent
We want to develop an application in directory @file{/app} that "with"s the
packages @code{GUI} and @code{Comm}, using the properties of the
corresponding project files (e.g. the qualifier settings and object directory).
Skeletal code for a main procedure might be something like the following:

@smallexample
@group
with GUI, Comm;
procedure App_Main is
   ...
begin
   ...
end App_Main;
@end group
@end smallexample

@noindent
Here is a project file, @file{app_proj.gpr}, that achieves the desired
effect:

@smallexample
@group
with "/gui/gui_proj", "/comm/comm_proj";
project App_Proj is
   for Main use ("app_main");
end App_Proj;
@end group
@end smallexample

@noindent
Building an executable is achieved through the command:
@smallexample
GNAT MAKE -P/app/app_proj
@end smallexample
@noindent
which will generate the @code{app_main} executable in the directory where
@file{app_proj.gpr} resides.

If an imported project file uses the standard extension (@code{gpr}) then
(as illustrated above) the @code{with} clause can omit the extension.

Our example specified an absolute path for each imported project file.
Alternatively, you can omit the directory if either
@itemize @bullet
@item
The imported project file is in the same directory as the importing project
file, or
@item
You have defined an environment variable @code{ADA_PROJECT_PATH} that
includes the directory containing the needed project file.
@end itemize

@noindent
Thus, if we define @code{ADA_PROJECT_PATH} to include @file{/gui} and
@file{/comm}, then our project file @file{app_proj.gpr} could be written as
follows:

@smallexample
@group
with "gui_proj", "comm_proj";
project App_Proj is
   for Main use ("app_main");
end App_Proj;
@end group
@end smallexample

@noindent
Importing other projects raises the possibility of ambiguities.  For
example, the same unit might be present in different imported projects, or
it might be present in both the importing project and an imported project.
Both of these conditions are errors.  Note that in the current version of
the Project Manager, it is illegal to have an ambiguous unit even if the
unit is never referenced by the importing project.  This restriction may be
relaxed in a future release.

@node Extending a Project
@subsection Extending a Project

@noindent
A common situation in large software systems is to have multiple
implementations for a common interface; in Ada terms, multiple versions of a
package body for the same specification.  For example, one implementation
might be safe for use in tasking programs, while another might only be used
in sequential applications.  This can be modeled in GNAT using the concept
of @emph{project extension}.  If one project (the "child") @emph{extends}
another project (the "parent") then by default all source files of the
parent project are inherited by the child, but the child project can
override any of the parent's source files with new versions, and can also
add new files.  This facility is the project analog of extension in
Object-Oriented Programming.  Project hierarchies are permitted (a child
project may be the parent of yet another project), and a project that
inherits one project can also import other projects.

As an example, suppose that directory @file{/seq} contains the project file
@file{seq_proj.gpr} and the source files @file{PACK.ADS}, @file{PACK.ADB},
and @file{PROC.ADB}:

@smallexample
@group
/seq
  PACK.ADS
  PACK.ADB
  PROC.ADB
  seq_proj.gpr
@end group
@end smallexample

@noindent
Note that the project file can simply be empty (that is, no attribute or
package is defined):

@smallexample
@group
project Seq_Proj is
end Seq_Proj;
@end group
@end smallexample

@noindent
implying that its source files are all the Ada source files in the project
directory.

Suppose we want to supply an alternate version of @file{PACK.ADB}, in
directory @file{/tasking}, but use the existing versions of @file{PACK.ADS}
and @file{PROC.ADB}.  We can define a project @code{Tasking_Proj} that
inherits @code{Seq_Proj}:

@smallexample
@group
/tasking
  PACK.ADB
  tasking_proj.gpr
@end group

@group
project Tasking_Proj extends "/seq/seq_proj" is
end Tasking_Proj;
@end group
@end smallexample

@noindent
The version of @file{PACK.ADB} used in a build depends on which project file
is specified.

Note that we could have designed this using project import rather than
project inheritance; a @code{base} project would contain the sources for
@file{PACK.ADS} and @file{PROC.ADB}, a sequential project would import
@code{base} and add @file{PACK.ADB}, and likewise a tasking project would
import @code{base} and add a different version of @file{PACK.ADB}.  The
choice depends on whether other sources in the original project need to be
overridden.  If they do, then project extension is necessary, otherwise,
importing is sufficient.


@c ***********************
@c * Project File Syntax *
@c ***********************

@node Project File Syntax
@section Project File Syntax

@menu
* Basic Syntax::
* Packages::
* Expressions::
* String Types::
* Variables::
* Attributes::
* Associative Array Attributes::
* case Constructions::
@end menu

@noindent
This section describes the structure of project files.

A project may be an @emph{independent project}, entirely defined by a single
project file. Any Ada source file in an independent project depends only
on the predefined library and other Ada source files in the same project.

@noindent
A project may also @dfn{depend on} other projects, in either or both of the following ways:
@itemize @bullet
@item It may import any number of projects
@item It may extend at most one other project
@end itemize

@noindent
The dependence relation is a directed acyclic graph (the subgraph reflecting
the "extends" relation is a tree).

A project's @dfn{immediate sources} are the source files directly defined by
that project, either implicitly by residing in the project file's directory,
or explicitly through any of the source-related attributes described below.
More generally, a project @var{proj}'s @dfn{sources} are the immediate sources
of @var{proj} together with the immediate sources (unless overridden) of any
project on which @var{proj} depends (either directly or indirectly).

@node Basic Syntax
@subsection Basic Syntax

@noindent
As seen in the earlier examples, project files have an Ada-like syntax.
The minimal project file is:
@smallexample
@group
project Empty is

end Empty;
@end group
@end smallexample

@noindent
The identifier @code{Empty} is the name of the project.
This project name must be present after the reserved
word @code{end} at the end of the project file, followed by a semi-colon.

Any name in a project file, such as the project name or a variable name,
has the same syntax as an Ada identifier.

The reserved words of project files are the Ada reserved words plus
@code{extends}, @code{external}, and @code{project}.  Note that the only Ada
reserved words currently used in project file syntax are:

@itemize @bullet
@item
@code{case}
@item
@code{end}
@item
@code{for}
@item
@code{is}
@item
@code{others}
@item
@code{package}
@item
@code{renames}
@item
@code{type}
@item
@code{use}
@item
@code{when}
@item
@code{with}
@end itemize

@noindent
Comments in project files have the same syntax as in Ada, two consecutives
hyphens through the end of the line.

@node Packages
@subsection Packages

@noindent
A project file may contain @emph{packages}. The name of a package must be one
of the identifiers (case insensitive) from a predefined list, and a package
with a given name may only appear once in a project file. The predefined list
includes the following packages:

@itemize @bullet
@item
@code{Naming}
@item
@code{Builder}
@item
@code{Compiler}
@item
@code{Binder}
@item
@code{Linker}
@item
@code{Finder}
@item
@code{Cross_Reference}
@item
@code{GNAT LIST}
@end itemize

@noindent
(The complete list of the package names and their attributes can be found
in file @file{PRJ-ATTR.ADB}).

@noindent
In its simplest form, a package may be empty:

@smallexample
@group
project Simple is
  package Builder is
  end Builder;
end Simple;
@end group
@end smallexample

@noindent
A package may contain @emph{attribute declarations},
@emph{variable declarations} and @emph{case constructions}, as will be
described below.

When there is ambiguity between a project name and a package name,
the name always designates the project. To avoid possible confusion, it is
always a good idea to avoid naming a project with one of the
names allowed for packages or any name that starts with @code{gnat}.


@node Expressions
@subsection Expressions

@noindent
An @emph{expression} is either a @emph{string expression} or a
@emph{string list expression}.

A @emph{string expression} is either a @emph{simple string expression} or a
@emph{compound string expression}.

A @emph{simple string expression} is one of the following:
@itemize @bullet
@item A literal string; e.g.@code{"comm/my_proj.gpr"}
@item A string-valued variable reference (see @ref{Variables})
@item A string-valued attribute reference (see @ref{Attributes})
@item An external reference (see @ref{External References in Project Files})
@end itemize

@noindent
A @emph{compound string expression} is a concatenation of string expressions,
using @code{"&"}
@smallexample
       Path & "/" & File_Name & ".ADS"
@end smallexample

@noindent
A @emph{string list expression} is either a
@emph{simple string list expression} or a
@emph{compound string list expression}.

A @emph{simple string list expression} is one of the following:
@itemize @bullet
@item A parenthesized list of zero or more string expressions, separated by commas
@smallexample
   File_Names := (File_Name, "GNAT.ADC", File_Name & ".orig");
   Empty_List := ();
@end smallexample
@item A string list-valued variable reference
@item A string list-valued attribute reference
@end itemize

@noindent
A @emph{compound string list expression} is the concatenation (using
@code{"&"}) of a simple string list expression and an expression.  Note that
each term in a compound string list expression, except the first, may be
either a string expression or a string list expression.

@smallexample
@group
   File_Name_List := () & File_Name; --  One string in this list
   Extended_File_Name_List := File_Name_List & (File_Name & ".orig");
   --  Two strings
   Big_List := File_Name_List & Extended_File_Name_List;
   --  Concatenation of two string lists: three strings
   Illegal_List := "GNAT.ADC" & Extended_File_Name_List;
   --  Illegal: must start with a string list
@end group
@end smallexample


@node String Types
@subsection String Types

@noindent
The value of a variable may be restricted to a list of string literals.
The restricted list of string literals is given in a
@emph{string type declaration}.

Here is an example of a string type declaration:

@smallexample
   type OS is ("NT, "nt", "Unix", "Linux", "other OS");
@end smallexample

@noindent
Variables of a string type are called @emph{typed variables}; all other
variables are called @emph{untyped variables}. Typed variables are
particularly useful in @code{case} constructions
(see @ref{case Constructions}).

A string type declaration starts with the reserved word @code{type}, followed
by the name of the string type (case-insensitive), followed by the reserved
word @code{is}, followed by a parenthesized list of one or more string literals
separated by commas, followed by a semicolon.

The string literals in the list are case sensitive and must all be different.
They may include any graphic characters allowed in Ada, including spaces.

A string type may only be declared at the project level, not inside a package.

A string type may be referenced by its name if it has been declared in the same
project file, or by its project name, followed by a dot,
followed by the string type name.


@node Variables
@subsection Variables

@noindent
A variable may be declared at the project file level, or in a package.
Here are some examples of variable declarations:

@smallexample
@group
   This_OS : OS := external ("OS"); --  a typed variable declaration
   That_OS := "Linux";              --  an untyped variable declaration
@end group
@end smallexample

@noindent
A @emph{typed variable declaration} includes the variable name, followed by a colon,
followed by the name of a string type, followed by @code{:=}, followed by
a simple string expression.

An @emph{untyped variable declaration} includes the variable name,
followed by @code{:=}, followed by an expression.  Note that, despite the
terminology, this form of "declaration" resembles more an assignment
than a declaration in Ada.  It is a declaration in several senses:
@itemize @bullet
@item
The variable name does not need to be defined previously
@item
The declaration establishes the @emph{kind} (string versus string list) of the
variable, and later declarations of the same variable need to be consistent
with this
@end itemize

@noindent
A string variable declaration (typed or untyped) declares a variable
whose value is a string. This variable may be used as a string expression.
@smallexample
   File_Name       := "readme.txt";
   Saved_File_Name := File_Name & ".saved";
@end smallexample

@noindent
A string list variable declaration declares a variable whose value is a list
of strings. The list may contain any number (zero or more) of strings.

@smallexample
   Empty_List := ();
   List_With_One_Element := ("/STYLE=");
   List_With_Two_Elements := List_With_One_Element & "/STYLE=GNAT";
   Long_List := ("MAIN.ADA", "PACK1_.ADA", "PACK1.ADA", "PACK2_.ADA"
                 "PACK2.ADA", "UTIL_.ADA", "UTIL.ADA");
@end smallexample

@noindent
The same typed variable may not be declared more than once at project level, and it may not be declared more than once in any package; it is in effect a constant or a readonly variable.

The same untyped variable may be declared several times.
In this case, the new value replaces the old one,
and any subsequent reference to the variable uses the new value.
However, as noted above, if a variable has been declared as a string, all subsequent
declarations must give it a string value. Similarly, if a variable has
been declared as a string list, all subsequent declarations
must give it a string list value.

A @emph{variable reference} may take several forms:

@itemize @bullet
@item The simple variable name, for a variable in the current package (if any) or in the current project
@item A context name, followed by a dot, followed by the variable name.
@end itemize

@noindent
A @emph{context} may be one of the following:

@itemize @bullet
@item The name of an existing package in the current project
@item The name of an imported project of the current project
@item The name of an ancestor project (i.e., a project extended by the current project, either directly or indirectly)
@item An imported/parent project name, followed by a dot, followed by a package name
@end itemize

@noindent
A variable reference may be used in an expression.


@node Attributes
@subsection Attributes

@noindent
A project (and its packages) may have @emph{attributes} that define the project's properties.
Some attributes have values that are strings;
others have values that are string lists.

There are two categories of attributes: @emph{simple attributes} and @emph{associative arrays}
(see @ref{Associative Array Attributes}).

The names of the attributes are restricted; there is a list of project
attributes, and a list of package attributes for each package.
The names are not case sensitive.

The project attributes are as follows (all are simple attributes):

@multitable @columnfractions .4 .3
@item @emph{Attribute Name}
@tab @emph{Value}
@item @code{Source_Files}
@tab string list
@item @code{Source_Dirs}
@tab string list
@item @code{Source_List_File}
@tab string
@item @code{Object_Dir}
@tab string
@item @code{Exec_Dir}
@tab string
@item @code{Main}
@tab string list
@item @code{Languages}
@tab string list
@item @code{Library_Dir}
@tab string
@item @code{Library_Name}
@tab string
@item @code{Library_Kind}
@tab string
@item @code{Library_Elaboration}
@tab string
@item @code{Library_Version}
@tab string
@end multitable

@noindent
The attributes for package @code{Naming} are as follows
(see @ref{Naming Schemes}):

@multitable @columnfractions .4 .2 .2 .2
@item Attribute Name @tab Category @tab Index @tab Value
@item @code{Specification_Suffix}
@tab associative array
@tab language name
@tab string
@item @code{Implementation_Suffix}
@tab associative array
@tab language name
@tab string
@item @code{Separate_Suffix}
@tab simple attribute
@tab n/a
@tab string
@item @code{Casing}
@tab simple attribute
@tab n/a
@tab string
@item @code{Dot_Replacement}
@tab simple attribute
@tab n/a
@tab string
@item @code{Specification}
@tab associative array
@tab Ada unit name
@tab string
@item @code{Implementation}
@tab associative array
@tab Ada unit name
@tab string
@item @code{Specification_Exceptions}
@tab associative array
@tab language name
@tab string list
@item @code{Implementation_Exceptions}
@tab associative array
@tab language name
@tab string list
@end multitable

@noindent
The attributes for package @code{Builder}, @code{Compiler}, @code{Binder},
@code{Linker}, @code{Cross_Reference}, and @code{Finder}
are as follows (see @ref{Qualifiers and Project Files}).

@multitable @columnfractions .4 .2 .2 .2
@item Attribute Name @tab Category @tab Index @tab Value
@item @code{Default_Qualifiers}
@tab associative array
@tab language name
@tab string list
@item @code{Qualifiers}
@tab associative array
@tab file name
@tab string list
@end multitable

@noindent
In addition, package @code{Builder} has a single string attribute
@code{Local_Configuration_Pragmas} and package @code{Builder} has a single
string attribute @code{Global_Configuration_Pragmas}.

@noindent
The attribute for package @code{Glide} are not documented: they are for
internal use only.

@noindent
Each simple attribute has a default value: the empty string (for string-valued
attributes) and the empty list (for string list-valued attributes).

Similar to variable declarations, an attribute declaration defines a new value
for an attribute.

Examples of simple attribute declarations:

@smallexample
   for Object_Dir use "objects";
   for Source_Dirs use ("units", "test/drivers");
@end smallexample

@noindent
A @dfn{simple attribute declaration} starts with the reserved word @code{for},
followed by the name of the attribute, followed by the reserved word
@code{use}, followed by an expression (whose kind depends on the attribute),
followed by a semicolon.

Attributes may be referenced in expressions.
The general form for such a reference is @code{<entity>'<attribute>}:
the entity for which the attribute is defined,
followed by an apostrophe, followed by the name of the attribute.
For associative array attributes, a litteral string between parentheses
need to be supplied as index.

Examples are:

@smallexample
  project'Object_Dir
  Naming'Dot_Replacement
  Imported_Project'Source_Dirs
  Imported_Project.Naming'Casing
  Builder'Default_Qualifiers("Ada")
@end smallexample

@noindent
The entity may be:
@itemize @bullet
@item @code{project} for an attribute of the current project
@item The name of an existing package of the current project
@item The name of an imported project
@item The name of a parent project (extended by the current project)
@item An imported/parent project name, followed by a dot,
      followed by a package name
@end itemize

@noindent
Example:
@smallexample
@group
   project Prj is
     for Source_Dirs use project'Source_Dirs & "units";
     for Source_Dirs use project'Source_Dirs & "test/drivers"
   end Prj;
@end group
@end smallexample

@noindent
In the first attribute declaration, initially the attribute @code{Source_Dirs}
has the default value: an empty string list. After this declaration,
@code{Source_Dirs} is a string list of one element: "units".
After the second attribute declaration @code{Source_Dirs} is a string list of
two elements: "units" and "test/drivers".

Note: this example is for illustration only. In practice,
the project file would contain only one attribute declaration:

@smallexample
   for Source_Dirs use ("units", "test/drivers");
@end smallexample


@node Associative Array Attributes
@subsection Associative Array Attributes

@noindent
Some attributes are defined as @emph{associative arrays}. An associative
array may be regarded as a function that takes a string as a parameter
and delivers a string or string list value as its result.

Here are some examples of associative array attribute declarations:

@smallexample
   for Implementation ("main") use "MAIN.ADA";
   for Qualifiers ("MAIN.ADA") use ("-v", "/REPORT_ERRORS=VERBOSE");
   for Qualifiers ("MAIN.ADA") use Builder'Qualifiers ("MAIN.ADA") & "-g";
@end smallexample

@noindent
Like untyped variables and simple attributes, associative array attributes may be declared several times. Each declaration supplies a new value for the
attribute, replacing the previous setting.


@node case Constructions
@subsection @code{case} Constructions

@noindent
A @code{case} construction is used in a project file to effect conditional
behavior.
Here is a typical example:

@smallexample
@group
project MyProj is
   type OS_Type is ("Linux", "Unix", "NT", "VMS");

   OS : OS_Type := external ("OS", "Linux");
@end group

@group
   package Compiler is
     case OS is
       when "Linux" | "Unix" =>
         for Default_Qualifiers ("Ada") use ("-gnath");
       when "NT" =>
         for Default_Qualifiers ("Ada") use ("/POLLING_ENABLE");
       when others =>
     end case;
   end Compiler;
end MyProj;
@end group
@end smallexample

@noindent
The syntax of a @code{case} construction is based on the Ada case statement
(although there is no @code{null} construction for empty alternatives).

Following the reserved word @code{case} there is the case variable (a typed
string variable), the reserved word @code{is}, and then a sequence of one or
more alternatives.
Each alternative comprises the reserved word @code{when}, either a list of
literal strings separated by the @code{"|"} character or the reserved word
@code{others},  and the @code{"=>"} token.
Each literal string must belong to the string type that is the type of the
case variable.
An @code{others} alternative, if present, must occur last.
The @code{end case;} sequence terminates the case construction.

After each @code{=>}, there are zero or more constructions.  The only
constructions allowed in a case construction are other case constructions and
attribute declarations. String type declarations, variable declarations and
package declarations are not allowed.

The value of the case variable is often given by an external reference
(see @ref{External References in Project Files}).


@c ****************************************
@c * Objects and Sources in Project Files *
@c ****************************************

@node Objects and Sources in Project Files
@section Objects and Sources in Project Files

@menu
* Object Directory::
* Exec Directory::
* Source Directories::
* Source File Names::
@end menu

@noindent
Each project has exactly one object directory and one or more source
directories. The source directories must contain at least one source file,
unless  the project file explicitly specifies that no source files are present
(see @ref{Source File Names}).


@node Object Directory
@subsection Object Directory

@noindent
The object directory for a project is the directory containing the compiler's
output (such as @file{ALI} files and object files) for the project's immediate
sources. Note that for inherited sources (when extending a parent project) the
parent project's object directory is used.

The object directory is given by the value of the attribute @code{Object_Dir}
in the project file.

@smallexample
   for Object_Dir use "objects";
@end smallexample

@noindent
The attribute @var{Object_Dir} has a string value, the path name of the object
directory. The path name may be absolute or relative to the directory of the
project file. This directory must already exist, and be readable and writable.

By default, when the attribute @code{Object_Dir} is not given an explicit value
or when its value is the empty string, the object directory is the same as the
directory containing the project file.


@node Exec Directory
@subsection Exec Directory

@noindent
The exec directory for a project is the directory containing the executables
for the project's main subprograms.

The exec directory is given by the value of the attribute @code{Exec_Dir}
in the project file.

@smallexample
   for Exec_Dir use "executables";
@end smallexample

@noindent
The attribute @var{Exec_Dir} has a string value, the path name of the exec
directory. The path name may be absolute or relative to the directory of the
project file. This directory must already exist, and be writable.

By default, when the attribute @code{Exec_Dir} is not given an explicit value
or when its value is the empty string, the exec directory is the same as the
object directory of the project file.


@node Source Directories
@subsection Source Directories

@noindent
The source directories of a project are specified by the project file
attribute @code{Source_Dirs}.

This attribute's value is a string list. If the attribute is not given an
explicit value, then there is only one source directory, the one where the
project file resides.

A @code{Source_Dirs} attribute that is explicitly defined to be the empty list,
as in

@smallexample
    for Source_Dirs use ();
@end smallexample

@noindent
indicates that the project contains no source files.

Otherwise, each string in the string list designates one or more
source directories.

@smallexample
   for Source_Dirs use ("sources", "test/drivers");
@end smallexample

@noindent
If a string in the list ends with @code{"/**"},  then the directory whose path
name precedes the two asterisks, as well as all its subdirectories
(recursively), are source directories.

@smallexample
   for Source_Dirs use ("/system/sources/**");
@end smallexample

@noindent
Here the directory @code{/system/sources} and all of its subdirectories
(recursively) are source directories.

To specify that the source directories are the directory of the project file
and all of its subdirectories, you can declare @code{Source_Dirs} as follows:
@smallexample
   for Source_Dirs use ("./**");
@end smallexample

@noindent
Each of the source directories must exist and be readable.


@node Source File Names
@subsection Source File Names

@noindent
In a project that contains source files, their names may be specified by the
attributes @code{Source_Files} (a string list) or @code{Source_List_File}
(a string). Source file names never include any directory information.

If the attribute @code{Source_Files} is given an explicit value, then each
element of the list is a source file name.

@smallexample
   for Source_Files use ("MAIN.ADB");
   for Source_Files use ("MAIN.ADB", "PACK1.ADS", "PACK2.ADB");
@end smallexample

@noindent
If the attribute @code{Source_Files} is not given an explicit value,
but the attribute @code{Source_List_File} is given a string value,
then the source file names are contained in the text file whose path name
(absolute or relative to the directory of the project file) is the
value of the attribute @code{Source_List_File}.

Each line in the file that is not empty or is not a comment
contains a source file name. A comment line starts with two hyphens.

@smallexample
   for Source_List_File use "source_list.txt";
@end smallexample

@noindent
By default, if neither the attribute @code{Source_Files} nor the attribute
@code{Source_List_File} is given an explicit value, then each file in the
source directories that conforms to the project's naming scheme
(see @ref{Naming Schemes}) is an immediate source of the project.

A warning is issued if both attributes @code{Source_Files} and
@code{Source_List_File} are given explicit values. In this case, the attribute
@code{Source_Files} prevails.

Each source file name must be the name of one and only one existing source file
in one of the source directories.

A @code{Source_Files} attribute defined with an empty list as its value
indicates that there are no source files in the project.

Except for projects that are clearly specified as containing no Ada source
files (@code{Source_Dirs} or @code{Source_Files} specified as an empty list,
or @code{Languages} specified without @code{"Ada"} in the list)
@smallexample
   for Source_Dirs use ();
   for Source_Files use ();
   for Languages use ("C", "C++");
@end smallexample

@noindent
a project must contain at least one immediate source.

Projects with no source files are useful as template packages
(see @ref{Packages in Project Files}) for other projects; in particular to
define a package @code{Naming} (see @ref{Naming Schemes}).


@c ****************************
@c * Importing Projects *
@c ****************************

@node  Importing Projects
@section Importing Projects

@noindent
An immediate source of a project P may depend on source files that
are neither immediate sources of P nor in the predefined library.
To get this effect, P must @emph{import} the projects that contain the needed
source files.

@smallexample
@group
  with "project1", "utilities.gpr";
  with "/namings/apex.gpr";
  project Main is
    ...
@end group
@end smallexample

@noindent
As can be seen in this example, the syntax for importing projects is similar
to the syntax for importing compilation units in Ada. However, project files
use literal strings instead of names, and the @code{with} clause identifies
project files rather than packages.

Each literal string is the file name or path name (absolute or relative) of a
project file. If a string is simply a file name, with no path, then its
location is determined by the @emph{project path}:

@itemize @bullet
@item
If the environment variable @env{ADA_PROJECT_PATH} exists, then the project
path includes all the directories in this environment variable, plus the
directory of the project file.

@item
If the environment variable @env{ADA_PROJECT_PATH} does not exist,
then the project path contains only one directory, namely the one where
the project file is located.
@end itemize

@noindent
If a relative pathname is used as in

@smallexample
  with "tests/proj";
@end smallexample

@noindent
then the path is relative to the directory where the importing project file is
located. Any symbolic link will be fully resolved in the directory
of the importing project file before the imported project file is looked up.

When the @code{with}'ed project file name does not have an extension,
the default is @file{.gpr}. If a file with this extension is not found, then
the file name as specified in the @code{with} clause (no extension) will be
used. In the above example, if a file @code{project1.gpr} is found, then it
will be used; otherwise, if a file @code{project1} exists then it will be used;
if neither file exists, this is an error.

A warning is issued if the name of the project file does not match the
name of the project; this check is case insensitive.

Any source file that is an immediate source of the imported project can be
used by the immediate sources of the importing project, and recursively. Thus
if @code{A} imports @code{B}, and @code{B} imports @code{C}, the immediate
sources of @code{A} may depend on the immediate sources of @code{C}, even if
@code{A} does not import @code{C} explicitly. However, this is not recommended,
because if and when @code{B} ceases to import @code{C}, some sources in
@code{A} will no longer compile.

A side effect of this capability is that cyclic dependences are not permitted:
if @code{A} imports @code{B} (directly or indirectly) then @code{B} is not
allowed to import @code{A}.


@c *********************
@c * Project Extension *
@c *********************

@node Project Extension
@section Project Extension

@noindent
During development of a large system, it is sometimes necessary to use
modified versions of some of the source files without changing the original
sources. This can be achieved through a facility known as
@emph{project extension}.

@smallexample
   project Modified_Utilities extends "/baseline/utilities.gpr" is ...
@end smallexample

@noindent
The project file for the project being extended (the @emph{parent}) is
identified by the literal string that follows the reserved word @code{extends},
which itself follows the name of the extending project (the @emph{child}).

By default, a child project inherits all the sources of its parent.
However, inherited sources can be overridden: a unit with the same name as one
in the parent will hide the original unit.
Inherited sources are considered to be sources (but not immediate sources)
of the child project; see @ref{Project File Syntax}.

An inherited source file retains any qualifiers specified in the parent project.

For example if the project @code{Utilities} contains the specification and the
body of an Ada package @code{Util_IO}, then the project
@code{Modified_Utilities} can contain a new body for package @code{Util_IO}.
The original body of @code{Util_IO} will not be considered in program builds.
However, the package specification will still be found in the project
@code{Utilities}.

A child project can have only one parent but it may import any number of other
projects.

A project is not allowed to import directly or indirectly at the same time a
child project and any of its ancestors.


@c ****************************************
@c * External References in Project Files *
@c ****************************************

@node  External References in Project Files
@section External References in Project Files

@noindent
A project file may contain references to external variables; such references
are called @emph{external references}.

An external variable is either defined as part of the environment (an
environment variable in Unix, for example) or else specified on the command
line via the @option{-X@emph{vbl}=@emph{value}} qualifier. If both, then the
command line value is used.

An external reference is denoted by the built-in function
@code{external}, which returns a string value.  This function has two forms:
@itemize @bullet
@item @code{external (external_variable_name)}
@item @code{external (external_variable_name, default_value)}
@end itemize

@noindent
Each parameter must be a string literal.  For example:

@smallexample
   external ("USER")
   external ("OS", "Linux")
@end smallexample

@noindent
In the form with one parameter, the function returns the value of
the external variable given as parameter. If this name is not present in the
environment, then the returned value is an empty string.

In the form with two string parameters, the second parameter is
the value returned when the variable given as the first parameter is not
present in the environment. In the example above, if @code{"OS"} is not
the name of an environment variable and is not passed on the command line,
then the returned value will be @code{"Linux"}.

An external reference may be part of a string expression or of a string
list expression, to define variables or attributes.

@smallexample
@group
   type Mode_Type is ("Debug", "Release");
   Mode : Mode_Type := external ("MODE");
   case Mode is
     when "Debug" =>
        ...
@end group
@end smallexample


@c *****************************
@c * Packages in Project Files *
@c *****************************

@node  Packages in Project Files
@section Packages in Project Files

@noindent
The @emph{package} is the project file feature that defines the settings for
project-aware tools.
For each such tool you can declare a corresponding package; the names for these
packages are preset (see @ref{Packages}) but are not case sensitive.
A package may contain variable declarations, attribute declarations, and case
constructions.

@smallexample
@group
   project Proj is
      package Builder is  -- used by GNAT MAKE
         for Default_Qualifiers ("Ada") use ("-v", "-g");
      end Builder;
   end Proj;
@end group
@end smallexample

@noindent
A package declaration starts with the reserved word @code{package},
followed by the package name (case insensitive), followed by the reserved word
@code{is}. It ends with the reserved word @code{end}, followed by the package
name, finally followed by a semi-colon.

Most of the packages have an attribute @code{Default_Qualifiers}.
This attribute is an associative array, and its value is a string list.
The index of the associative array is the name of a programming language (case
insensitive). This attribute indicates the qualifier or qualifiers to be used
with the corresponding tool.

Some packages also have another attribute, @code{Qualifiers}, an associative
array whose value is a string list. The index is the name of a source file.
This attribute indicates the qualifier or qualifiers to be used by the corresponding
tool when dealing with this specific file.

Further information on these qualifier-related attributes is found in
@ref{Qualifiers and Project Files}.

A package may be declared as a @emph{renaming} of another package; e.g., from
the project file for an imported project.

@smallexample
@group
  with "/global/apex.gpr";
  project Example is
    package Naming renames Apex.Naming;
    ...
  end Example;
@end group
@end smallexample

@noindent
Packages that are renamed in other project files often come from project files
that have no sources: they are just used as templates. Any modification in the
template will be reflected automatically in all the project files that rename
a package from the template.

In addition to the tool-oriented packages, you can also declare a package
named @code{Naming} to establish specialized source file naming conventions
(see @ref{Naming Schemes}).


@c ************************************
@c * Variables from Imported Projects *
@c ************************************

@node Variables from Imported Projects
@section Variables from Imported Projects

@noindent
An attribute or variable defined in an imported or parent project can
be used in expressions in the importing / extending project.
Such an attribute or variable is prefixed with the name of the project
and (if relevant) the name of package where it is defined.

@smallexample
@group
  with "imported";
  project Main extends "base" is
     Var1 := Imported.Var;
     Var2 := Base.Var & ".new";
@end group

@group
     package Builder is
        for Default_Qualifiers ("Ada") use Imported.Builder.Ada_Qualifiers &
                         "/STYLE=GNAT" & "-v";
     end Builder;
@end group

@group
     package Compiler is
        for Default_Qualifiers ("Ada") use Base.Compiler.Ada_Qualifiers;
     end Compiler;
  end Main;
@end group
@end smallexample

@noindent
In this example:

@itemize @bullet
@item
@code{Var1} is a copy of the variable @code{Var} defined in the project file
@file{"imported.gpr"}
@item
the value of @code{Var2} is a copy of the value of variable @code{Var}
defined in the project file @file{base.gpr}, concatenated with @code{".new"}
@item
attribute @code{Default_Qualifiers ("Ada")} in package @code{Builder}
is a string list that includes in its value a copy of variable
@code{Ada_Qualifiers} defined in the @code{Builder} package in project file
@file{imported.gpr} plus two new elements: @option{"/STYLE=GNAT"} and @option{"-v"};
@item
attribute @code{Default_Qualifiers ("Ada")} in package @code{Compiler}
is a copy of the variable @code{Ada_Qualifiers} defined in the @code{Compiler}
package in project file @file{base.gpr}, the project being extended.
@end itemize


@c ******************
@c * Naming Schemes *
@c ******************

@node  Naming Schemes
@section Naming Schemes

@noindent
Sometimes an Ada software system is ported from a foreign compilation
environment to GNAT, with file names that do not use the default GNAT
conventions. Instead of changing all the file names (which for a variety of
reasons might not be possible), you can define the relevant file naming scheme
in the @code{Naming} package in your project file.  For example, the following
package models the Apex file naming rules:

@smallexample
@group
  package Naming is
    for Casing                        use "lowercase";
    for Dot_Replacement               use ".";
    for Specification_Suffix ("Ada")  use ".1.ADA";
    for Implementation_Suffix ("Ada") use ".2.ADA";
  end Naming;
@end group
@end smallexample

@noindent
You can define the following attributes in package @code{Naming}:

@table @code

@item @var{Casing}
This must be a string with one of the three values @code{"lowercase"},
@code{"uppercase"} or @code{"mixedcase"}; these strings are case insensitive.

@noindent
If @var{Casing} is not specified, then the default is @code{"lowercase"}.

@item @var{Dot_Replacement}
This must be a string whose value satisfies the following conditions:

@itemize @bullet
@item It must not be empty
@item It cannot start or end with an alphanumeric character
@item It cannot be a single underscore
@item It cannot start with an underscore followed by an alphanumeric
@item It cannot contain a dot @code{'.'} except if it the entire string is @code{"."}
@end itemize

@noindent
If @code{Dot_Replacement} is not specified, then the default is @code{"-"}.

@item @var{Specification_Suffix}
This is an associative array (indexed by the programming language name, case
insensitive) whose value is a string that must satisfy the following
conditions:

@itemize @bullet
@item It must not be empty
@item It cannot start with an alphanumeric character
@item It cannot start with an underscore followed by an alphanumeric character
@end itemize
@noindent
If @code{Specification_Suffix ("Ada")} is not specified, then the default is
@code{".ADS"}.

@item @var{Implementation_Suffix}
This is an associative array (indexed by the programming language name, case
insensitive) whose value is a string that must satisfy the following
conditions:

@itemize @bullet
@item It must not be empty
@item It cannot start with an alphanumeric character
@item It cannot start with an underscore followed by an alphanumeric character
@item It cannot be a suffix of @code{Specification_Suffix}
@end itemize
@noindent
If @code{Implementation_Suffix ("Ada")} is not specified, then the default is
@code{".ADB"}.

@item @var{Separate_Suffix}
This must be a string whose value satisfies the same conditions as
@code{Implementation_Suffix}.

@noindent
If @code{Separate_Suffix ("Ada")} is not specified, then it defaults to same
value as @code{Implementation_Suffix ("Ada")}.

@item @var{Specification}
@noindent
You can use the @code{Specification} attribute, an associative array, to define
the source file name for an individual Ada compilation unit's spec. The array
index must be a string literal that identifies the Ada unit (case insensitive).
The value of this attribute must be a string that identifies the file that
contains this unit's spec (case sensitive or insensitive depending on the
operating system).

@smallexample
   for Specification ("MyPack.MyChild") use "mypack.mychild.spec";
@end smallexample

@item @var{Implementation}

You can use the @code{Implementation} attribute, an associative array, to
define the source file name for an individual Ada compilation unit's body
(possibly a subunit).  The array index must be a string literal that identifies
the Ada unit (case insensitive).  The value of this attribute must be a string
that identifies the file that contains this unit's body or subunit (case
sensitive or insensitive depending on the operating system).

@smallexample
   for Implementation ("MyPack.MyChild") use "mypack.mychild.body";
@end smallexample
@end table


@c ********************
@c * Library Projects *
@c ********************

@node Library Projects
@section Library Projects

@noindent
@emph{Library projects} are projects whose object code is placed in a library.
(Note that this facility is not yet supported on all platforms)

To create a library project, you need to define in its project file
two project-level attributes: @code{Library_Name} and @code{Library_Dir}.
Additionally, you may define the library-related attributes
@code{Library_Kind}, @code{Library_Version} and @code{Library_Elaboration}.

The @code{Library_Name} attribute has a string value that must start with a
letter and include only letters and digits.

The @code{Library_Dir} attribute has a string value that designates the path
(absolute or relative) of the directory where the library will reside.
It must designate an existing directory, and this directory needs to be
different from the project's object directory. It also needs to be writable.

If both @code{Library_Name} and @code{Library_Dir} are specified and
are legal, then the project file defines a library project.  The optional
library-related attributes are checked only for such project files.

The @code{Library_Kind} attribute has a string value that must be one of the
following (case insensitive): @code{"static"}, @code{"dynamic"} or
@code{"relocatable"}. If this attribute is not specified, the library is a
static library. Otherwise, the library may be dynamic or relocatable.
Depending on the operating system, there may or may not be a distinction
between dynamic and relocatable libraries. For example, on Unix there is no
such distinction.

The @code{Library_Version} attribute has a string value whose interpretation
is platform dependent. On Unix, it is used only for dynamic/relocatable
libraries as the internal name of the library (the @code{"soname"}). If the
library file name (built from the @code{Library_Name}) is different from the
@code{Library_Version}, then the library file will be a symbolic link to the
actual file whose name will be @code{Library_Version}.

Example (on Unix):

@smallexample
@group
project Plib is

   Version := "1";

   for Library_Dir use "lib_dir";
   for Library_Name use "dummy";
   for Library_Kind use "relocatable";
   for Library_Version use "libdummy.so." & Version;

end Plib;
@end group
@end smallexample

@noindent
Directory @file{lib_dir} will contain the internal library file whose name
will be @file{libdummy.so.1}, and @file{libdummy.so} will be a symbolic link to
@file{libdummy.so.1}.

When @command{GNAT MAKE} detects that a project file (not the main project file)
is a library project file, it will check all immediate sources of the project
and rebuild the library if any of the sources have been recompiled.
All @file{ALI} files will also be copied from the object directory to the
library directory. To build executables, @command{GNAT MAKE} will use the
library rather than the individual object files.


@c *************************************
@c * Qualifiers Related to Project Files *
@c *************************************
@node Qualifiers Related to Project Files
@section Qualifiers Related to Project Files

@noindent
The following qualifiers are used by GNAT tools that support project files:

@table @code

@item @option{-P@var{project}}
Indicates the name of a project file. This project file will be parsed with
the verbosity indicated by @option{-vP@emph{x}}, if any, and using the external
references indicated by @option{-X} qualifiers, if any.

@noindent
There must be only one @option{-P} qualifier on the command line.

@noindent
Since the Project Manager parses the project file only after all the qualifiers
on the command line are checked, the order of the qualifiers @option{-P},
@option{-Vp@emph{x}} or @option{-X} is not significant.

@item @option{-X@var{name=value}}
Indicates that external variable @var{name} has the value @var{value}.
The Project Manager will use this value for occurrences of
@code{external(name)} when parsing the project file.

@noindent
If @var{name} or @var{value} includes a space, then @var{name=value} should be
put between quotes.
@smallexample
  -XOS=NT
  -X"user=John Doe"
@end smallexample

@noindent
Several @option{-X} qualifiers can be used simultaneously.
If several @option{-X} qualifiers specify the same @var{name}, only the last one
is used.

@noindent
An external variable specified with a @option{-X} qualifier takes precedence
over the value of the same name in the environment.

@item @option{-vP@emph{x}}
Indicates the verbosity of the parsing of GNAT project files.
@option{-vP0} means Default (no output for syntactically correct project
files);
@option{-vP1} means Medium;
@option{-vP2} means High.
@noindent
The default is Default.
@noindent
If several @option{-vP@emph{x}} qualifiers are present, only the last one is
used.

@end table


@c **********************************
@c * Tools Supporting Project Files *
@c **********************************

@node  Tools Supporting Project Files
@section Tools Supporting Project Files

@menu
* GNAT MAKE and Project Files::
* The GNAT Driver and Project Files::
@end menu

@node GNAT MAKE and Project Files
@subsection GNAT MAKE and Project Files

@noindent
This section covers two topics related to @command{GNAT MAKE} and project files:
defining qualifiers for @command{GNAT MAKE} and for the tools that it invokes;
and the use of the @code{Main} attribute.

@menu
* Qualifiers and Project Files::
* Project Files and Main Subprograms::
@end menu

@node Qualifiers and Project Files
@subsubsection Qualifiers and Project Files

@noindent
For each of the packages @code{Builder}, @code{Compiler}, @code{Binder}, and
@code{Linker}, you can specify a @code{Default_Qualifiers} attribute, a
@code{Qualifiers} attribute, or both; as their names imply, these qualifier-related
attributes affect which qualifiers are used for which files when
@command{GNAT MAKE} is invoked.  As will be explained below, these
package-contributed qualifiers precede the qualifiers passed on the
@command{GNAT MAKE} command line.

The @code{Default_Qualifiers} attribute is an associative array indexed by
language name (case insensitive) and returning a string list.  For example:

@smallexample
@group
package Compiler is
  for Default_Qualifiers ("Ada") use ("/STYLE=", "-v");
end Compiler;
@end group
@end smallexample

@noindent
The @code{Qualifiers} attribute is also an associative array, indexed by a file
name (which may or may not be case sensitive, depending on the operating
system) and returning a string list.  For example:

@smallexample
@group
package Builder is
   for Qualifiers ("MAIN1.ADB") use ("/OPTIMIZE=ALL");
   for Qualifiers ("MAIN2.ADB") use ("-g");
end Builder;
@end group
@end smallexample

@noindent
For the @code{Builder} package, the file names should designate source files
for main subprograms.  For the @code{Binder} and @code{Linker} packages, the
file names should designate @file{ALI} or source files for main subprograms.
In each case just the file name (without explicit extension) is acceptable.

For each tool used in a program build (@command{GNAT MAKE}, the compiler, the
binder, and the linker), its corresponding package @dfn{contributes} a set of
qualifiers for each file on which the tool is invoked, based on the
qualifier-related attributes defined in the package. In particular, the qualifiers
that each of these packages contributes for a given file @var{f} comprise:

@itemize @bullet
@item
the value of attribute @code{Qualifiers (@var{f})}, if it is specified in the
package for the given file,
@item
otherwise, the value of @code{Default_Qualifiers ("Ada")}, if it is specified in
the package.
@end itemize

@noindent
If neither of these attributes is defined in the package, then the package does
not contribute any qualifiers for the given file.

When @command{GNAT MAKE} is invoked on a file, the qualifiers comprise two sets,
in the following order: those contributed for the file by the @code{Builder}
package; and the qualifiers passed on the command line.

When @command{GNAT MAKE} invokes a tool (compiler, binder, linker) on a file,
the qualifiers passed to the tool comprise three sets, in the following order:

@enumerate
@item
the applicable qualifiers contributed for the file by the @code{Builder} package
in the project file supplied on the command line;

@item
those contributed for the file by the package (in the relevant project file --
see below) corresponding to the tool; and

@item
the applicable qualifiers passed on the command line.
@end enumerate

@noindent
The term @emph{applicable qualifiers} reflects the fact that @command{GNAT MAKE}
qualifiers may or may not be passed to individual tools, depending on the
individual qualifier.

@command{GNAT MAKE} may invoke the compiler on source files from different
projects. The Project Manager will use the appropriate project file to
determine the @code{Compiler} package for each source file being compiled.
Likewise for the @code{Binder} and @code{Linker} packages.

As an example, consider the following package in a project file:

@smallexample
@group
project Proj1 is
   package Compiler is
      for Default_Qualifiers ("Ada") use ("-g");
      for Qualifiers ("A.ADB") use ("/OPTIMIZE=SOME");
      for Qualifiers ("B.ADB") use ("/OPTIMIZE=ALL", "/STYLE=");
   end Compiler;
end Proj1;
@end group
@end smallexample

@noindent
If @command{GNAT MAKE} is invoked with this project file, and it needs to
compile, say, the files @file{A.ADB}, @file{B.ADB}, and @file{C.ADB}, then
@file{A.ADB} will be compiled with the qualifier @option{/OPTIMIZE=SOME}, @file{B.ADB}
with qualifiers @option{/OPTIMIZE=ALL} and @option{/STYLE=}, and @file{C.ADB} with
@option{-g}.

Another example illustrates the ordering of the qualifiers contributed by
different packages:

@smallexample
@group
project Proj2 is
   package Builder is
      for Qualifiers ("MAIN.ADB") use ("-g", "/OPTIMIZE=SOME", "-f");
   end Builder;
@end group

@group
   package Compiler is
      for Qualifiers ("MAIN.ADB") use ("/OPTIMIZE=ALL");
   end Compiler;
end Proj2;
@end group
@end smallexample

@noindent
If you issue the command:

@smallexample
    GNAT MAKE -PProj2 /OPTIMIZE=NONE main
@end smallexample

@noindent
then the compiler will be invoked on @file{MAIN.ADB} with the following sequence of qualifiers

@smallexample
   -g /OPTIMIZE=SOME /OPTIMIZE=ALL /OPTIMIZE=NONE
@end smallexample

with the last @option{-O} qualifier having precedence over the earlier ones;
several other qualifiers (such as @option{-c}) are added implicitly.

The qualifiers @option{-g} and @option{/OPTIMIZE=SOME} are contributed by package
@code{Builder},  @option{/OPTIMIZE=ALL} is contributed by the package @code{Compiler}
and @option{/OPTIMIZE=NONE} comes from the command line.

The @option{-g} qualifier will also be passed in the invocation of
@command{GNAT LINK.}

A final example illustrates qualifier contributions from packages in different
project files:

@smallexample
@group
project Proj3 is
   for Source_Files use ("PACK.ADS", "PACK.ADB");
   package Compiler is
      for Default_Qualifiers ("Ada") use ("/CHECKS=ASSERTIONS");
   end Compiler;
end Proj3;
@end group

@group
with "Proj3";
project Proj4 is
   for Source_Files use ("FOO_MAIN.ADB", "BAR_MAIN.ADB");
   package Builder is
      for Qualifiers ("FOO_MAIN.ADB") use ("-s", "-g");
   end Builder;
end Proj4;
@end group

@group
-- Ada source file:
with Pack;
procedure Foo_Main is
   ...
end Foo_Main;
@end group
@end smallexample

If the command is
@smallexample
GNAT MAKE -PProj4 FOO_MAIN.ADB /COMPILER_QUALIFIERS /CHECKS=OVERFLOW
@end smallexample

@noindent
then the qualifiers passed to the compiler for @file{FOO_MAIN.ADB} are
@option{-g} (contributed by the package @code{Proj4.Builder}) and
@option{/CHECKS=OVERFLOW} (passed on the command line).
When the imported package @code{Pack} is compiled, the qualifiers used are
@option{-g} from @code{Proj4.Builder}, @option{/CHECKS=ASSERTIONS} (contributed from
package @code{Proj3.Compiler}, and @option{/CHECKS=OVERFLOW} from the command line.


@node Project Files and Main Subprograms
@subsubsection Project Files and Main Subprograms

@noindent
When using a project file, you can invoke @command{GNAT MAKE}
with several main subprograms, by specifying their source files on the command
line.  Each of these needs to be an immediate source file of the project.

@smallexample
    GNAT MAKE -Pprj main1 main2 main3
@end smallexample

@noindent
When using a project file, you can also invoke @command{GNAT MAKE} without
explicitly specifying any main, and the effect depends on whether you have
defined the @code{Main} attribute.  This attribute has a string list value,
where each element in the list is the name of a source file (the file
extension is optional) containing a main subprogram.

If the @code{Main} attribute is defined in a project file as a non-empty
string list and the qualifier @option{-u} is not used on the command line, then
invoking @command{GNAT MAKE} with this project file but without any main on the
command line is equivalent to invoking @command{GNAT MAKE} with all the file
names in the @code{Main} attribute on the command line.

Example:
@smallexample
@group
   project Prj is
      for Main use ("main1", "main2", "main3");
   end Prj;
@end group
@end smallexample

@noindent
With this project file, @code{"GNAT MAKE -Pprj"} is equivalent to
@code{"GNAT MAKE -Pprj main1 main2 main3"}.

When the project attribute @code{Main} is not specified, or is specified
as an empty string list, or when the qualifier @option{-u} is used on the command
line, then invoking @command{GNAT MAKE} with no main on the command line will
result in all immediate sources of the project file being checked, and
potentially recompiled. Depending on the presence of the qualifier @option{-u},
sources from other project files on which the immediate sources of the main
project file depend are also checked and potentially recompiled. In other
words, the @option{-u} qualifier is applied to all of the immediate sources of themain project file.


@node The GNAT Driver and Project Files
@subsection The GNAT Driver and Project Files

@noindent
A number of GNAT tools, other than @command{GNAT MAKE} are project-aware:
@command{GNAT BIND}, @command{GNAT FIND}, @command{GNAT LINK}, @command{GNAT LIST}
and @command{GNAT XREF}. However, none of these tools can be invoked directly
with a project file qualifier (@code{-P}). They need to be invoke through the
@command{gnat} driver.

The @command{gnat} driver is a front-end that accepts a number of commands and
call the corresponding tool. It has been designed initially for VMS to convert
VMS style qualifiers to Unix style qualifiers, but it is now available to all
the GNAT supported platforms.

On non VMS platforms, the @command{gnat} driver accepts the following commands
(case insensitive):

@itemize @bullet
@item
BIND to invoke @command{GNAT BIND}
@item
CHOP to invoke @command{GNAT CHOP}
@item
COMP or COMPILE to invoke the compiler
@item
ELIM to invoke @command{GNAT ELIM}
@item
FIND to invoke @command{GNAT FIND}
@item
KR or KRUNCH to invoke @command{GNAT KRUNCH}
@item
LINK to invoke @command{GNAT LINK}
@item
LS or LIST to invoke @command{GNAT LIST}
@item
MAKE to invoke @command{GNAT MAKE}
@item
NAME to invoke @command{gnatname}
@item
PREP or PREPROCESS to invoke @command{GNAT PREPROCESS}
@item
PSTA or STANDARD to invoke @command{GNAT STANDARD}
@item
STUB to invoke @command{GNAT STUB}
@item
XREF to invoke @command{GNAT XREF}
@end itemize

@noindent
Note that the compiler is invoked using the command @command{GNAT MAKE -f -u}.

@noindent
Following the command, you may put qualifiers and arguments for the invoked
tool.

@smallexample
  gnat bind -C MAIN.ALI
  gnat ls -a main
  gnat chop foo.txt
@end smallexample

@noindent
In addition, for command BIND, FIND, LS or LIST, LINK and XREF, the project
file related qualifiers (@code{-P}, @code{-X} and @code{-vPx}) may be used in
addition to the qualifiers of the invoking tool.

@noindent
For each of these command, there is possibly a package in the main project that
corresponds to the invoked tool.

@itemize @bullet
@item
package @code{Binder} for command BIND (invoking @code{GNAT BIND})

@item
package @code{Finder} for command FIND (invoking @code{GNAT FIND})

@item
package @code{GNAT LIST} for command LS or LIST (invoking @code{GNAT LIST})

@item
package @code{Linker} for command LINK (invoking @code{GNAT LINK})

@item
package @code{Cross_Reference} for command XREF (invoking @code{GNAT LINK})

@end itemize

@noindent
Package @code{GNAT LIST} has a unique attribute @code{Qualifiers}, a simple variable
with a string list value. It contains qualifiers for the invocation of
@code{GNAT LIST}.

@smallexample
@group
project Proj1 is
   package GNAT LIST is
      for Qualifiers use ("-a", "-v");
   end GNAT LIST;
end Proj1;
@end group
@end smallexample

@noindent
All other packages contains a qualifier @code{Default_Qualifiers}, an associative
array, indexed by the programming language (case insensitive) and having a
string list value. @code{Default_Qualifiers ("Ada")} contains the qualifiers for
the invocation of the tool corresponding to the package.

@smallexample
@group
project Proj is

   for Source_Dirs use ("./**");

   package GNAT LIST is
      for Qualifiers use ("-a", "-v");
   end GNAT LIST;
@end group
@group

   package Binder is
      for Default_Qualifiers ("Ada") use ("-C", "-e");
   end Binder;
@end group
@group

   package Linker is
      for Default_Qualifiers ("Ada") use ("-C");
   end Linker;
@end group
@group

   package Finder is
      for Default_Qualifiers ("Ada") use ("-a", "-f");
   end Finder;
@end group
@group

   package Cross_Reference is
      for Default_Qualifiers ("Ada") use ("-a", "-f", "-d", "-u");
   end Cross_Reference;
end Proj;
@end group
@end smallexample

@noindent
With the above project file, commands such as

@smallexample
   gnat ls -Pproj main
   gnat xref -Pproj main
   gnat bind -Pproj MAIN.ALI
@end smallexample

@noindent
will set up the environment properly and invoke the tool with the qualifiers
found in the package corresponding to the tool.




@node An Extended Example
@section An Extended Example

@noindent
Suppose that we have two programs, @var{prog1} and @var{prog2}, with the sources
in the respective directories. We would like to build them with a single
@command{GNAT MAKE} command, and we would like to place their object files into
@file{.build} subdirectories of the source directories. Furthermore, we would
like to have to have two separate subdirectories in @file{.build}  --
@file{release} and @file{debug} -- which will contain the object files compiled with
different set of compilation flags.

In other words, we have the following structure:

@smallexample
@group
   main
     |- prog1
     |    |- .build
     |         | debug
     |         | release
     |- prog2
          |- .build
               | debug
               | release
@end group
@end smallexample

@noindent
Here are the project files that we need to create in a directory @file{main}
to maintain this structure:

@enumerate

@item We create a @code{Common} project with a package @code{Compiler} that
specifies the compilation qualifiers:

@smallexample
File "common.gpr":
@group
@b{project} Common @b{is}

   @b{for} Source_Dirs @b{use} (); -- No source files
@end group

@group
   @b{type} Build_Type @b{is} ("release", "debug");
   Build : Build_Type := External ("BUILD", "debug");
@end group
@group
   @b{package} Compiler @b{is}
      @b{case} Build @b{is}
         @b{when} "release" =>
           @b{for} Default_Qualifiers ("Ada") @b{use} ("/OPTIMIZE=ALL");
         @b{when} "debug"   =>
           @b{for} Default_Qualifiers ("Ada") @b{use} ("-g");
      @b{end case};
   @b{end} Compiler;

@b{end} Common;
@end group
@end smallexample

@item We create separate projects for the two programs:

@smallexample
@group
File "prog1.gpr":

@b{with} "common";
@b{project} Prog1 @b{is}

    @b{for} Source_Dirs @b{use} ("prog1");
    @b{for} Object_Dir  @b{use} "prog1/.build/" & Common.Build;

    @b{package} Compiler @b{renames} Common.Compiler;

@b{end} Prog1;
@end group
@end smallexample

@smallexample
@group
File "prog2.gpr":

@b{with} "common";
@b{project} Prog2 @b{is}

    @b{for} Source_Dirs @b{use} ("prog2");
    @b{for} Object_Dir  @b{use} "prog2/.build/" & Common.Build;

    @b{package} Compiler @b{renames} Common.Compiler;

@end group
@b{end} Prog2;
@end smallexample

@item We create a wrapping project @var{Main}:

@smallexample
@group
File "main.gpr":

@b{with} "common";
@b{with} "prog1";
@b{with} "prog2";
@b{project} Main @b{is}

   @b{package} Compiler @b{renames} Common.Compiler;

@b{end} Main;
@end group
@end smallexample

@item Finally we need to create a dummy procedure that @code{with}s (either
explicitly or implicitly) all the sources of our two programs.

@end enumerate

@noindent
Now we can build the programs using the command

@smallexample
   GNAT MAKE -Pmain dummy
@end smallexample

@noindent
for the Debug mode, or

@smallexample
   GNAT MAKE -Pmain -XBUILD=release
@end smallexample

@noindent
for the Release mode.


@c ********************************
@c * Project File Complete Syntax *
@c ********************************

@node Project File Complete Syntax
@section Project File Complete Syntax

@smallexample
project ::=
  context_clause project_declaration

context_clause ::=
  @{with_clause@}

with_clause ::=
  @b{with} literal_string @{ , literal_string @} ;

project_declaration ::=
  @b{project} <project_>simple_name [ @b{extends} literal_string ] @b{is}
    @{declarative_item@}
  @b{end} <project_>simple_name;

declarative_item ::=
  package_declaration |
  typed_string_declaration |
  other_declarative_item

package_declaration ::=
  @b{package} <package_>simple_name package_completion

package_completion ::=
  package_body | package_renaming

package body ::=
  @b{is}
    @{other_declarative_item@}
  @b{end} <package_>simple_name ;

package_renaming ::==
  @b{renames} <project_>simple_name.<package_>simple_name ;

typed_string_declaration ::=
  @b{type} <typed_string_>_simple_name @b{is}
   ( literal_string @{, literal_string@} );

other_declarative_item ::=
  attribute_declaration |
  typed_variable_declaration |
  variable_declaration |
  case_construction

attribute_declaration ::=
  @b{for} attribute @b{use} expression ;

attribute ::=
  <simple_attribute_>simple_name |
  <associative_array_attribute_>simple_name ( literal_string )

typed_variable_declaration ::=
  <typed_variable_>simple_name : <typed_string_>name :=  string_expression ;

variable_declaration ::=
  <variable_>simple_name := expression;

expression ::=
  term @{& term@}

term ::=
  literal_string |
  string_list |
  <variable_>name |
  external_value |
  attribute_reference

literal_string ::=
  (same as Ada)

string_list ::=
  ( <string_>expression @{ , <string_>expression @} )

external_value ::=
  @b{external} ( literal_string [, literal_string] )

attribute_reference ::=
  attribute_parent ' <simple_attribute_>simple_name [ ( literal_string ) ]

attribute_parent ::=
  @b{project} |
  <project_or_package>simple_name |
  <project_>simple_name . <package_>simple_name

case_construction ::=
  @b{case} <typed_variable_>name @b{is}
    @{case_item@}
  @b{end case} ;

case_item ::=
  @b{when} discrete_choice_list => @{case_construction | attribute_declaration@}

discrete_choice_list ::=
  literal_string @{| literal_string@}

name ::=
  simple_name @{. simple_name@}

simple_name ::=
  identifier (same as Ada)

@end smallexample


@node Elaboration Order Handling in GNAT
@chapter Elaboration Order Handling in GNAT
@cindex Order of elaboration
@cindex Elaboration control

@menu
* Elaboration Code in Ada 95::
* Checking the Elaboration Order in Ada 95::
* Controlling the Elaboration Order in Ada 95::
* Controlling Elaboration in GNAT - Internal Calls::
* Controlling Elaboration in GNAT - External Calls::
* Default Behavior in GNAT - Ensuring Safety::
* Elaboration Issues for Library Tasks::
* Mixing Elaboration Models::
* What to Do If the Default Elaboration Behavior Fails::
* Elaboration for Access-to-Subprogram Values::
* Summary of Procedures for Elaboration Control::
* Other Elaboration Order Considerations::
@end menu

@noindent
This chapter describes the handling of elaboration code in Ada 95 and
in GNAT, and discusses how the order of elaboration of program units can
be controlled in GNAT, either automatically or with explicit programming
features.

@node Elaboration Code in Ada 95
@section Elaboration Code in Ada 95

@noindent
Ada 95 provides rather general mechanisms for executing code at elaboration
time, that is to say before the main program starts executing. Such code arises
in three contexts:

@table @asis
@item Initializers for variables.
Variables declared at the library level, in package specs or bodies, can
require initialization that is performed at elaboration time, as in:
@smallexample
@cartouche
Sqrt_Half : Float := Sqrt (0.5);
@end cartouche
@end smallexample

@item Package initialization code
Code in a @code{BEGIN-END} section at the outer level of a package body is
executed as part of the package body elaboration code.

@item Library level task allocators
Tasks that are declared using task allocators at the library level
start executing immediately and hence can execute at elaboration time.
@end table

@noindent
Subprogram calls are possible in any of these contexts, which means that
any arbitrary part of the program may be executed as part of the elaboration
code. It is even possible to write a program which does all its work at
elaboration time, with a null main program, although stylistically this
would usually be considered an inappropriate way to structure
a program.

An important concern arises in the context of elaboration code:
we have to be sure that it is executed in an appropriate order. What we
have is a series of elaboration code sections, potentially one section
for each unit in the program. It is important that these execute
in the correct order. Correctness here means that, taking the above
example of the declaration of @code{Sqrt_Half},
if some other piece of
elaboration code references @code{Sqrt_Half},
then it must run after the
section of elaboration code that contains the declaration of
@code{Sqrt_Half}.

There would never be any order of elaboration problem if we made a rule
that whenever you @code{with} a unit, you must elaborate both the spec and body
of that unit before elaborating the unit doing the @code{with}'ing:

@smallexample
@group
@cartouche
@b{with} Unit_1;
@b{package} Unit_2 @b{is} ...
@end cartouche
@end group
@end smallexample

@noindent
would require that both the body and spec of @code{Unit_1} be elaborated
before the spec of @code{Unit_2}. However, a rule like that would be far too
restrictive. In particular, it would make it impossible to have routines
in separate packages that were mutually recursive.

You might think that a clever enough compiler could look at the actual
elaboration code and determine an appropriate correct order of elaboration,
but in the general case, this is not possible. Consider the following
example.

In the body of @code{Unit_1}, we have a procedure @code{Func_1}
that references
the variable @code{Sqrt_1}, which is declared in the elaboration code
of the body of @code{Unit_1}:

@smallexample
@cartouche
Sqrt_1 : Float := Sqrt (0.1);
@end cartouche
@end smallexample

@noindent
The elaboration code of the body of @code{Unit_1} also contains:

@smallexample
@group
@cartouche
@b{if} expression_1 = 1 @b{then}
   Q := Unit_2.Func_2;
@b{end if};
@end cartouche
@end group
@end smallexample

@noindent
@code{Unit_2} is exactly parallel,
it has a procedure @code{Func_2} that references
the variable @code{Sqrt_2}, which is declared in the elaboration code of
the body @code{Unit_2}:

@smallexample
@cartouche
Sqrt_2 : Float := Sqrt (0.1);
@end cartouche
@end smallexample

@noindent
The elaboration code of the body of @code{Unit_2} also contains:

@smallexample
@group
@cartouche
@b{if} expression_2 = 2 @b{then}
   Q := Unit_1.Func_1;
@b{end if};
@end cartouche
@end group
@end smallexample

@noindent
Now the question is, which of the following orders of elaboration is
acceptable:

@smallexample
@group
Spec of Unit_1
Spec of Unit_2
Body of Unit_1
Body of Unit_2
@end group
@end smallexample

@noindent
or

@smallexample
@group
Spec of Unit_2
Spec of Unit_1
Body of Unit_2
Body of Unit_1
@end group
@end smallexample

@noindent
If you carefully analyze the flow here, you will see that you cannot tell
at compile time the answer to this question.
If @code{expression_1} is not equal to 1,
and @code{expression_2} is not equal to 2,
then either order is acceptable, because neither of the function calls is
executed. If both tests evaluate to true, then neither order is acceptable
and in fact there is no correct order.

If one of the two expressions is true, and the other is false, then one
of the above orders is correct, and the other is incorrect. For example,
if @code{expression_1} = 1 and @code{expression_2} /= 2,
then the call to @code{Func_2}
will occur, but not the call to @code{Func_1.}
This means that it is essential
to elaborate the body of @code{Unit_1} before
the body of @code{Unit_2}, so the first
order of elaboration is correct and the second is wrong.

By making @code{expression_1} and @code{expression_2}
depend on input data, or perhaps
the time of day, we can make it impossible for the compiler or binder
to figure out which of these expressions will be true, and hence it
is impossible to guarantee a safe order of elaboration at run time.

@node Checking the Elaboration Order in Ada 95
@section Checking the Elaboration Order in Ada 95

@noindent
In some languages that involve the same kind of elaboration problems,
e.g. Java and C++, the programmer is expected to worry about these
ordering problems himself, and it is common to
write a program in which an incorrect elaboration order  gives
surprising results, because it references variables before they
are initialized.
Ada 95 is designed to be a safe language, and a programmer-beware approach is
clearly not sufficient. Consequently, the language provides three lines
of defense:

@table @asis
@item Standard rules
Some standard rules restrict the possible choice of elaboration
order. In particular, if you @code{with} a unit, then its spec is always
elaborated before the unit doing the @code{with}. Similarly, a parent
spec is always elaborated before the child spec, and finally
a spec is always elaborated before its corresponding body.

@item Dynamic elaboration checks
@cindex Elaboration checks
@cindex Checks, elaboration
Dynamic checks are made at run time, so that if some entity is accessed
before it is elaborated (typically  by means of a subprogram call)
then the exception (@code{Program_Error}) is raised.

@item Elaboration control
Facilities are provided for the programmer to specify the desired order
of elaboration.
@end table

Let's look at these facilities in more detail. First, the rules for
dynamic checking. One possible rule would be simply to say that the
exception is raised if you access a variable which has not yet been
elaborated. The trouble with this approach is that it could require
expensive checks on every variable reference. Instead Ada 95 has two
rules which are a little more restrictive, but easier to check, and
easier to state:

@table @asis
@item Restrictions on calls
A subprogram can only be called at elaboration time if its body
has been elaborated. The rules for elaboration given above guarantee
that the spec of the subprogram has been elaborated before the
call, but not the body. If this rule is violated, then the
exception @code{Program_Error} is raised.

@item Restrictions on instantiations
A generic unit can only be instantiated if the body of the generic
unit has been elaborated. Again, the rules for elaboration given above
guarantee that the spec of the generic unit has been elaborated
before the instantiation, but not the body. If this rule is
violated, then the exception @code{Program_Error} is raised.
@end table

@noindent
The idea is that if the body has been elaborated, then any variables
it references must have been elaborated; by checking for the body being
elaborated we guarantee that none of its references causes any
trouble. As we noted above, this is a little too restrictive, because a
subprogram that has no non-local references in its body may in fact be safe
to call. However, it really would be unsafe to rely on this, because
it would mean that the caller was aware of details of the implementation
in the body. This goes against the basic tenets of Ada.

A plausible implementation can be described as follows.
A Boolean variable is associated with each subprogram
and each generic unit. This variable is initialized to False, and is set to
True at the point body is elaborated. Every call or instantiation checks the
variable, and raises @code{Program_Error} if the variable is False.

Note that one might think that it would be good enough to have one Boolean
variable for each package, but that would not deal with cases of trying
to call a body in the same package as the call
that has not been elaborated yet.
Of course a compiler may be able to do enough analysis to optimize away
some of the Boolean variables as unnecessary, and @code{GNAT} indeed
does such optimizations, but still the easiest conceptual model is to
think of there being one variable per subprogram.

@node Controlling the Elaboration Order in Ada 95
@section Controlling the Elaboration Order in Ada 95

@noindent
In the previous section we discussed the rules in Ada 95 which ensure
that @code{Program_Error} is raised if an incorrect elaboration order is
chosen. This prevents erroneous executions, but we need mechanisms to
specify a correct execution and avoid the exception altogether.
To achieve this, Ada 95 provides a number of features for controlling
the order of elaboration. We discuss these features in this section.

First, there are several ways of indicating to the compiler that a given
unit has no elaboration problems:

@table @asis
@item packages that do not require a body
In Ada 95, a library package that does not require a body does not permit
a body. This means that if we have a such a package, as in:

@smallexample
@group
@cartouche
@b{package} Definitions @b{is}
   @b{generic}
      @b{type} m @b{is new} integer;
   @b{package} Subp @b{is}
      @b{type} a @b{is array} (1 .. 10) @b{of} m;
      @b{type} b @b{is array} (1 .. 20) @b{of} m;
   @b{end} Subp;
@b{end} Definitions;
@end cartouche
@end group
@end smallexample

@noindent
A package that @code{with}'s @code{Definitions} may safely instantiate
@code{Definitions.Subp} because the compiler can determine that there
definitely is no package body to worry about in this case

@item pragma Pure
@cindex pragma Pure
@findex Pure
Places sufficient restrictions on a unit to guarantee that
no call to any subprogram in the unit can result in an
elaboration problem. This means that the compiler does not need
to worry about the point of elaboration of such units, and in
particular, does not need to check any calls to any subprograms
in this unit.

@item pragma Preelaborate
@findex Preelaborate
@cindex pragma Preelaborate
This pragma places slightly less stringent restrictions on a unit than
does pragma Pure,
but these restrictions are still sufficient to ensure that there
are no elaboration problems with any calls to the unit.

@item pragma Elaborate_Body
@findex Elaborate_Body
@cindex pragma Elaborate_Body
This pragma requires that the body of a unit be elaborated immediately
after its spec. Suppose a unit @code{A} has such a pragma,
and unit @code{B} does
a @code{with} of unit @code{A}. Recall that the standard rules require
the spec of unit @code{A}
to be elaborated before the @code{with}'ing unit; given the pragma in
@code{A}, we also know that the body of @code{A}
will be elaborated before @code{B}, so
that calls to @code{A} are safe and do not need a check.
@end table

@noindent
Note that,
unlike pragma @code{Pure} and pragma @code{Preelaborate},
the use of
@code{Elaborate_Body} does not guarantee that the program is
free of elaboration problems, because it may not be possible
to satisfy the requested elaboration order.
Let's go back to the example with @code{Unit_1} and @code{Unit_2}.
If a programmer
marks @code{Unit_1} as @code{Elaborate_Body},
and not @code{Unit_2,} then the order of
elaboration will be:

@smallexample
@group
Spec of Unit_2
Spec of Unit_1
Body of Unit_1
Body of Unit_2
@end group
@end smallexample

@noindent
Now that means that the call to @code{Func_1} in @code{Unit_2}
need not be checked,
it must be safe. But the call to @code{Func_2} in
@code{Unit_1} may still fail if
@code{Expression_1} is equal to 1,
and the programmer must still take
responsibility for this not being the case.

If all units carry a pragma @code{Elaborate_Body}, then all problems are
eliminated, except for calls entirely within a body, which are
in any case fully under programmer control. However, using the pragma
everywhere is not always possible.
In particular, for our @code{Unit_1}/@code{Unit_2} example, if
we marked both of them as having pragma @code{Elaborate_Body}, then
clearly there would be no possible elaboration order.

The above pragmas allow a server to guarantee safe use by clients, and
clearly this is the preferable approach. Consequently a good rule in
Ada 95 is to mark units as @code{Pure} or @code{Preelaborate} if possible,
and if this is not possible,
mark them as @code{Elaborate_Body} if possible.
As we have seen, there are situations where neither of these
three pragmas can be used.
So we also provide methods for clients to control the
order of elaboration of the servers on which they depend:

@table @asis
@item pragma Elaborate (unit)
@findex Elaborate
@cindex pragma Elaborate
This pragma is placed in the context clause, after a @code{with} clause,
and it requires that the body of the named unit be elaborated before
the unit in which the pragma occurs. The idea is to use this pragma
if the current unit calls at elaboration time, directly or indirectly,
some subprogram in the named unit.

@item pragma Elaborate_All (unit)
@findex Elaborate_All
@cindex pragma Elaborate_All
This is a stronger version of the Elaborate pragma. Consider the
following example:

@smallexample
Unit A @code{with}'s unit B and calls B.Func in elab code
Unit B @code{with}'s unit C, and B.Func calls C.Func
@end smallexample

@noindent
Now if we put a pragma @code{Elaborate (B)}
in unit @code{A}, this ensures that the
body of @code{B} is elaborated before the call, but not the
body of @code{C}, so
the call to @code{C.Func} could still cause @code{Program_Error} to
be raised.

The effect of a pragma @code{Elaborate_All} is stronger, it requires
not only that the body of the named unit be elaborated before the
unit doing the @code{with}, but also the bodies of all units that the
named unit uses, following @code{with} links transitively. For example,
if we put a pragma @code{Elaborate_All (B)} in unit @code{A},
then it requires
not only that the body of @code{B} be elaborated before @code{A},
but also the
body of @code{C}, because @code{B} @code{with}'s @code{C}.
@end table

@noindent
We are now in a position to give a usage rule in Ada 95 for avoiding
elaboration problems, at least if dynamic dispatching and access to
subprogram values are not used. We will handle these cases separately
later.

The rule is simple. If a unit has elaboration code that can directly or
indirectly make a call to a subprogram in a @code{with}'ed unit, or instantiate
a generic unit in a @code{with}'ed unit,
then if the @code{with}'ed unit does not have
pragma @code{Pure} or @code{Preelaborate}, then the client should have
a pragma @code{Elaborate_All}
for the @code{with}'ed unit. By following this rule a client is
assured that calls can be made without risk of an exception.
If this rule is not followed, then a program may be in one of four
states:

@table @asis
@item No order exists
No order of elaboration exists which follows the rules, taking into
account any @code{Elaborate}, @code{Elaborate_All},
or @code{Elaborate_Body} pragmas. In
this case, an Ada 95 compiler must diagnose the situation at bind
time, and refuse to build an executable program.

@item One or more orders exist, all incorrect
One or more acceptable elaboration orders exists, and all of them
generate an elaboration order problem. In this case, the binder
can build an executable program, but @code{Program_Error} will be raised
when the program is run.

@item Several orders exist, some right, some incorrect
One or more acceptable elaboration orders exists, and some of them
work, and some do not. The programmer has not controlled
the order of elaboration, so the binder may or may not pick one of
the correct orders, and the program may or may not raise an
exception when it is run. This is the worst case, because it means
that the program may fail when moved to another compiler, or even
another version of the same compiler.

@item One or more orders exists, all correct
One ore more acceptable elaboration orders exist, and all of them
work. In this case the program runs successfully. This state of
affairs can be guaranteed by following the rule we gave above, but
may be true even if the rule is not followed.
@end table

@noindent
Note that one additional advantage of following our Elaborate_All rule
is that the program continues to stay in the ideal (all orders OK) state
even if maintenance
changes some bodies of some subprograms. Conversely, if a program that does
not follow this rule happens to be safe at some point, this state of affairs
may deteriorate silently as a result of maintenance changes.

You may have noticed that the above discussion did not mention
the use of @code{Elaborate_Body}. This was a deliberate omission. If you
@code{with} an @code{Elaborate_Body} unit, it still may be the case that
code in the body makes calls to some other unit, so it is still necessary
to use @code{Elaborate_All} on such units.

@node Controlling Elaboration in GNAT - Internal Calls
@section Controlling Elaboration in GNAT - Internal Calls

@noindent
In the case of internal calls, i.e. calls within a single package, the
programmer has full control over the order of elaboration, and it is up
to the programmer to elaborate declarations in an appropriate order. For
example writing:

@smallexample
@group
@cartouche
@b{function} One @b{return} Float;

Q : Float := One;

@b{function} One @b{return} Float @b{is}
@b{begin}
     return 1.0;
@b{end} One;
@end cartouche
@end group
@end smallexample

@noindent
will obviously raise @code{Program_Error} at run time, because function
One will be called before its body is elaborated. In this case GNAT will
generate a warning that the call will raise @code{Program_Error}:

@smallexample
@group
@cartouche
 1. procedure y is
 2.    function One return Float;
 3.
 4.    Q : Float := One;
                    |
    >>> warning: cannot call "One" before body is elaborated
    >>> warning: Program_Error will be raised at run time

 5.
 6.    function One return Float is
 7.    begin
 8.         return 1.0;
 9.    end One;
10.
11. begin
12.    null;
13. end;
@end cartouche
@end group
@end smallexample

@noindent
Note that in this particular case, it is likely that the call is safe, because
the function @code{One} does not access any global variables.
Nevertheless in Ada 95, we do not want the validity of the check to depend on
the contents of the body (think about the separate compilation case), so this
is still wrong, as we discussed in the previous sections.

The error is easily corrected by rearranging the declarations so that the
body of One appears before the declaration containing the call
(note that in Ada 95,
declarations can appear in any order, so there is no restriction that
would prevent this reordering, and if we write:

@smallexample
@group
@cartouche
@b{function} One @b{return} Float;

@b{function} One @b{return} Float @b{is}
@b{begin}
     return 1.0;
@b{end} One;

Q : Float := One;
@end cartouche
@end group
@end smallexample

@noindent
then all is well, no warning is generated, and no
@code{Program_Error} exception
will be raised.
Things are more complicated when a chain of subprograms is executed:

@smallexample
@group
@cartouche
@b{function} A @b{return} Integer;
@b{function} B @b{return} Integer;
@b{function} C @b{return} Integer;

@b{function} B @b{return} Integer @b{is begin return} A; @b{end};
@b{function} C @b{return} Integer @b{is begin return} B; @b{end};

X : Integer := C;

@b{function} A @b{return} Integer @b{is begin return} 1; @b{end};
@end cartouche
@end group
@end smallexample

@noindent
Now the call to @code{C}
at elaboration time in the declaration of @code{X} is correct, because
the body of @code{C} is already elaborated,
and the call to @code{B} within the body of
@code{C} is correct, but the call
to @code{A} within the body of @code{B} is incorrect, because the body
of @code{A} has not been elaborated, so @code{Program_Error}
will be raised on the call to @code{A}.
In this case GNAT will generate a
warning that @code{Program_Error} may be
raised at the point of the call. Let's look at the warning:

@smallexample
@group
@cartouche
 1. procedure x is
 2.    function A return Integer;
 3.    function B return Integer;
 4.    function C return Integer;
 5.
 6.    function B return Integer is begin return A; end;
                                                    |
    >>> warning: call to "A" before body is elaborated may
                 raise Program_Error
    >>> warning: "B" called at line 7
    >>> warning: "C" called at line 9

 7.    function C return Integer is begin return B; end;
 8.
 9.    X : Integer := C;
10.
11.    function A return Integer is begin return 1; end;
12.
13. begin
14.    null;
15. end;
@end cartouche
@end group
@end smallexample

@noindent
Note that the message here says "may raise", instead of the direct case,
where the message says "will be raised". That's because whether
@code{A} is
actually called depends in general on run-time flow of control.
For example, if the body of @code{B} said

@smallexample
@group
@cartouche
@b{function} B @b{return} Integer @b{is}
@b{begin}
   @b{if} some-condition-depending-on-input-data @b{then}
      @b{return} A;
   @b{else}
      @b{return} 1;
   @b{end if};
@b{end} B;
@end cartouche
@end group
@end smallexample

@noindent
then we could not know until run time whether the incorrect call to A would
actually occur, so @code{Program_Error} might
or might not be raised. It is possible for a compiler to
do a better job of analyzing bodies, to
determine whether or not @code{Program_Error}
might be raised, but it certainly
couldn't do a perfect job (that would require solving the halting problem
and is provably impossible), and because this is a warning anyway, it does
not seem worth the effort to do the analysis. Cases in which it
would be relevant are rare.

In practice, warnings of either of the forms given
above will usually correspond to
real errors, and should be examined carefully and eliminated.
In the rare case where a warning is bogus, it can be suppressed by any of
the following methods:

@itemize @bullet
@item
Compile with the @option{/WARNINGS=SUPPRESS} qualifier set

@item
Suppress @code{Elaboration_Checks} for the called subprogram

@item
Use pragma @code{Warnings_Off} to turn warnings off for the call
@end itemize

@noindent
For the internal elaboration check case,
GNAT by default generates the
necessary run-time checks to ensure
that @code{Program_Error} is raised if any
call fails an elaboration check. Of course this can only happen if a
warning has been issued as described above. The use of pragma
@code{Suppress (Elaboration_Checks)} may (but is not guaranteed to) suppress
some of these checks, meaning that it may be possible (but is not
guaranteed) for a program to be able to call a subprogram whose body
is not yet elaborated, without raising a @code{Program_Error} exception.

@node Controlling Elaboration in GNAT - External Calls
@section Controlling Elaboration in GNAT - External Calls

@noindent
The previous section discussed the case in which the execution of a
particular thread of elaboration code occurred entirely within a
single unit. This is the easy case to handle, because a programmer
has direct and total control over the order of elaboration, and
furthermore, checks need only be generated in cases which are rare
and which the compiler can easily detect.
The situation is more complex when separate compilation is taken into account.
Consider the following:

@smallexample
@cartouche
@group
@b{package} Math @b{is}
   @b{function} Sqrt (Arg : Float) @b{return} Float;
@b{end} Math;

@b{package body} Math @b{is}
   @b{function} Sqrt (Arg : Float) @b{return} Float @b{is}
   @b{begin}
         ...
   @b{end} Sqrt;
@b{end} Math;
@end group
@group
@b{with} Math;
@b{package} Stuff @b{is}
   X : Float := Math.Sqrt (0.5);
@b{end} Stuff;

@b{with} Stuff;
@b{procedure} Main @b{is}
@b{begin}
   ...
@b{end} Main;
@end group
@end cartouche
@end smallexample

@noindent
where @code{Main} is the main program. When this program is executed, the
elaboration code must first be executed, and one of the jobs of the
binder is to determine the order in which the units of a program are
to be elaborated. In this case we have four units: the spec and body
of @code{Math},
the spec of @code{Stuff} and the body of @code{Main}).
In what order should the four separate sections of elaboration code
be executed?

There are some restrictions in the order of elaboration that the binder
can choose. In particular, if unit U has a @code{with}
for a package @code{X}, then you
are assured that the spec of @code{X}
is elaborated before U , but you are
not assured that the body of @code{X}
is elaborated before U.
This means that in the above case, the binder is allowed to choose the
order:

@smallexample
spec of Math
spec of Stuff
body of Math
body of Main
@end smallexample

@noindent
but that's not good, because now the call to @code{Math.Sqrt}
that happens during
the elaboration of the @code{Stuff}
spec happens before the body of @code{Math.Sqrt} is
elaborated, and hence causes @code{Program_Error} exception to be raised.
At first glance, one might say that the binder is misbehaving, because
obviously you want to elaborate the body of something you @code{with}
first, but
that is not a general rule that can be followed in all cases. Consider

@smallexample
@group
@cartouche
@b{package} X @b{is} ...

@b{package} Y @b{is} ...

@b{with} X;
@b{package body} Y @b{is} ...

@b{with} Y;
@b{package body} X @b{is} ...
@end cartouche
@end group
@end smallexample

@noindent
This is a common arrangement, and, apart from the order of elaboration
problems that might arise in connection with elaboration code, this works fine.
A rule that says that you must first elaborate the body of anything you
@code{with} cannot work in this case:
the body of @code{X} @code{with}'s @code{Y},
which means you would have to
elaborate the body of @code{Y} first, but that @code{with}'s @code{X},
which means
you have to elaborate the body of @code{X} first, but ... and we have a
loop that cannot be broken.

It is true that the binder can in many cases guess an order of elaboration
that is unlikely to cause a @code{Program_Error}
exception to be raised, and it tries to do so (in the
above example of @code{Math/Stuff/Spec}, the GNAT binder will
by default
elaborate the body of @code{Math} right after its spec, so all will be well).

However, a program that blindly relies on the binder to be helpful can
get into trouble, as we discussed in the previous sections, so
GNAT
provides a number of facilities for assisting the programmer in
developing programs that are robust with respect to elaboration order.

@node Default Behavior in GNAT - Ensuring Safety
@section Default Behavior in GNAT - Ensuring Safety

@noindent
The default behavior in GNAT ensures elaboration safety. In its
default mode GNAT implements the
rule we previously described as the right approach. Let's restate it:

@itemize
@item
@emph{If a unit has elaboration code that can directly or indirectly make a
call to a subprogram in a @code{with}'ed unit, or instantiate a generic unit
in a @code{with}'ed unit, then if the @code{with}'ed unit
does not have pragma @code{Pure} or
@code{Preelaborate}, then the client should have an
@code{Elaborate_All} for the @code{with}'ed unit.}
@end itemize

@noindent
By following this rule a client
is assured that calls and instantiations can be made without risk of an exception.

In this mode GNAT traces all calls that are potentially made from
elaboration code, and puts in any missing implicit @code{Elaborate_All}
pragmas.
The advantage of this approach is that no elaboration problems
are possible if the binder can find an elaboration order that is
consistent with these implicit @code{Elaborate_All} pragmas. The
disadvantage of this approach is that no such order may exist.

If the binder does not generate any diagnostics, then it means that it
has found an elaboration order that is guaranteed to be safe. However,
the binder may still be relying on implicitly generated
@code{Elaborate_All} pragmas so portability to other compilers than
GNAT is not guaranteed.

If it is important to guarantee portability, then the compilations should
use the
@option{/WARNINGS=ELABORATION}
(warn on elaboration problems) qualifier. This will cause warning messages
to be generated indicating the missing @code{Elaborate_All} pragmas.
Consider the following source program:

@smallexample
@group
@cartouche
@b{with} k;
@b{package} j @b{is}
  m : integer := k.r;
@b{end};
@end cartouche
@end group
@end smallexample

@noindent
where it is clear that there
should be a pragma @code{Elaborate_All}
for unit @code{k}. An implicit pragma will be generated, and it is
likely that the binder will be able to honor it. However,
it is safer to include the pragma explicitly in the source. If this
unit is compiled with the
@option{/WARNINGS=ELABORATION}
qualifier, then the compiler outputs a warning:

@smallexample
@group
@cartouche
1. with k;
2. package j is
3.   m : integer := k.r;
                     |
   >>> warning: call to "r" may raise Program_Error
   >>> warning: missing pragma Elaborate_All for "k"

4. end;
@end cartouche
@end group
@end smallexample

@noindent
and these warnings can be used as a guide for supplying manually
the missing pragmas.

This default mode is more restrictive than the Ada Reference
Manual, and it is possible to construct programs which will compile
using the dynamic model described there, but will run into a
circularity using the safer static model we have described.

Of course any Ada compiler must be able to operate in a mode
consistent with the requirements of the Ada Reference Manual,
and in particular must have the capability of implementing the
standard dynamic model of elaboration with run-time checks.

In GNAT, this standard mode can be achieved either by the use of
the @option{/CHECKS=ELABORATION} qualifier on the compiler (@code{GNAT COMPILE} or @code{GNAT MAKE})
command, or by the use of the configuration pragma:

@smallexample
pragma Elaboration_Checks (RM);
@end smallexample

@noindent
Either approach will cause the unit affected to be compiled using the
standard dynamic run-time elaboration checks described in the Ada
Reference Manual. The static model is generally preferable, since it
is clearly safer to rely on compile and link time checks rather than
run-time checks. However, in the case of legacy code, it may be
difficult to meet the requirements of the static model. This
issue is further discussed in
@ref{What to Do If the Default Elaboration Behavior Fails}.

Note that the static model provides a strict subset of the allowed
behavior and programs of the Ada Reference Manual, so if you do
adhere to the static model and no circularities exist,
then you are assured that your program will
work using the dynamic model.

@node Elaboration Issues for Library Tasks
@section Elaboration Issues for Library Tasks
@cindex Library tasks, elaboration issues
@cindex Elaboration of library tasks

@noindent
In this section we examine special elaboration issues that arise for
programs that declare library level tasks.

Generally the model of execution of an Ada program is that all units are
elaborated, and then execution of the program starts. However, the
declaration of library tasks definitely does not fit this model. The
reason for this is that library tasks start as soon as they are declared
(more precisely, as soon as the statement part of the enclosing package
body is reached), that is to say before elaboration
of the program is complete. This means that if such a task calls a
subprogram, or an entry in another task, the callee may or may not be
elaborated yet, and in the standard
Reference Manual model of dynamic elaboration checks, you can even
get timing dependent Program_Error exceptions, since there can be
a race between the elaboration code and the task code.

The static model of elaboration in GNAT seeks to avoid all such
dynamic behavior, by being conservative, and the conservative
approach in this particular case is to assume that all the code
in a task body is potentially executed at elaboration time if
a task is declared at the library level.

This can definitely result in unexpected circularities. Consider
the following example

@smallexample
package Decls is
  task Lib_Task is
     entry Start;
  end Lib_Task;

  type My_Int is new Integer;

  function Ident (M : My_Int) return My_Int;
end Decls;

with Utils;
package body Decls is
  task body Lib_Task is
  begin
     accept Start;
     Utils.Put_Val (2);
  end Lib_Task;

  function Ident (M : My_Int) return My_Int is
  begin
     return M;
  end Ident;
end Decls;

with Decls;
package Utils is
  procedure Put_Val (Arg : Decls.My_Int);
end Utils;

with Text_IO;
package body Utils is
  procedure Put_Val (Arg : Decls.My_Int) is
  begin
     Text_IO.Put_Line (Decls.My_Int'Image (Decls.Ident (Arg)));
  end Put_Val;
end Utils;

with Decls;
procedure Main is
begin
   Decls.Lib_Task.Start;
end;
@end smallexample

@noindent
If the above example is compiled in the default static elaboration
mode, then a circularity occurs. The circularity comes from the call
@code{Utils.Put_Val} in the task body of @code{Decls.Lib_Task}. Since
this call occurs in elaboration code, we need an implicit pragma
@code{Elaborate_All} for @code{Utils}. This means that not only must
the spec and body of @code{Utils} be elaborated before the body
of @code{Decls}, but also the spec and body of any unit that is
@code{with'ed} by the body of @code{Utils} must also be elaborated before
the body of @code{Decls}. This is the transitive implication of
pragma @code{Elaborate_All} and it makes sense, because in general
the body of @code{Put_Val} might have a call to something in a
@code{with'ed} unit.

In this case, the body of Utils (actually its spec) @code{with's}
@code{Decls}. Unfortunately this means that the body of @code{Decls}
must be elaborated before itself, in case there is a call from the
body of @code{Utils}.

Here is the exact chain of events we are worrying about:

@enumerate
@item
In the body of @code{Decls} a call is made from within the body of a library
task to a subprogram in the package @code{Utils}. Since this call may
occur at elaboration time (given that the task is activated at elaboration
time), we have to assume the worst, i.e. that the
call does happen at elaboration time.

@item
This means that the body and spec of @code{Util} must be elaborated before
the body of @code{Decls} so that this call does not cause an access before
elaboration.

@item
Within the body of @code{Util}, specifically within the body of
@code{Util.Put_Val} there may be calls to any unit @code{with}'ed
by this package.

@item
One such @code{with}'ed package is package @code{Decls}, so there
might be a call to a subprogram in @code{Decls} in @code{Put_Val}.
In fact there is such a call in this example, but we would have to
assume that there was such a call even if it were not there, since
we are not supposed to write the body of @code{Decls} knowing what
is in the body of @code{Utils}; certainly in the case of the
static elaboration model, the compiler does not know what is in
other bodies and must assume the worst.

@item
This means that the spec and body of @code{Decls} must also be
elaborated before we elaborate the unit containing the call, but
that unit is @code{Decls}! This means that the body of @code{Decls}
must be elaborated before itself, and that's a circularity.
@end enumerate

@noindent
Indeed, if you add an explicit pragma Elaborate_All for @code{Utils} in
the body of @code{Decls} you will get a true Ada Reference Manual
circularity that makes the program illegal.

In practice, we have found that problems with the static model of
elaboration in existing code often arise from library tasks, so
we must address this particular situation.

Note that if we compile and run the program above, using the dynamic model of
elaboration (that is to say use the @option{/CHECKS=ELABORATION} qualifier),
then it compiles, binds,
links, and runs, printing the expected result of 2. Therefore in some sense
the circularity here is only apparent, and we need to capture
the properties of this program that  distinguish it from other library-level
tasks that have real elaboration problems.

We have four possible answers to this question:

@itemize @bullet

@item
Use the dynamic model of elaboration.

If we use the @option{/CHECKS=ELABORATION} qualifier, then as noted above, the program works.
Why is this? If we examine the task body, it is apparent that the task cannot
proceed past the
@code{accept} statement until after elaboration has been completed, because
the corresponding entry call comes from the main program, not earlier.
This is why the dynamic model works here. But that's really giving
up on a precise analysis, and we prefer to take this approach only if we cannot
solve the
problem in any other manner. So let us examine two ways to reorganize
the program to avoid the potential elaboration problem.

@item
Split library tasks into separate packages.

Write separate packages, so that library tasks are isolated from
other declarations as much as possible. Let us look at a variation on
the above program.

@smallexample
package Decls1 is
  task Lib_Task is
     entry Start;
  end Lib_Task;
end Decls1;

with Utils;
package body Decls1 is
  task body Lib_Task is
  begin
     accept Start;
     Utils.Put_Val (2);
  end Lib_Task;
end Decls1;

package Decls2 is
  type My_Int is new Integer;
  function Ident (M : My_Int) return My_Int;
end Decls2;

with Utils;
package body Decls2 is
  function Ident (M : My_Int) return My_Int is
  begin
     return M;
  end Ident;
end Decls2;

with Decls2;
package Utils is
  procedure Put_Val (Arg : Decls2.My_Int);
end Utils;

with Text_IO;
package body Utils is
  procedure Put_Val (Arg : Decls2.My_Int) is
  begin
     Text_IO.Put_Line (Decls2.My_Int'Image (Decls2.Ident (Arg)));
  end Put_Val;
end Utils;

with Decls1;
procedure Main is
begin
   Decls1.Lib_Task.Start;
end;
@end smallexample

@noindent
All we have done is to split @code{Decls} into two packages, one
containing the library task, and one containing everything else. Now
there is no cycle, and the program compiles, binds, links and executes
using the default static model of elaboration.

@item
Declare separate task types.

A significant part of the problem arises because of the use of the
single task declaration form. This means that the elaboration of
the task type, and the elaboration of the task itself (i.e. the
creation of the task) happen at the same time. A good rule
of style in Ada 95 is to always create explicit task types. By
following the additional step of placing task objects in separate
packages from the task type declaration, many elaboration problems
are avoided. Here is another modified example of the example program:

@smallexample
package Decls is
  task type Lib_Task_Type is
     entry Start;
  end Lib_Task_Type;

  type My_Int is new Integer;

  function Ident (M : My_Int) return My_Int;
end Decls;

with Utils;
package body Decls is
  task body Lib_Task_Type is
  begin
     accept Start;
     Utils.Put_Val (2);
  end Lib_Task_Type;

  function Ident (M : My_Int) return My_Int is
  begin
     return M;
  end Ident;
end Decls;

with Decls;
package Utils is
  procedure Put_Val (Arg : Decls.My_Int);
end Utils;

with Text_IO;
package body Utils is
  procedure Put_Val (Arg : Decls.My_Int) is
  begin
     Text_IO.Put_Line (Decls.My_Int'Image (Decls.Ident (Arg)));
  end Put_Val;
end Utils;

with Decls;
package Declst is
   Lib_Task : Decls.Lib_Task_Type;
end Declst;

with Declst;
procedure Main is
begin
   Declst.Lib_Task.Start;
end;
@end smallexample

@noindent
What we have done here is to replace the @code{task} declaration in
package @code{Decls} with a @code{task type} declaration. Then we
introduce a separate package @code{Declst} to contain the actual
task object. This separates the elaboration issues for
the @code{task type}
declaration, which causes no trouble, from the elaboration issues
of the task object, which is also unproblematic, since it is now independent
of the elaboration of  @code{Utils}.
This separation of concerns also corresponds to
a generally sound engineering principle of separating declarations
from instances. This version of the program also compiles, binds, links,
and executes, generating the expected output.

@item
Use No_Entry_Calls_In_Elaboration_Code restriction.
@cindex No_Entry_Calls_In_Elaboration_Code

The previous two approaches described how a program can be restructured
to avoid the special problems caused by library task bodies. in practice,
however, such restructuring may be difficult to apply to existing legacy code,
so we must consider solutions that do not require massive rewriting.

Let us consider more carefully why our original sample program works
under the dynamic model of elaboration. The reason is that the code
in the task body blocks immediately on the @code{accept}
statement. Now of course there is nothing to prohibit elaboration
code from making entry calls (for example from another library level task),
so we cannot tell in isolation that
the task will not execute the accept statement  during elaboration.

However, in practice it is very unusual to see elaboration code
make any entry calls, and the pattern of tasks starting
at elaboration time and then immediately blocking on @code{accept} or
@code{select} statements is very common. What this means is that
the compiler is being too pessimistic when it analyzes the
whole package body as though it might be executed at elaboration
time.

If we know that the elaboration code contains no entry calls, (a very safe
assumption most of the time, that could almost be made the default
behavior), then we can compile all units of the program under control
of the following configuration pragma:

@smallexample
pragma Restrictions (No_Entry_Calls_In_Elaboration_Code);
@end smallexample

@noindent
This pragma can be placed in the @file{GNAT.ADC} file in the usual
manner. If we take our original unmodified program and compile it
in the presence of a @file{GNAT.ADC} containing the above pragma,
then once again, we can compile, bind, link, and execute, obtaining
the expected result. In the presence of this pragma, the compiler does
not trace calls in a task body, that appear after the first @code{accept}
or @code{select} statement, and therefore does not report a potential
circularity in the original program.

The compiler will check to the extent it can that the above
restriction is not violated, but it is not always possible to do a
complete check at compile time, so it is important to use this
pragma only if the stated restriction is in fact met, that is to say
no task receives an entry call before elaboration of all units is completed.

@end itemize

@node Mixing Elaboration Models
@section Mixing Elaboration Models
@noindent
So far, we have assumed that the entire program is either compiled
using the dynamic model or static model, ensuring consistency. It
is possible to mix the two models, but rules have to be followed
if this mixing is done to ensure that elaboration checks are not
omitted.

The basic rule is that @emph{a unit compiled with the static model cannot
be @code{with'ed} by a unit compiled with the dynamic model}. The
reason for this is that in the static model, a unit assumes that
its clients guarantee to use (the equivalent of) pragma
@code{Elaborate_All} so that no elaboration checks are required
in inner subprograms, and this assumption is violated if the
client is compiled with dynamic checks.

The precise rule is as follows. A unit that is compiled with dynamic
checks can only @code{with} a unit that meets at least one of the
following criteria:

@itemize @bullet

@item
The @code{with'ed} unit is itself compiled with dynamic elaboration
checks (that is with the @option{/CHECKS=ELABORATION} qualifier.

@item
The @code{with'ed} unit is an internal GNAT implementation unit from
the System, Interfaces, Ada, or GNAT hierarchies.

@item
The @code{with'ed} unit has pragma Preelaborate or pragma Pure.

@item
The @code{with'ing} unit (that is the client) has an explicit pragma
@code{Elaborate_All} for the @code{with'ed} unit.

@end itemize

@noindent
If this rule is violated, that is if a unit with dynamic elaboration
checks @code{with's} a unit that does not meet one of the above four
criteria, then the binder (@code{GNAT BIND}) will issue a warning
similar to that in the following example:

@smallexample
warning: "X.ADS" has dynamic elaboration checks and with's
warning:   "Y.ADS" which has static elaboration checks
@end smallexample

@noindent
These warnings indicate that the rule has been violated, and that as a result
elaboration checks may be missed in the resulting executable file.
This warning may be suppressed using the @code{-ws} binder qualifier
in the usual manner.

One useful application of this mixing rule is in the case of a subsystem
which does not itself @code{with} units from the remainder of the
application. In this case, the entire subsystem can be compiled with
dynamic checks to resolve a circularity in the subsystem, while
allowing the main application that uses this subsystem to be compiled
using the more reliable default static model.

@node What to Do If the Default Elaboration Behavior Fails
@section What to Do If the Default Elaboration Behavior Fails

@noindent
If the binder cannot find an acceptable order, it outputs detailed
diagnostics. For example:
@smallexample
@group
@iftex
@leftskip=0cm
@end iftex
error: elaboration circularity detected
info:   "proc (body)" must be elaborated before "pack (body)"
info:     reason: Elaborate_All probably needed in unit "pack (body)"
info:     recompile "pack (body)" with /WARNINGS=ELABORATION
info:                             for full details
info:       "proc (body)"
info:         is needed by its spec:
info:       "proc (spec)"
info:         which is withed by:
info:       "pack (body)"
info:  "pack (body)" must be elaborated before "proc (body)"
info:     reason: pragma Elaborate in unit "proc (body)"
@end group

@end smallexample

@noindent
In this case we have a cycle that the binder cannot break. On the one
hand, there is an explicit pragma Elaborate in @code{proc} for
@code{pack}. This means that the body of @code{pack} must be elaborated
before the body of @code{proc}. On the other hand, there is elaboration
code in @code{pack} that calls a subprogram in @code{proc}. This means
that for maximum safety, there should really be a pragma
Elaborate_All in @code{pack} for @code{proc} which would require that
the body of @code{proc} be elaborated before the body of
@code{pack}. Clearly both requirements cannot be satisfied.
Faced with a circularity of this kind, you have three different options.

@table @asis
@item Fix the program
The most desirable option from the point of view of long-term maintenance
is to rearrange the program so that the elaboration problems are avoided.
One useful technique is to place the elaboration code into separate
child packages. Another is to move some of the initialization code to
explicitly called subprograms, where the program controls the order
of initialization explicitly. Although this is the most desirable option,
it may be impractical and involve too much modification, especially in
the case of complex legacy code.

@item Perform dynamic checks
If the compilations are done using the
@option{/CHECKS=ELABORATION}
(dynamic elaboration check) qualifier, then GNAT behaves in
a quite different manner. Dynamic checks are generated for all calls
that could possibly result in raising an exception. With this qualifier,
the compiler does not generate implicit @code{Elaborate_All} pragmas.
The behavior then is exactly as specified in the Ada 95 Reference Manual.
The binder will generate an executable program that may or may not
raise @code{Program_Error}, and then it is the programmer's job to ensure
that it does not raise an exception. Note that it is important to
compile all units with the qualifier, it cannot be used selectively.

@item Suppress checks
The drawback of dynamic checks is that they generate a
significant overhead at run time, both in space and time. If you
are absolutely sure that your program cannot raise any elaboration
exceptions, and you still want to use the dynamic elaboration model,
then you can use the configuration pragma
@code{Suppress (Elaboration_Checks)} to suppress all such checks. For
example this pragma could be placed in the @file{GNAT.ADC} file.

@item Suppress checks selectively
When you know that certain calls in elaboration code cannot possibly
lead to an elaboration error, and the binder nevertheless generates warnings
on those calls and inserts Elaborate_All pragmas that lead to elaboration
circularities, it is possible to remove those warnings locally and obtain
a program that will bind. Clearly this can be unsafe, and it is the
responsibility of the programmer to make sure that the resulting program has
no elaboration anomalies. The pragma @code{Suppress (Elaboration_Check)} can
be used with different granularity to suppress warnings and break
elaboration circularities:

@itemize @bullet
@item
Place the pragma that names the called subprogram in the declarative part
that contains the call.

@item
Place the pragma in the declarative part, without naming an entity. This
disables warnings on all calls in the corresponding  declarative region.

@item
Place the pragma in the package spec that declares the called subprogram,
and name the subprogram. This disables warnings on all elaboration calls to
that subprogram.

@item
Place the pragma in the package spec that declares the called subprogram,
without naming any entity. This disables warnings on all elaboration calls to
all subprograms declared in this spec.
@end itemize

@noindent
These four cases are listed in order of decreasing safety, and therefore
require increasing programmer care in their application. Consider the
following program:
@smallexample

package Pack1 is
  function F1 return Integer;
  X1 : Integer;
end Pack1;

package Pack2 is
  function F2 return Integer;
  function Pure (x : integer) return integer;
  --  pragma Suppress (Elaboration_Check, On => Pure);  -- (3)
  --  pragma Suppress (Elaboration_Check);              -- (4)
end Pack2;

with Pack2;
package body Pack1 is
  function F1 return Integer is
  begin
    return 100;
  end F1;
  Val : integer := Pack2.Pure (11);    --  Elab. call (1)
begin
  declare
    --  pragma Suppress(Elaboration_Check, Pack2.F2);   -- (1)
    --  pragma Suppress(Elaboration_Check);             -- (2)
  begin
    X1 := Pack2.F2 + 1;                --  Elab. call (2)
  end;
end Pack1;

with Pack1;
package body Pack2 is
  function F2 return Integer is
  begin
     return Pack1.F1;
  end F2;
  function Pure (x : integer) return integer is
  begin
     return x ** 3 - 3 * x;
  end;
end Pack2;

with Pack1, Ada.Text_IO;
procedure Proc3 is
begin
  Ada.Text_IO.Put_Line(Pack1.X1'Img); -- 101
end Proc3;
@end smallexample
In the absence of any pragmas, an attempt to bind this program produces
the following diagnostics:
@smallexample
@group
@iftex
@leftskip=.5cm
@end iftex
error: elaboration circularity detected
info:    "pack1 (body)" must be elaborated before "pack1 (body)"
info:       reason: Elaborate_All probably needed in unit "pack1 (body)"
info:       recompile "pack1 (body)" with /WARNINGS=ELABORATION for full details
info:          "pack1 (body)"
info:             must be elaborated along with its spec:
info:          "pack1 (spec)"
info:             which is withed by:
info:          "pack2 (body)"
info:             which must be elaborated along with its spec:
info:          "pack2 (spec)"
info:             which is withed by:
info:          "pack1 (body)"
@end group
@end smallexample
The sources of the circularity are the two calls to @code{Pack2.Pure} and
@code{Pack2.F2} in the body of @code{Pack1}. We can see that the call to
F2 is safe, even though F2 calls F1, because the call appears after the
elaboration of the body of F1. Therefore the pragma (1) is safe, and will
remove the warning on the call. It is also possible to use pragma (2)
because there are no other potentially unsafe calls in the block.

@noindent
The call to @code{Pure} is safe because this function does not depend on the
state of @code{Pack2}. Therefore any call to this function is safe, and it
is correct to place pragma (3) in the corresponding package spec.

@noindent
Finally, we could place pragma (4) in the spec of @code{Pack2} to disable
warnings on all calls to functions declared therein. Note that this is not
necessarily safe, and requires more detailed examination of the subprogram
bodies involved. In particular, a call to @code{F2} requires that @code{F1}
be already elaborated.
@end table

@noindent
It is hard to generalize on which of these four approaches should be
taken. Obviously if it is possible to fix the program so that the default
treatment works, this is preferable, but this may not always be practical.
It is certainly simple enough to use
@option{/CHECKS=ELABORATION}
but the danger in this case is that, even if the GNAT binder
finds a correct elaboration order, it may not always do so,
and certainly a binder from another Ada compiler might not. A
combination of testing and analysis (for which the warnings generated
with the
@option{/WARNINGS=ELABORATION}
qualifier can be useful) must be used to ensure that the program is free
of errors. One qualifier that is useful in this testing is the
@code{/PESSIMISTIC_ELABORATION_ORDER}
qualifier for
@code{GNAT BIND}.
Normally the binder tries to find an order that has the best chance of
of avoiding elaboration problems. With this qualifier, the binder
plays a devil's advocate role, and tries to choose the order that
has the best chance of failing. If your program works even with this
qualifier, then it has a better chance of being error free, but this is still
not a guarantee.

For an example of this approach in action, consider the C-tests (executable
tests) from the ACVC suite. If these are compiled and run with the default
treatment, then all but one of them succeed without generating any error
diagnostics from the binder. However, there is one test that fails, and
this is not surprising, because the whole point of this test is to ensure
that the compiler can handle cases where it is impossible to determine
a correct order statically, and it checks that an exception is indeed
raised at run time.

This one test must be compiled and run using the
@option{/CHECKS=ELABORATION}
qualifier, and then it passes. Alternatively, the entire suite can
be run using this qualifier. It is never wrong to run with the dynamic
elaboration qualifier if your code is correct, and we assume that the
C-tests are indeed correct (it is less efficient, but efficiency is
not a factor in running the ACVC tests.)

@node Elaboration for Access-to-Subprogram Values
@section Elaboration for Access-to-Subprogram Values
@cindex Access-to-subprogram

@noindent
The introduction of access-to-subprogram types in Ada 95 complicates
the handling of elaboration. The trouble is that it becomes
impossible to tell at compile time which procedure
is being called. This means that it is not possible for the binder
to analyze the elaboration requirements in this case.

If at the point at which the access value is created
(i.e., the evaluation of @code{P'Access} for a subprogram @code{P}),
the body of the subprogram is
known to have been elaborated, then the access value is safe, and its use
does not require a check. This may be achieved by appropriate arrangement
of the order of declarations if the subprogram is in the current unit,
or, if the subprogram is in another unit, by using pragma
@code{Pure}, @code{Preelaborate}, or @code{Elaborate_Body}
on the referenced unit.

If the referenced body is not known to have been elaborated at the point
the access value is created, then any use of the access value must do a
dynamic check, and this dynamic check will fail and raise a
@code{Program_Error} exception if the body has not been elaborated yet.
GNAT will generate the necessary checks, and in addition, if the
@option{/WARNINGS=ELABORATION}
qualifier is set, will generate warnings that such checks are required.

The use of dynamic dispatching for tagged types similarly generates
a requirement for dynamic checks, and premature calls to any primitive
operation of a tagged type before the body of the operation has been elaborated,
will result in the raising of @code{Program_Error}.

@node Summary of Procedures for Elaboration Control
@section Summary of Procedures for Elaboration Control
@cindex Elaboration control

@noindent
First, compile your program with the default options, using none of
the special elaboration control qualifiers. If the binder successfully
binds your program, then you can be confident that, apart from issues
raised by the use of access-to-subprogram types and dynamic dispatching,
the program is free of elaboration errors. If it is important that the
program be portable, then use the
@option{/WARNINGS=ELABORATION}
qualifier to generate warnings about missing @code{Elaborate_All}
pragmas, and supply the missing pragmas.

If the program fails to bind using the default static elaboration
handling, then you can fix the program to eliminate the binder
message, or recompile the entire program with the
@option{/CHECKS=ELABORATION} qualifier to generate dynamic elaboration checks,
and, if you are sure there really are no elaboration problems,
use a global pragma @code{Suppress (Elaboration_Checks)}.

@node Other Elaboration Order Considerations
@section Other Elaboration Order Considerations
@noindent
This section has been entirely concerned with the issue of finding a valid
elaboration order, as defined by the Ada Reference Manual. In a case
where several elaboration orders are valid, the task is to find one
of the possible valid elaboration orders (and the static model in GNAT
will ensure that this is achieved).

The purpose of the elaboration rules in the Ada Reference Manual is to
make sure that no entity is accessed before it has been elaborated. For
a subprogram, this means that the spec and body must have been elaborated
before the subprogram is called. For an object, this means that the object
must have been elaborated before its value is read or written. A violation
of either of these two requirements is an access before elaboration order,
and this section has been all about avoiding such errors.

In the case where more than one order of elaboration is possible, in the
sense that access before elaboration errors are avoided, then any one of
the orders is "correct" in the sense that it meets the requirements of
the Ada Reference Manual, and no such error occurs.

However, it may be the case for a given program, that there are
constraints on the order of elaboration that come not from consideration
of avoiding elaboration errors, but rather from extra-lingual logic
requirements. Consider this example:

@smallexample
with Init_Constants;
package Constants is
   X : Integer := 0;
   Y : Integer := 0;
end Constants;

package Init_Constants is
   procedure Calc;
end Init_Constants;

with Constants;
package body Init_Constants is
   procedure Calc is begin null; end;
begin
   Constants.X := 3;
   Constants.Y := 4;
end Init_Constants;

with Constants;
package Calc is
   Z : Integer := Constants.X + Constants.Y;
end Calc;

with Calc;
with Text_IO; use Text_IO;
procedure Main is
begin
   Put_Line (Calc.Z'Img);
end Main;
@end smallexample

@noindent
In this example, there is more than one valid order of elaboration. For
example both the following are correct orders:

@smallexample
Init_Constants spec
Constants spec
Calc spec
Main body
Init_Constants body

  and

Init_Constants spec
Init_Constants body
Constants spec
Calc spec
Main body
@end smallexample

@noindent
There is no language rule to prefer one or the other, both are correct
from an order of elaboration point of view. But the programmatic effects
of the two orders are very different. In the first, the elaboration routine
of @code{Calc} initializes @code{Z} to zero, and then the main program
runs with this value of zero. But in the second order, the elaboration
routine of @code{Calc} runs after the body of Init_Constants has set
@code{X} and @code{Y} and thus @code{Z} is set to 7 before @code{Main}
runs.

One could perhaps by applying pretty clever non-artificial intelligence
to the situation guess that it is more likely that the second order of
elaboration is the one desired, but there is no formal linguistic reason
to prefer one over the other. In fact in this particular case, GNAT will
prefer the second order, because of the rule that bodies are elaborated
as soon as possible, but it's just luck that this is what was wanted
(if indeed the second order was preferred).

If the program cares about the order of elaboration routines in a case like
this, it is important to specify the order required. In this particular
case, that could have been achieved by adding to the spec of Calc:

@smallexample
pragma Elaborate_All (Constants);
@end smallexample

@noindent
which requires that the body (if any) and spec of @code{Constants},
as well as the body and spec of any unit @code{with}'ed by
@code{Constants} be elaborated before @code{Calc} is elaborated.

Clearly no automatic method can always guess which alternative you require,
and if you are working with legacy code that had constraints of this kind
which were not properly specified by adding @code{Elaborate} or
@code{Elaborate_All} pragmas, then indeed it is possible that two different
compilers can choose different orders.

The @code{GNAT BIND}
@code{/PESSIMISTIC_ELABORATION} qualifier may be useful in smoking
out problems. This qualifier causes bodies to be elaborated as late as possible
instead of as early as possible. In the example above, it would have forced
the choice of the first elaboration order. If you get different results
when using this qualifier, and particularly if one set of results is right,
and one is wrong as far as you are concerned, it shows that you have some
missing @code{Elaborate} pragmas. For the example above, we have the
following output:

@smallexample
GNAT MAKE -f -q main
main
 7
GNAT MAKE -f -q main /BINDER_QUALIFIERS -p
main
 0
@end smallexample

@noindent
It is of course quite unlikely that both these results are correct, so
it is up to you in a case like this to investigate the source of the
difference, by looking at the two elaboration orders that are chosen,
and figuring out which is correct, and then adding the necessary
@code{Elaborate_All} pragmas to ensure the desired order.

@node The Cross-Referencing Tools GNAT XREF and GNAT FIND
@chapter  The Cross-Referencing Tools @code{GNAT XREF} and @code{GNAT FIND}
@findex GNAT XREF
@findex GNAT FIND

@noindent
The compiler generates cross-referencing information (unless
you set the @samp{/XREF=SUPPRESS} qualifier), which are saved in the @file{.ALI} files.
This information indicates where in the source each entity is declared and
referenced. Note that entities in package Standard are not included, but
entities in all other predefined units are included in the output.

Before using any of these two tools, you need to compile successfully your
application, so that GNAT gets a chance to generate the cross-referencing
information.

The two tools @code{GNAT XREF} and @code{GNAT FIND} take advantage of this
information to provide the user with the capability to easily locate the
declaration and references to an entity. These tools are quite similar,
the difference being that @code{GNAT FIND} is intended for locating
definitions and/or references to a specified entity or entities, whereas
@code{GNAT XREF} is oriented to generating a full report of all
cross-references.

To use these tools, you must not compile your application using the
@option{/XREF=SUPPRESS} qualifier on the @file{GNAT MAKE} command line (@inforef{The
GNAT Make Program GNAT MAKE,,gnat_ug}). Otherwise, cross-referencing
information will not be generated.

@menu
* GNAT XREF Qualifiers::
* GNAT FIND Qualifiers::
* Project Files for GNAT XREF and GNAT FIND::
* Regular Expressions in GNAT FIND and GNAT XREF::
* Examples of GNAT XREF Usage::
* Examples of GNAT FIND Usage::
@end menu

@node GNAT XREF Qualifiers
@section @code{GNAT XREF} Qualifiers

@noindent
The command lines for @code{GNAT XREF} is:
@smallexample
$ GNAT XREF [qualifiers] sourcefile1 [sourcefile2 ...]
@end smallexample

@noindent
where

@table @code
@item sourcefile1, sourcefile2
identifies the source files for which a report is to be generated. The
'with'ed units will be processed too. You must provide at least one file.

These file names are considered to be regular expressions, so for instance
specifying 'source*.ADB' is the same as giving every file in the current
directory whose name starts with 'source' and whose extension is 'adb'.

@end table

@noindent
The qualifiers can be :
@table @code
@item /ALL_FILES
If this qualifier is present, @code{GNAT FIND} and @code{GNAT XREF} will parse
the read-only files found in the library search path. Otherwise, these files
will be ignored. This option can be used to protect Gnat sources or your own
libraries from being parsed, thus making @code{GNAT FIND} and @code{GNAT XREF}
much faster, and their output much smaller.

@item /SOURCE_SEARCH=direc
When looking for source files also look in directory DIR. The order in which
source file search is undertaken is the same as for @file{GNAT MAKE}.

@item /OBJECT_SEARCH=direc
When searching for library and object files, look in directory
DIR. The order in which library files are searched is the same as for
@file{GNAT MAKE}.

@item /NOSTD_INCLUDES
Do not look for sources in the system default directory.

@item /NOSTD_LIBRARIES
Do not look for library files in the system default directory.

@item /RUNTIME_SYSTEM=@var{rts-path}
@cindex @code{/RUNTIME_SYSTEM} (@code{GNAT XREF})
Specifies the default location of the runtime library. Same meaning as the
equivalent @code{GNAT MAKE} flag (see @ref{Qualifiers for GNAT MAKE}).

@item -d
If this qualifier is set @code{GNAT XREF} will output the parent type
reference for each matching derived types.

@item /FULL_PATHNAME
If this qualifier is set, the output file names will be preceded by their
directory (if the file was found in the search path). If this qualifier is
not set, the directory will not be printed.

@item /IGNORE_LOCALS
If this qualifier is set, information is output only for library-level
entities, ignoring local entities. The use of this qualifier may accelerate
@code{GNAT FIND} and @code{GNAT XREF}.

@item /SEARCH=direc
Equivalent to @samp{/OBJECT_SEARCH=direc /SOURCE_SEARCH=direc}.

@item /PROJECT=file
Specify a project file to use @xref{Project Files}.
By default, @code{GNAT XREF} and @code{GNAT FIND} will try to locate a
project file in the current directory.

If a project file is either specified or found by the tools, then the content
of the source directory and object directory lines are added as if they
had been specified respectively by @samp{/SOURCE_SEARCH}
and @samp{OBJECT_SEARCH}.
@item /UNUSED
Output only unused symbols. This may be really useful if you give your
main compilation unit on the command line, as @code{GNAT XREF} will then
display every unused entity and 'with'ed package.


@end table

All these qualifiers may be in any order on the command line, and may even
appear after the file names. They need not be separated by spaces, thus
you can say @samp{GNAT XREF /ALL_FILES/IGNORE_LOCALS} instead of
@samp{GNAT XREF /ALL_FILES /IGNORE_LOCALS}.

@node GNAT FIND Qualifiers
@section @code{GNAT FIND} Qualifiers

@noindent
The command line for @code{GNAT FIND} is:

@smallexample
$ GNAT FIND [qualifiers] pattern[:sourcefile[:line[:column]]]
      [file1 file2 ...]
@end smallexample

@noindent
where

@table @code
@item pattern
An entity will be output only if it matches the regular expression found
in @samp{pattern}, see @xref{Regular Expressions in GNAT FIND and GNAT XREF}.

Omitting the pattern is equivalent to specifying @samp{*}, which
will match any entity. Note that if you do not provide a pattern, you
have to provide both a sourcefile and a line.

Entity names are given in Latin-1, with uppercase/lowercase equivalence
for matching purposes. At the current time there is no support for
8-bit codes other than Latin-1, or for wide characters in identifiers.

@item sourcefile
@code{GNAT FIND} will look for references, bodies or declarations
of symbols referenced in @file{sourcefile}, at line @samp{line}
and column @samp{column}. See @pxref{Examples of GNAT FIND Usage}
for syntax examples.

@item line
is a decimal integer identifying the line number containing
the reference to the entity (or entities) to be located.

@item column
is a decimal integer identifying the exact location on the
line of the first character of the identifier for the
entity reference. Columns are numbered from 1.

@item file1 file2 ...
The search will be restricted to these files. If none are given, then
the search will be done for every library file in the search path.
These file must appear only after the pattern or sourcefile.

These file names are considered to be regular expressions, so for instance
specifying 'source*.ADB' is the same as giving every file in the current
directory whose name starts with 'source' and whose extension is 'adb'.

Not that if you specify at least one file in this part, @code{GNAT FIND} may
sometimes not be able to find the body of the subprograms...

@end table

At least one of 'sourcefile' or 'pattern' has to be present on
the command line.

The following qualifiers are available:
@table @code

@item /ALL_FILES
If this qualifier is present, @code{GNAT FIND} and @code{GNAT XREF} will parse
the read-only files found in the library search path. Otherwise, these files
will be ignored. This option can be used to protect Gnat sources or your own
libraries from being parsed, thus making @code{GNAT FIND} and @code{GNAT XREF}
much faster, and their output much smaller.

@item /SOURCE_SEARCH=direc
When looking for source files also look in directory DIR. The order in which
source file search is undertaken is the same as for @file{GNAT MAKE}.

@item /OBJECT_SEARCH=direc
When searching for library and object files, look in directory
DIR. The order in which library files are searched is the same as for
@file{GNAT MAKE}.

@item /NOSTD_INCLUDES
Do not look for sources in the system default directory.

@item /NOSTD_LIBRARIES
Do not look for library files in the system default directory.

@item /RUNTIME_SYSTEM=@var{rts-path}
@cindex @code{/RUNTIME_SYSTEM} (@code{GNAT FIND})
Specifies the default location of the runtime library. Same meaning as the
equivalent @code{GNAT MAKE} flag (see @ref{Qualifiers for GNAT MAKE}).

@item -d
If this qualifier is set, then @code{GNAT FIND} will output the parent type
reference for each matching derived types.

@item /EXPRESSIONS
By default, @code{GNAT FIND} accept the simple regular expression set for
@samp{pattern}. If this qualifier is set, then the pattern will be
considered as full Unix-style regular expression.

@item /FULL_PATHNAME
If this qualifier is set, the output file names will be preceded by their
directory (if the file was found in the search path). If this qualifier is
not set, the directory will not be printed.

@item /IGNORE_LOCALS
If this qualifier is set, information is output only for library-level
entities, ignoring local entities. The use of this qualifier may accelerate
@code{GNAT FIND} and @code{GNAT XREF}.

@item /SEARCH=direc
Equivalent to @samp{/OBJECT_SEARCH=direc /SOURCE_SEARCH=direc}.

@item /PROJECT=file
Specify a project file (@pxref{Project Files}) to use.
By default, @code{GNAT XREF} and @code{GNAT FIND} will try to locate a
project file in the current directory.

If a project file is either specified or found by the tools, then the content
of the source directory and object directory lines are added as if they
had been specified respectively by @samp{/SOURCE_SEARCH} and
@samp{/OBJECT_SEARCH}.

@item /REFERENCES
By default, @code{GNAT FIND} will output only the information about the
declaration, body or type completion of the entities. If this qualifier is
set, the @code{GNAT FIND} will locate every reference to the entities in
the files specified on the command line (or in every file in the search
path if no file is given on the command line).

@item /PRINT_LINES
If this qualifier is set, then @code{GNAT FIND} will output the content
of the Ada source file lines were the entity was found.

@item -t
If this qualifier is set, then @code{GNAT FIND} will output the type hierarchy for
the specified type. It act like -d option but recursively from parent
type to parent type. When this qualifier is set it is not possible to
specify more than one file.

@end table

All these qualifiers may be in any order on the command line, and may even
appear after the file names. They need not be separated by spaces, thus
you can say @samp{GNAT XREF /ALL_FILES/IGNORE_LOCALS} instead of
@samp{GNAT XREF /ALL_FILES /IGNORE_LOCALS}.

As stated previously, GNAT FIND will search in every directory in the
search path. You can force it to look only in the current directory if
you specify @code{*} at the end of the command line.


@node Project Files for GNAT XREF and GNAT FIND
@section Project Files for @command{GNAT XREF} and @command{GNAT FIND}

@noindent
Project files allow a programmer to specify how to compile its
application, where to find sources,... These files are used primarily by
the Glide Ada mode, but they can also be used by the two tools
@code{GNAT XREF} and @code{GNAT FIND}.

A project file name must end with @file{.adp}. If a single one is
present in the current directory, then @code{GNAT XREF} and @code{GNAT FIND} will
extract the information from it. If multiple project files are found, none of
them is read, and you have to use the @samp{-p} qualifier to specify the one
you want to use.

The following lines can be included, even though most of them have default
values which can be used in most cases.
The lines can be entered in any order in the file.
Except for @samp{src_dir} and @samp{obj_dir}, you can only have one instance of
each line. If you have multiple instances, only the last one is taken into
account.

@table @code
@item src_dir=DIR         [default: "[]"]
specifies a directory where to look for source files. Multiple src_dir lines
can be specified and they will be searched in the order they
are specified.

@item obj_dir=DIR         [default: "[]"]
specifies a directory where to look for object and library files. Multiple
obj_dir lines can be specified and they will be searched in the order they
are specified

@item comp_opt=SWITCHES   [default: ""]
creates a variable which can be referred to subsequently by using
the @samp{$@{comp_opt@}} notation. This is intended to store the default
qualifiers given to @file{GNAT MAKE} and @file{GNAT COMPILE}.

@item bind_opt=SWITCHES   [default: ""]
creates a variable which can be referred to subsequently by using
the @samp{$@{bind_opt@}} notation. This is intended to store the default
qualifiers given to @file{GNAT BIND}.

@item link_opt=SWITCHES   [default: ""]
creates a variable which can be referred to subsequently by using
the @samp{$@{link_opt@}} notation. This is intended to store the default
qualifiers given to @file{GNAT LINK}.

@item main=EXECUTABLE     [default: ""]
specifies the name of the executable for the application. This variable can
be referred to in the following lines by using the @samp{$@{main@}} notation.

@item comp_cmd=COMMAND    [default: "GNAT COMPILE /SEARCH=$@{src_dir@} /DEBUG /TRY_SEMANTICS"]
specifies the command used to compile a single file in the application.

@item make_cmd=COMMAND    [default: "GNAT MAKE $@{main@} /SOURCE_SEARCH=$@{src_dir@} /OBJECT_SEARCH=$@{obj_dir@} /DEBUG /TRY_SEMANTICS /COMPILER_QUALIFIERS $@{comp_opt@} /BINDER_QUALIFIERS $@{bind_opt@} /LINKER_QUALIFIERS $@{link_opt@}"]
specifies the command used to recompile the whole application.

@item run_cmd=COMMAND     [default: "$@{main@}"]
specifies the command used to run the application.

@item debug_cmd=COMMAND   [default: "GDB $@{main@}"]
specifies the command used to debug the application

@end table

@code{GNAT XREF} and @code{GNAT FIND} only take into account the @samp{src_dir}
and @samp{obj_dir} lines, and ignore the others.

@node Regular Expressions in GNAT FIND and GNAT XREF
@section  Regular Expressions in @code{GNAT FIND} and @code{GNAT XREF}

@noindent
As specified in the section about @code{GNAT FIND}, the pattern can be a
regular expression. Actually, there are to set of regular expressions
which are recognized by the program :

@table @code
@item globbing patterns
These are the most usual regular expression. They are the same that you
generally used in a Unix shell command line, or in a DOS session.

Here is a more formal grammar :
@smallexample
@group
@iftex
@leftskip=.5cm
@end iftex
regexp ::= term
term   ::= elmt            -- matches elmt
term   ::= elmt elmt       -- concatenation (elmt then elmt)
term   ::= *               -- any string of 0 or more characters
term   ::= ?               -- matches any character
term   ::= [char @{char@}] -- matches any character listed
term   ::= [char - char]   -- matches any character in range
@end group
@end smallexample

@item full regular expression
The second set of regular expressions is much more powerful. This is the
type of regular expressions recognized by utilities such a @file{grep}.

The following is the form of a regular expression, expressed in Ada
reference manual style BNF is as follows

@smallexample
@iftex
@leftskip=.5cm
@end iftex
@group
regexp ::= term @{| term@} -- alternation (term or term ...)

term ::= item @{item@}     -- concatenation (item then item)

item ::= elmt              -- match elmt
item ::= elmt *            -- zero or more elmt's
item ::= elmt +            -- one or more elmt's
item ::= elmt ?            -- matches elmt or nothing
@end group
@group
elmt ::= nschar            -- matches given character
elmt ::= [nschar @{nschar@}]   -- matches any character listed
elmt ::= [^ nschar @{nschar@}] -- matches any character not listed
elmt ::= [char - char]     -- matches chars in given range
elmt ::= \ char            -- matches given character
elmt ::= .                 -- matches any single character
elmt ::= ( regexp )        -- parens used for grouping

char ::= any character, including special characters
nschar ::= any character except ()[].*+?^
@end group
@end smallexample

Following are a few examples :

@table @samp
@item abcde|fghi
will match any of the two strings 'abcde' and 'fghi'.

@item abc*d
will match any string like 'abd', 'abcd', 'abccd', 'abcccd', and so on

@item [a-z]+
will match any string which has only lowercase characters in it (and at
least one character

@end table
@end table

@node Examples of GNAT XREF Usage
@section Examples of @code{GNAT XREF} Usage

@subsection General Usage

@noindent
For the following examples, we will consider the following units :

@smallexample
@group
@cartouche
MAIN.ADS:
1: @b{with} Bar;
2: @b{package} Main @b{is}
3:     @b{procedure} Foo (B : @b{in} Integer);
4:     C : Integer;
5: @b{private}
6:     D : Integer;
7: @b{end} Main;

MAIN.ADB:
1: @b{package body} Main @b{is}
2:     @b{procedure} Foo (B : @b{in} Integer) @b{is}
3:     @b{begin}
4:        C := B;
5:        D := B;
6:        Bar.Print (B);
7:        Bar.Print (C);
8:     @b{end} Foo;
9: @b{end} Main;

BAR.ADS:
1: @b{package} Bar @b{is}
2:     @b{procedure} Print (B : Integer);
3: @b{end} bar;
@end cartouche
@end group
@end smallexample

@table @code

@noindent
The first thing to do is to recompile your application (for instance, in
that case just by doing a @samp{GNAT MAKE main}, so that GNAT generates
the cross-referencing information.
You can then issue any of the following commands:

@item GNAT XREF MAIN.ADB
@code{GNAT XREF} generates cross-reference information for MAIN.ADB
and every unit 'with'ed by MAIN.ADB.

The output would be:
@smallexample
@iftex
@leftskip=0cm
@end iftex
B                                                      Type: Integer
  Decl: BAR.ADS           2:22
B                                                      Type: Integer
  Decl: MAIN.ADS          3:20
  Body: MAIN.ADB          2:20
  Ref:  MAIN.ADB          4:13     5:13     6:19
Bar                                                    Type: Unit
  Decl: BAR.ADS           1:9
  Ref:  MAIN.ADB          6:8      7:8
       MAIN.ADS           1:6
C                                                      Type: Integer
  Decl: MAIN.ADS          4:5
  Modi: MAIN.ADB          4:8
  Ref:  MAIN.ADB          7:19
D                                                      Type: Integer
  Decl: MAIN.ADS          6:5
  Modi: MAIN.ADB          5:8
Foo                                                    Type: Unit
  Decl: MAIN.ADS          3:15
  Body: MAIN.ADB          2:15
Main                                                    Type: Unit
  Decl: MAIN.ADS          2:9
  Body: MAIN.ADB          1:14
Print                                                   Type: Unit
  Decl: BAR.ADS           2:15
  Ref:  MAIN.ADB          6:12     7:12
@end smallexample

@noindent
that is the entity @code{Main} is declared in MAIN.ADS, line 2, column 9,
its body is in MAIN.ADB, line 1, column 14 and is not referenced any where.

The entity @code{Print} is declared in BAR.ADS, line 2, column 15 and it
it referenced in MAIN.ADB, line 6 column 12 and line 7 column 12.

@item GNAT XREF PACKAGE1.ADB PACKAGE2.ADS
@code{GNAT XREF} will generates cross-reference information for
PACKAGE1.ADB, PACKAGE2.ADS and any other package 'with'ed by any
of these.

@end table


@node Examples of GNAT FIND Usage
@section Examples of @code{GNAT FIND} Usage

@table @code

@item GNAT FIND /FULL_PATHNAME xyz:MAIN.ADB
Find declarations for all entities xyz referenced at least once in
MAIN.ADB. The references are search in every library file in the search
path.

The directories will be printed as well (as the @samp{/FULL_PATHNAME}
qualifier is set)

The output will look like:
@smallexample
[directory]MAIN.ADS:106:14: xyz <= declaration
[directory]MAIN.ADB:24:10: xyz <= body
[directory]FOO.ADS:45:23: xyz <= declaration
@end smallexample

@noindent
that is to say, one of the entities xyz found in MAIN.ADB is declared at
line 12 of MAIN.ADS (and its body is in MAIN.ADB), and another one is
declared at line 45 of FOO.ADS

@item GNAT FIND /FULL_PATHNAME/SOURCE_LINE xyz:MAIN.ADB
This is the same command as the previous one, instead @code{GNAT FIND} will
display the content of the Ada source file lines.

The output will look like:

@smallexample
[directory]MAIN.ADS:106:14: xyz <= declaration
   procedure xyz;
[directory]MAIN.ADB:24:10: xyz <= body
   procedure xyz is
[directory]FOO.ADS:45:23: xyz <= declaration
   xyz : Integer;
@end smallexample

@noindent
This can make it easier to find exactly the location your are looking
for.

@item GNAT FIND /REFERENCES "*x*":MAIN.ADS:123 FOO.ADB
Find references to all entities containing an x that are
referenced on line 123 of MAIN.ADS.
The references will be searched only in MAIN.ADB and FOO.ADB.

@item GNAT FIND MAIN.ADS:123
Find declarations and bodies for all entities that are referenced on
line 123 of MAIN.ADS.

This is the same as @code{GNAT FIND "*":MAIN.ADB:123}.

@item GNAT FIND [mydir]MAIN.ADB:123:45
Find the declaration for the entity referenced at column 45 in
line 123 of file MAIN.ADB in directory mydir. Note that it
is usual to omit the identifier name when the column is given,
since the column position identifies a unique reference.

The column has to be the beginning of the identifier, and should not
point to any character in the middle of the identifier.

@end table

@node File Name Krunching Using GNAT KRUNCH
@chapter File Name Krunching Using @code{GNAT KRUNCH}
@findex GNAT KRUNCH

@noindent
This chapter discusses the method used by the compiler to shorten
the default file names chosen for Ada units so that they do not
exceed the maximum length permitted. It also describes the
@code{GNAT KRUNCH} utility that can be used to determine the result of
applying this shortening.
@menu
* About GNAT KRUNCH::
* Using GNAT KRUNCH::
* Krunching Method::
* Examples of GNAT KRUNCH Usage::
@end menu

@node About GNAT KRUNCH
@section About @code{GNAT KRUNCH}

@noindent
The default file naming rule in GNAT
is that the file name must be derived from
the unit name. The exact default rule is as follows:
@itemize @bullet
@item
Take the unit name and replace all dots by hyphens.
@item
If such a replacement occurs in the
second character position of a name, and the first character is
A, G, S, or I then replace the dot by the character
$ (dollar sign)
instead of a minus.
@end itemize
The reason for this exception is to avoid clashes
with the standard names for children of System, Ada, Interfaces,
and GNAT, which use the prefixes S- A- I- and G-
respectively.

The @code{/FILE_NAME_MAX_LENGTH=@var{nn}}
qualifier of the compiler activates a "krunching"
circuit that limits file names to nn characters (where nn is a decimal
integer). For example, using OpenVMS,
where the maximum file name length is
39, the value of nn is usually set to 39, but if you want to generate
a set of files that would be usable if ported to a system with some
different maximum file length, then a different value can be specified.
The default value of 39 for OpenVMS need not be specified.

The @code{GNAT KRUNCH} utility can be used to determine the krunched name for
a given file, when krunched to a specified maximum length.

@node Using GNAT KRUNCH
@section Using @code{GNAT KRUNCH}

@noindent
The @code{GNAT KRUNCH} command has the form


@smallexample
$ GNAT KRUNCH @var{name} /COUNT=nn
@end smallexample

@noindent
@var{name} can be an Ada name with dots or the GNAT name of the unit,
where the dots representing child units or subunit are replaced by
hyphens. The only confusion arises if a name ends in @code{.ADS} or
@code{.ADB}. @code{GNAT KRUNCH} takes this to be an extension if there are
no other dots in the name.

@var{length} represents the length of the krunched name. The default
when no argument is given is 39 characters. A length of zero stands for
unlimited, in other words do not chop except for system files which are
always 39.

@noindent
The output is the krunched name. The output has an extension only if the
original argument was a file name with an extension.

@node Krunching Method
@section Krunching Method

@noindent
The initial file name is determined by the name of the unit that the file
contains. The name is formed by taking the full expanded name of the
unit and replacing the separating dots with hyphens and
using uppercase
for all letters, except that a hyphen in the second character position is
replaced by a dollar sign if the first character is
A, I, G, or S.
The extension is @code{.ADS} for a
specification and @code{.ADB} for a body.
Krunching does not affect the extension, but the file name is shortened to
the specified length by following these rules:

@itemize @bullet
@item
The name is divided into segments separated by hyphens, tildes or
underscores and all hyphens, tildes, and underscores are
eliminated. If this leaves the name short enough, we are done.

@item
If the name is too long, the longest segment is located (left-most if there are two
of equal length), and shortened by dropping its last character. This is
repeated until the name is short enough.

As an example, consider the krunching of @*@file{OUR-STRINGS-WIDE_FIXED.ADB}
to fit the name into 8 characters as required by some operating systems.

@smallexample
our-strings-wide_fixed 22
our strings wide fixed 19
our string  wide fixed 18
our strin   wide fixed 17
our stri    wide fixed 16
our stri    wide fixe  15
our str     wide fixe  14
our str     wid  fixe  13
our str     wid  fix   12
ou  str     wid  fix   11
ou  st      wid  fix   10
ou  st      wi   fix   9
ou  st      wi   fi    8
Final file name: OUSTWIFI.ADB
@end smallexample

@item
The file names for all predefined units are always krunched to eight
characters. The krunching of these predefined units uses the following
special prefix replacements:

@table @file
@item ada-
replaced by @file{A-}

@item gnat-
replaced by @file{G-}

@item interfaces-
replaced by @file{I-}

@item system-
replaced by @file{S-}
@end table

These system files have a hyphen in the second character position. That
is why normal user files replace such a character with a
dollar sign, to
avoid confusion with system file names.

As an example of this special rule, consider
@*@file{ADA-STRINGS-WIDE_FIXED.ADB}, which gets krunched as follows:

@smallexample
ada-strings-wide_fixed 22
a-  strings wide fixed 18
a-  string  wide fixed 17
a-  strin   wide fixed 16
a-  stri    wide fixed 15
a-  stri    wide fixe  14
a-  str     wide fixe  13
a-  str     wid  fixe  12
a-  str     wid  fix   11
a-  st      wid  fix   10
a-  st      wi   fix   9
a-  st      wi   fi    8
Final file name: A-STWIFI.ADB
@end smallexample
@end itemize

Of course no file shortening algorithm can guarantee uniqueness over all
possible unit names, and if file name krunching is used then it is your
responsibility to ensure that no name clashes occur. The utility
program @code{GNAT KRUNCH} is supplied for conveniently determining the
krunched name of a file.

@node Examples of GNAT KRUNCH Usage
@section Examples of @code{GNAT KRUNCH} Usage

@smallexample
@iftex
@leftskip=0cm
@end iftex
$ GNAT KRUNCH VERY_LONG_UNIT_NAME.ADS/count=6 --> VLUNNA.ADS
$ GNAT KRUNCH VERY_LONG_UNIT_NAME.ADS/count=0 --> VERY_LONG_UNIT_NAME.ADS
@end smallexample

@node Preprocessing Using GNAT PREPROCESS
@chapter Preprocessing Using @code{GNAT PREPROCESS}
@findex GNAT PREPROCESS

@noindent
The @code{GNAT PREPROCESS} utility provides
a simple preprocessing capability for Ada programs.
It is designed for use with GNAT, but is not dependent on any special
features of GNAT.

@menu
* Using GNAT PREPROCESS::
* Qualifiers for GNAT PREPROCESS::
* Form of Definitions File::
* Form of Input Text for GNAT PREPROCESS::
@end menu

@node Using GNAT PREPROCESS
@section Using @code{GNAT PREPROCESS}

@noindent
To call @code{GNAT PREPROCESS} use

@smallexample
$ GNAT PREPROCESS [-bcrsu] [-Dsymbol=value] infile outfile [deffile]
@end smallexample

@noindent
where
@table @code
@item infile
is the full name of the input file, which is an Ada source
file containing preprocessor directives.

@item outfile
is the full name of the output file, which is an Ada source
in standard Ada form. When used with GNAT, this file name will
normally have an ads or adb suffix.

@item deffile
is the full name of a text file containing definitions of
symbols to be referenced by the preprocessor. This argument is
optional, and can be replaced by the use of the @code{-D} qualifier.

@item qualifiers
is an optional sequence of qualifiers as described in the next section.
@end table

@node Qualifiers for GNAT PREPROCESS
@section Qualifiers for @code{GNAT PREPROCESS}

@table @code

@item /BLANK_LINES
Causes both preprocessor lines and the lines deleted by
preprocessing to be replaced by blank lines in the output source file,
preserving line numbers in the output file.

@item /COMMENTS
Causes both preprocessor lines and the lines deleted
by preprocessing to be retained in the output source as comments marked
with the special string "--! ". This option will result in line numbers
being preserved in the output file.

@item -Dsymbol=value
Defines a new symbol, associated with value. If no value is given on the
command line, then symbol is considered to be @code{True}. This qualifier
can be used in place of a definition file.

@item /REMOVE (default)
This is the default setting which causes lines deleted by preprocessing
to be entirely removed from the output file.

@item /REFERENCE
Causes a @code{Source_Reference} pragma to be generated that
references the original input file, so that error messages will use
the file name of this original file. The use of this qualifier implies
that preprocessor lines are not to be removed from the file, so its
use will force @code{/BLANK_LINES} mode if
@code{/COMMENTS}
has not been specified explicitly.

Note that if the file to be preprocessed contains multiple units, then
it will be necessary to @code{GNAT CHOP} the output file from
@code{GNAT PREPROCESS}. If a @code{Source_Reference} pragma is present
in the preprocessed file, it will be respected by
@code{GNAT CHOP /REFERENCE}
so that the final chopped files will correctly refer to the original
input source file for @code{GNAT PREPROCESS}.

@item /SYMBOLS
Causes a sorted list of symbol names and values to be
listed on the standard output file.

@item /UNDEFINED
Causes undefined symbols to be treated as having the value FALSE in the context
of a preprocessor test. In the absence of this option, an undefined symbol in
a @code{#if} or @code{#elsif} test will be treated as an error.

@end table


@node Form of Definitions File
@section Form of Definitions File

@noindent
The definitions file contains lines of the form

@smallexample
symbol := value
@end smallexample

@noindent
where symbol is an identifier, following normal Ada (case-insensitive)
rules for its syntax, and value is one of the following:

@itemize @bullet
@item
Empty, corresponding to a null substitution
@item
A string literal using normal Ada syntax
@item
Any sequence of characters from the set
(letters, digits, period, underline).
@end itemize

@noindent
Comment lines may also appear in the definitions file, starting with
the usual @code{--},
and comments may be added to the definitions lines.

@node Form of Input Text for GNAT PREPROCESS
@section Form of Input Text for @code{GNAT PREPROCESS}

@noindent
The input text may contain preprocessor conditional inclusion lines,
as well as general symbol substitution sequences.

The preprocessor conditional inclusion commands have the form

@smallexample
@group
@cartouche
#if @i{expression} [then]
   lines
#elsif @i{expression} [then]
   lines
#elsif @i{expression} [then]
   lines
...
#else
   lines
#end if;
@end cartouche
@end group
@end smallexample

@noindent
In this example, @i{expression} is defined by the following grammar:
@smallexample
@i{expression} ::=  <symbol>
@i{expression} ::=  <symbol> = "<value>"
@i{expression} ::=  <symbol> = <symbol>
@i{expression} ::=  <symbol> 'Defined
@i{expression} ::=  not @i{expression}
@i{expression} ::=  @i{expression} and @i{expression}
@i{expression} ::=  @i{expression} or @i{expression}
@i{expression} ::=  @i{expression} and then @i{expression}
@i{expression} ::=  @i{expression} or else @i{expression}
@i{expression} ::=  ( @i{expression} )
@end smallexample

@noindent
For the first test (@i{expression} ::= <symbol>) the symbol must have
either the value true or false, that is to say the right-hand of the
symbol definition must be one of the (case-insensitive) literals
@code{True} or @code{False}. If the value is true, then the
corresponding lines are included, and if the value is false, they are
excluded.

The test (@i{expression} ::= <symbol> @code{'Defined}) is true only if
the symbol has been defined in the definition file or by a @code{-D}
qualifier on the command line. Otherwise, the test is false.

The equality tests are case insensitive, as are all the preprocessor lines.

If the symbol referenced is not defined in the symbol definitions file,
then the effect depends on whether or not qualifier @code{-u}
is specified. If so, then the symbol is treated as if it had the value
false and the test fails. If this qualifier is not specified, then
it is an error to reference an undefined symbol. It is also an error to
reference a symbol that is defined with a value other than @code{True}
or @code{False}.

The use of the @code{not} operator inverts the sense of this logical test, so
that the lines are included only if the symbol is not defined.
The @code{then} keyword is optional as shown

The @code{#} must be the first non-blank character on a line, but
otherwise the format is free form. Spaces or tabs may appear between
the @code{#} and the keyword. The keywords and the symbols are case
insensitive as in normal Ada code. Comments may be used on a
preprocessor line, but other than that, no other tokens may appear on a
preprocessor line. Any number of @code{elsif} clauses can be present,
including none at all. The @code{else} is optional, as in Ada.

The @code{#} marking the start of a preprocessor line must be the first
non-blank character on the line, i.e. it must be preceded only by
spaces or horizontal tabs.

Symbol substitution outside of preprocessor lines is obtained by using
the sequence

@smallexample
$symbol
@end smallexample

@noindent
anywhere within a source line, except in a comment or within a
string literal. The identifier
following the @code{$} must match one of the symbols defined in the symbol
definition file, and the result is to substitute the value of the
symbol in place of @code{$symbol} in the output file.

Note that although the substitution of strings within a string literal
is not possible, it is possible to have a symbol whose defined value is
a string literal. So instead of setting XYZ to @code{hello} and writing:

@smallexample
Header : String := "$XYZ";
@end smallexample

@noindent
you should set XYZ to @code{"hello"} and write:

@smallexample
Header : String := $XYZ;
@end smallexample

@noindent
and then the substitution will occur as desired.

@node The GNAT Run-Time Library Builder GNAT LIBRARY
@chapter The GNAT Run-Time Library Builder @code{GNAT LIBRARY}
@findex GNAT LIBRARY
@cindex Library builder

@noindent
@code{GNAT LIBRARY} is a tool for rebuilding the GNAT run time with user
supplied configuration pragmas.

@menu
* Running GNAT LIBRARY::
* Qualifiers for GNAT LIBRARY::
* Examples of GNAT LIBRARY Usage::
@end menu

@node Running GNAT LIBRARY
@section Running @code{GNAT LIBRARY}

@noindent
The @code{GNAT LIBRARY} command has the form

@smallexample
$ GNAT LIBRARY /[CREATE | SET | DELETE]=directory [/CONFIG=file]
@end smallexample

@node Qualifiers for GNAT LIBRARY
@section Qualifiers for @code{GNAT LIBRARY}

@noindent
@code{GNAT LIBRARY} recognizes the following qualifiers:

@table @code
@item /CREATE=directory
@cindex @code{/CREATE=directory} (@code{GNAT LIBRARY})
     Create the new run-time library in the specified directory.

@item /SET=directory
@cindex @code{/SET=directory} (@code{GNAT LIBRARY})
     Make the library in the specified directory the current run-time
     library.

@item /DELETE=directory
@cindex @code{/DELETE=directory} (@code{GNAT LIBRARY})
     Delete the run-time library in the specified directory.

@item /CONFIG=file
@cindex @code{/CONFIG=file} (@code{GNAT LIBRARY})
     With /CREATE:
     Use the configuration pragmas in the specified file when building
     the library.

     With /SET:
     Use the configuration pragmas in the specified file when compiling.

@end table

@node Examples of GNAT LIBRARY Usage
@section Example of @code{GNAT LIBRARY} Usage

@smallexample
Contents of VAXFLOAT.ADC:
pragma Float_Representation (VAX_Float);

$ GNAT LIBRARY /CREATE=[.VAXFLOAT] /CONFIG=VAXFLOAT.ADC

GNAT LIBRARY rebuilds the run-time library in directory [.VAXFLOAT]

@end smallexample

@node The GNAT Library Browser GNAT LIST
@chapter The GNAT Library Browser @code{GNAT LIST}
@findex GNAT LIST
@cindex Library browser

@noindent
@code{GNAT LIST} is a tool that outputs information about compiled
units. It gives the relationship between objects, unit names and source
files. It can also be used to check the source dependencies of a unit
as well as various characteristics.

@menu
* Running GNAT LIST::
* Qualifiers for GNAT LIST::
* Examples of GNAT LIST Usage::
@end menu

@node Running GNAT LIST
@section Running @code{GNAT LIST}

@noindent
The @code{GNAT LIST} command has the form

@smallexample
$ GNAT LIST qualifiers @var{object_or_ali_file}
@end smallexample

@noindent
The main argument is the list of object or @file{ali} files
(@pxref{The Ada Library Information Files})
for which information is requested.

In normal mode, without additional option, @code{GNAT LIST} produces a
four-column listing. Each line represents information for a specific
object. The first column gives the full path of the object, the second
column gives the name of the principal unit in this object, the third
column gives the status of the source and the fourth column gives the
full path of the source representing this unit.
Here is a simple example of use:

@smallexample
$ GNAT LIST *.OBJ
[]DEMO1.OBJ            demo1            DIF DEMO1.ADB
[]DEMO2.OBJ            demo2             OK DEMO2.ADB
[]HELLO.OBJ            h1                OK HELLO.ADB
[]INSTR-CHILD.OBJ      instr.child      MOK INSTR-CHILD.ADB
[]INSTR.OBJ            instr             OK INSTR.ADB
[]TEF.OBJ              tef              DIF TEF.ADB
[]TEXT_IO_EXAMPLE.OBJ  text_io_example   OK TEXT_IO_EXAMPLE.ADB
[]TGEF.OBJ             tgef             DIF TGEF.ADB
@end smallexample

@noindent
The first line can be interpreted as follows: the main unit which is
contained in
object file @file{DEMO1.OBJ} is demo1, whose main source is in
@file{DEMO1.ADB}. Furthermore, the version of the source used for the
compilation of demo1 has been modified (DIF). Each source file has a status
qualifier which can be:

@table @code
@item OK (unchanged)
The version of the source file used for the compilation of the
specified unit corresponds exactly to the actual source file.

@item MOK (slightly modified)
The version of the source file used for the compilation of the
specified unit differs from the actual source file but not enough to
require recompilation. If you use GNAT MAKE with the qualifier
@code{/MINIMAL_RECOMPILATION}, a file marked
MOK will not be recompiled.

@item DIF (modified)
No version of the source found on the path corresponds to the source
used to build this object.

@item ??? (file not found)
No source file was found for this unit.

@item HID (hidden,  unchanged version not first on PATH)
The version of the source that corresponds exactly to the source used
for compilation has been found on the path but it is hidden by another
version of the same source that has been modified.

@end table

@node Qualifiers for GNAT LIST
@section Qualifiers for @code{GNAT LIST}

@noindent
@code{GNAT LIST} recognizes the following qualifiers:

@table @code
@item /ALL_UNITS
@cindex @code{/ALL_UNITS} (@code{GNAT LIST})
Consider all units, including those of the predefined Ada library.
Especially useful with @code{/DEPENDENCIES}.

@item /DEPENDENCIES
@cindex @code{/DEPENDENCIES} (@code{GNAT LIST})
List sources from which specified units depend on.

@item /OUTPUT=OPTIONS
@cindex @code{/OUTPUT=OPTIONS} (@code{GNAT LIST})
Output the list of options.

@item /OUTPUT=OBJECTS
@cindex @code{/OUTPUT=OBJECTS} (@code{GNAT LIST})
Only output information about object files.

@item /OUTPUT=SOURCES
@cindex @code{/OUTPUT=SOURCES} (@code{GNAT LIST})
Only output information about source files.

@item /OUTPUT=UNITS
@cindex @code{/OUTPUT=UNITS} (@code{GNAT LIST})
Only output information about compilation units.

@item /OBJECT_SEARCH=@var{dir}
@itemx /SOURCE_SEARCH=@var{dir}
@itemx /SEARCH=@var{dir}
@itemx  /NOCURRENT_DIRECTORY
@itemx /NOSTD_INCLUDES
Source path manipulation. Same meaning as the equivalent @code{GNAT MAKE} flags
(see @ref{Qualifiers for GNAT MAKE}).

@item /RUNTIME_SYSTEM=@var{rts-path}
@cindex @code{/RUNTIME_SYSTEM} (@code{GNAT LIST})
Specifies the default location of the runtime library. Same meaning as the
equivalent @code{GNAT MAKE} flag (see @ref{Qualifiers for GNAT MAKE}).

@item /OUTPUT=VERBOSE
@cindex @code{/OUTPUT=VERBOSE} (@code{GNAT LIST})
Verbose mode. Output the complete source and object paths. Do not use
the default column layout but instead use long format giving as much as
information possible on each requested units, including special
characteristics such as:

@table @code
@item  Preelaborable
The unit is preelaborable in the Ada 95 sense.

@item No_Elab_Code
No elaboration code has been produced by the compiler for this unit.

@item Pure
The unit is pure in the Ada 95 sense.

@item Elaborate_Body
The unit contains a pragma Elaborate_Body.

@item Remote_Types
The unit contains a pragma Remote_Types.

@item Shared_Passive
The unit contains a pragma Shared_Passive.

@item Predefined
This unit is part of the predefined environment and cannot be modified
by the user.

@item Remote_Call_Interface
The unit contains a pragma Remote_Call_Interface.

@end table

@end table

@node Examples of GNAT LIST Usage
@section Example of @code{GNAT LIST} Usage

@smallexample
GNAT LIST /DEPENDENCIES /OUTPUT=SOURCES /ALL_UNITS DEMO1.ADB

GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]ADA.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]A-FINALI.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]A-FILICO.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]A-STREAM.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]A-TAGS.ADS
DEMO1.ADB
GEN_LIST.ADS
GEN_LIST.ADB
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]GNAT.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]G-IO.ADS
INSTR.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]SYSTEM.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-EXCTAB.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-FINIMP.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-FINROO.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-SECSTA.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-STALIB.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-STOELE.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-STRATT.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-TASOLI.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]S-UNSTYP.ADS
GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB]UNCHCONV.ADS
@end smallexample


@node Finding Memory Problems with GNAT Debug Pool
@chapter Finding Memory Problems with GNAT Debug Pool
@findex Debug Pool
@cindex storage, pool, memory corruption

@noindent
The use of unchecked deallocation and unchecked conversion can easily
lead to incorrect memory references. The problems generated by such
references are usually difficult to tackle because the symptoms can be
very remote from the origin of the problem. In such cases, it is
very helpful to detect the problem as early as possible. This is the
purpose of the Storage Pool provided by @code{GNAT.Debug_Pools}.

@noindent
In order to use the GNAT specific debugging pool, the user must
associate a debug pool object with each of the access types that may be
related to suspected memory problems. See Ada Reference Manual
13.11.
@smallexample
@b{type} Ptr @b{is} @b{access} Some_Type;
Pool : GNAT.Debug_Pools.Debug_Pool;
@b{for} Ptr'Storage_Pool @b{use} Pool;
@end smallexample

@code{GNAT.Debug_Pools} is derived from of a GNAT-specific kind of
pool: the Checked_Pool. Such pools, like standard Ada storage pools,
allow the user to redefine allocation and deallocation strategies. They
also provide a checkpoint for each dereference, through the use of
the primitive operation @code{Dereference} which is implicitly called at
each dereference of an access value.

Once an access type has been associated with a debug pool, operations on
values of the type may raise four distinct exceptions,
which correspond to four potential kinds of memory corruption:
@itemize @bullet
@item
@code{GNAT.Debug_Pools.Accessing_Not_Allocated_Storage}
@item
@code{GNAT.Debug_Pools.Accessing_Deallocated_Storage}
@item
@code{GNAT.Debug_Pools.Freeing_Not_Allocated_Storage}
@item
@code{GNAT.Debug_Pools.Freeing_Deallocated_Storage }
@end itemize

@noindent
For types associated with a Debug_Pool, dynamic allocation is performed using
the standard
GNAT allocation routine. References to all allocated chunks of memory
are kept in an internal dictionary. The deallocation strategy consists
in not releasing the memory to the underlying system but rather to fill
it with a memory pattern easily recognizable during debugging sessions:
The memory pattern is the old IBM hexadecimal convention: 16#DEADBEEF#.
Upon each dereference, a check is made that the access value denotes a properly
allocated memory location. Here is a complete example of use of
@code{Debug_Pools}, that includes typical instances of  memory corruption:
@smallexample
@iftex
@leftskip=0cm
@end iftex
@b{with} Gnat.Io; @b{use} Gnat.Io;
@b{with} Unchecked_Deallocation;
@b{with} Unchecked_Conversion;
@b{with} GNAT.Debug_Pools;
@b{with} System.Storage_Elements;
@b{with} Ada.Exceptions; @b{use} Ada.Exceptions;
@b{procedure} Debug_Pool_Test @b{is}

   @b{type} T @b{is} @b{access} Integer;
   @b{type} U @b{is} @b{access} @b{all} T;

   P : GNAT.Debug_Pools.Debug_Pool;
   @b{for} T'Storage_Pool @b{use} P;

   @b{procedure} Free @b{is} @b{new} Unchecked_Deallocation (Integer, T);
   @b{function} UC @b{is} @b{new} Unchecked_Conversion (U, T);
   A, B : @b{aliased} T;

   @b{procedure} Info @b{is} @b{new} GNAT.Debug_Pools.Print_Info(Put_Line);

@b{begin}
   Info (P);
   A := @b{new} Integer;
   B := @b{new} Integer;
   B := A;
   Info (P);
   Free (A);
   @b{begin}
      Put_Line (Integer'Image(B.@b{all}));
   @b{exception}
      @b{when} E : @b{others} => Put_Line ("raised: " & Exception_Name (E));
   @b{end};
   @b{begin}
      Free (B);
   @b{exception}
      @b{when} E : @b{others} => Put_Line ("raised: " & Exception_Name (E));
   @b{end};
   B := UC(A'Access);
   @b{begin}
      Put_Line (Integer'Image(B.@b{all}));
   @b{exception}
      @b{when} E : @b{others} => Put_Line ("raised: " & Exception_Name (E));
   @b{end};
   @b{begin}
      Free (B);
   @b{exception}
      @b{when} E : @b{others} => Put_Line ("raised: " & Exception_Name (E));
   @b{end};
   Info (P);
@b{end} Debug_Pool_Test;
@end smallexample
@noindent
The debug pool mechanism provides the following precise diagnostics on the
execution of this erroneous program:
@smallexample
Debug Pool info:
  Total allocated bytes :  0
  Total deallocated bytes :  0
  Current Water Mark:  0
  High Water Mark:  0

Debug Pool info:
  Total allocated bytes :  8
  Total deallocated bytes :  0
  Current Water Mark:  8
  High Water Mark:  8

raised: GNAT.DEBUG_POOLS.ACCESSING_DEALLOCATED_STORAGE
raised: GNAT.DEBUG_POOLS.FREEING_DEALLOCATED_STORAGE
raised: GNAT.DEBUG_POOLS.ACCESSING_NOT_ALLOCATED_STORAGE
raised: GNAT.DEBUG_POOLS.FREEING_NOT_ALLOCATED_STORAGE
Debug Pool info:
  Total allocated bytes :  8
  Total deallocated bytes :  4
  Current Water Mark:  4
  High Water Mark:  8

@end smallexample

@node Creating Sample Bodies Using GNAT STUB
@chapter Creating Sample Bodies Using @code{GNAT STUB}
@findex GNAT STUB

@noindent
@code{GNAT STUB} creates body stubs, that is, empty but compilable bodies
for library unit declarations.

To create a body stub, @code{GNAT STUB} has to compile the library
unit declaration. Therefore, bodies can be created only for legal
library units. Moreover, if a library unit depends semantically upon
units located outside the current directory, you have to provide
the source search path when calling @code{GNAT STUB}, see the description
of @code{GNAT STUB} qualifiers below.

@menu
* Running GNAT STUB::
* Qualifiers for GNAT STUB::
@end menu

@node Running GNAT STUB
@section Running @code{GNAT STUB}

@noindent
@code{GNAT STUB} has the command-line interface of the form

@smallexample
$ GNAT STUB [qualifiers] filename [directory]
@end smallexample

@noindent
where
@table @code
@item filename
is the name of the source file that contains a library unit declaration
for which a body must be created. This name should follow the GNAT file name
conventions. No crunching is allowed for this file name. The file
name may contain the path information.

@item directory
indicates the directory to place a body stub (default is the
current directory)

@item qualifiers
is an optional sequence of qualifiers as described in the next section
@end table

@node Qualifiers for GNAT STUB
@section Qualifiers for @code{GNAT STUB}

@table @code

@item /FULL
If the destination directory already contains a file with a name of the body file
for the argument spec file, replace it with the generated body stub.

@item /HEADER=SPEC
Put the comment header (i.e. all the comments preceding the
compilation unit) from the source of the library unit declaration
into the body stub.

@item /HEADER=GENERAL
Put a sample comment header into the body stub.

@item /SEARCH=direc
@itemx /NOCURRENT_DIRECTORY
These qualifiers have the same meaning as in calls to GNAT COMPILE.
They define the source search path in the call to GNAT COMPILE issued
by @code{GNAT STUB} to compile an argument source file.

@item /INDENTATION=@var{n}
(@var{n} is a decimal natural number). Set the indentation level in the
generated body sample to n, '/INDENTATION=0' means "no indentation",
the default indentation is 3.

@item /TREE_FILE=SAVE
Do not remove the tree file (i.e. the snapshot of the compiler internal
structures used by @code{GNAT STUB}) after creating the body stub.

@item /LINE_LENGTH=@var{n}
(@var{n} is a decimal positive number) Set the maximum line length in the
body stub to n, the default is 78.

@item /QUIET
Quiet mode: do not generate a confirmation when a body is
successfully created or a message when a body is not required for an
argument unit.

@item /TREE_FILE=REUSE
Reuse the tree file (if it exists) instead of creating it: instead of
creating the tree file for the library unit declaration, GNAT STUB
tries to find it in the current directory and use it for creating
a body. If the tree file is not found, no body is created. @code{/REUSE}
also implies @code{/SAVE}, whether or not
@code{/SAVE} is set explicitly.

@item /TREE_FILE=OVERWRITE
Overwrite the existing tree file: if the current directory already
contains the file which, according to the GNAT file name rules should
be considered as a tree file for the argument source file, GNAT STUB
will refuse to create the tree file needed to create a body sampler,
unless @code{-t} option is set

@item /VERBOSE
Verbose mode: generate version information.

@end table

@node Reducing the Size of Ada Executables with GNAT ELIM
@chapter Reducing the Size of Ada Executables with @code{GNAT ELIM}
@findex GNAT ELIM

@menu
* About GNAT ELIM::
* Eliminate Pragma::
* Tree Files::
* Preparing Tree and Bind Files for GNAT ELIM::
* Running GNAT ELIM::
* Correcting the List of Eliminate Pragmas::
* Making Your Executables Smaller::
* Summary of the GNAT ELIM Usage Cycle::
@end menu

@node About GNAT ELIM
@section About @code{GNAT ELIM}

@noindent
When a program shares a set of Ada
packages with other programs, it may happen that this program uses
only a fraction of the subprograms defined in these packages. The code
created for these unused subprograms increases the size of the executable.

@code{GNAT ELIM} tracks unused subprograms in an Ada program and
outputs a list of GNAT-specific @code{Eliminate} pragmas (see next
section) marking all the subprograms that are declared but never called.
By placing the list of @code{Eliminate} pragmas in the GNAT configuration
file @file{GNAT.ADC} and recompiling your program, you may decrease the
size of its executable, because the compiler will not generate the code
for 'eliminated' subprograms.

@code{GNAT ELIM} needs as its input data a set of tree files
(see @ref{Tree Files}) representing all the components of a program to
process and a bind file for a main subprogram (see
@ref{Preparing Tree and Bind Files for GNAT ELIM}).

@node Eliminate Pragma
@section @code{Eliminate} Pragma
@findex Eliminate

@noindent
The simplified syntax of the Eliminate pragma used by @code{GNAT ELIM} is:

@smallexample
@cartouche
@b{pragma} Eliminate (Library_Unit_Name, Subprogram_Name);
@end cartouche
@end smallexample

@noindent
where
@table @code
@item Library_Unit_Name
full expanded Ada name of a library unit

@item Subprogram_Name
a simple or expanded name of a subprogram declared within this
compilation unit

@end table

@noindent
The effect of an @code{Eliminate} pragma placed in the GNAT configuration
file @file{GNAT.ADC} is:

@itemize @bullet

@item
If the subprogram @code{Subprogram_Name} is declared within
the library unit @code{Library_Unit_Name}, the compiler will not generate
code for this subprogram. This applies to all overloaded subprograms denoted
by @code{Subprogram_Name}.

@item
If a subprogram marked by the pragma @code{Eliminate} is used (called)
in a program, the compiler will produce an error message in the place where
it is called.
@end itemize

@node Tree Files
@section Tree Files
@cindex Tree file

@noindent
A tree file stores a snapshot of the compiler internal data
structures at the very end of a successful compilation. It contains all the
syntactic and semantic information for the compiled unit and all the
units upon which it depends semantically.
To use tools that make use of tree files, you
need to first produce the right set of tree files.

GNAT produces correct tree files when /TREE_OUTPUT /NOLOAD options are set
in a GNAT COMPILE call. The tree files have an .adt extension.
Therefore, to produce a tree file for the compilation unit contained in a file
named @file{FOO.ADB}, you must use the command

@smallexample
$ GNAT COMPILE /NOLOAD /TREE_OUTPUT FOO.ADB
@end smallexample

@noindent
and you will get the tree file @file{foo.adt}.
compilation.

@node Preparing Tree and Bind Files for GNAT ELIM
@section Preparing Tree and Bind Files for @code{GNAT ELIM}

@noindent
A set of tree files covering the program to be analyzed with
@code{GNAT ELIM} and
the bind file for the main subprogram does not have to
be in the current directory.
'-T' GNAT ELIM option may be used to provide
the search path for tree files, and '-b'
option may be used to point to the bind
file to process (see @ref{Running GNAT ELIM})

If you do not have the appropriate set of tree
files and the right bind file, you
may create them in the current directory using the following procedure.

Let @code{Main_Prog} be the name of a main subprogram, and suppose
this subprogram is in a file named @file{MAIN_PROG.ADB}.

To create a bind file for @code{GNAT ELIM}, run @code{GNAT BIND} for
the main subprogram. @code{GNAT ELIM} can work with both Ada and C
bind files; when both are present, it uses the Ada bind file.
The following commands will build the program and create the bind file:

@smallexample
$ GNAT MAKE /ACTIONS=COMPILE MAIN_PROG
$ GNAT BIND main_prog
@end smallexample

@noindent
To create a minimal set of tree files covering the whole program, call
@code{GNAT MAKE} for this program as follows:

@smallexample
$ GNAT MAKE /FORCE_COMPILE /ACTIONS=COMPILE /NOLOAD /TREE_OUTPUT MAIN_PROG
@end smallexample

@noindent
The @code{/ACTIONS=COMPILE} GNAT MAKE option turns off the bind and link
steps, that are useless anyway because the sources are compiled with
@option{/NOLOAD} option which turns off code generation.

The @code{/FORCE_COMPILE} GNAT MAKE option forces
recompilation of all the needed sources.

This sequence of actions will create all the data needed by @code{GNAT ELIM}
from scratch and therefore guarantee its consistency. If you would like to
use some existing set of files as @code{GNAT ELIM} output, you must make
sure that the set of files is complete and consistent. You can use the
@code{-m} qualifier to check if there are missed tree files

Note, that @code{GNAT ELIM} needs neither object nor ALI files.

@node Running GNAT ELIM
@section Running @code{GNAT ELIM}

@noindent
@code{GNAT ELIM} has the following command-line interface:

@smallexample
$ GNAT ELIM [options] name
@end smallexample

@noindent
@code{name} should be a full expanded Ada name of a main subprogram
of a program (partition).

@code{GNAT ELIM} options:

@table @code
@item /QUIET
Quiet mode: by default @code{GNAT ELIM} generates to the standard error
stream a trace of the source file names of the compilation units being
processed. This option turns this trace off.

@item /VERBOSE
Verbose mode: @code{GNAT ELIM} version information is printed as Ada
comments to the standard output stream.

@item /ALL
Also look for subprograms from the GNAT run time that can be eliminated.

@item /MISSED
Check if any tree files are missing for an accurate result.

@item /TREE_DIRS=@var{dir}
When looking for tree files also look in directory @var{dir}

@item /BIND_FILE=@var{bind_file}
Specifies @var{bind_file} as the bind file to process. If not set, the name
of the bind file is computed from the full expanded Ada name of a main subprogram.

@item -d@var{x}
Activate internal debugging qualifiers. @var{x} is a letter or digit, or
string of letters or digits, which specifies the type of debugging
mode desired.  Normally these are used only for internal development
or system debugging purposes. You can find full documentation for these
qualifiers in the body of the @code{GNAT ELIM.Options} unit in the compiler
source file @file{GNATELIM-OPTIONS.ADB}.
@end table

@noindent
@code{GNAT ELIM} sends its output to the standard output stream, and all the
tracing and debug information is sent to the standard error stream.
In order to produce a proper GNAT configuration file
@file{GNAT.ADC}, redirection must be used:

@smallexample
$ PIPE GNAT ELIM MAIN_PROG > GNAT.ADC
@end smallexample


@noindent
In order to append the @code{GNAT ELIM} output to the existing contents of
@file{GNAT.ADC}.

@node Correcting the List of Eliminate Pragmas
@section Correcting the List of Eliminate Pragmas

@noindent
In some rare cases it may happen that @code{GNAT ELIM} will try to eliminate
subprograms which are actually called in the program. In this case, the
compiler will generate an error message of the form:

@smallexample
FILE.ADB:106:07: cannot call eliminated subprogram "My_Prog"
@end smallexample

@noindent
You will need to manually remove the wrong @code{Eliminate} pragmas from
the @file{GNAT.ADC} file. It is advised that you recompile your program
from scratch after that because you need a consistent @file{GNAT.ADC} file
during the entire compilation.

@node Making Your Executables Smaller
@section Making Your Executables Smaller

@noindent
In order to get a smaller executable for your program you now have to
recompile the program completely with the new @file{GNAT.ADC} file
created by @code{GNAT ELIM} in your current directory:

@smallexample
$ GNAT MAKE /FORCE_COMPILE MAIN_PROG
@end smallexample

@noindent
(you will need @code{/FORCE_COMPILE} option for GNAT MAKE to
recompile everything
with the set of pragmas @code{Eliminate} you have obtained with
@code{GNAT ELIM}).

Be aware that the set of @code{Eliminate} pragmas is specific to each
program. It is not recommended to merge sets of @code{Eliminate}
pragmas created for different programs in one @file{GNAT.ADC} file.

@node Summary of the GNAT ELIM Usage Cycle
@section Summary of the GNAT ELIM Usage Cycle

@noindent
Here is a quick summary of the steps to be taken in order to reduce
the size of your executables with @code{GNAT ELIM}. You may use
other GNAT options to control the optimization level,
to produce the debugging information, to set search path, etc.

@enumerate
@item
Produce a bind file and a set of tree files

@smallexample
$ GNAT MAKE /ACTIONS=COMPILE MAIN_PROG
$ GNAT BIND main_prog
$ GNAT MAKE /FORCE_COMPILE /NO_LINK /NOLOAD /TREE_OUTPUT MAIN_PROG
@end smallexample

@item
Generate a list of @code{Eliminate} pragmas
@smallexample
$ PIPE GNAT ELIM MAIN_PROG > GNAT.ADC
@end smallexample

@item
Recompile the application

@smallexample
$ GNAT MAKE /FORCE_COMPILE MAIN_PROG
@end smallexample

@end enumerate

@node Other Utility Programs
@chapter Other Utility Programs

@noindent
This chapter discusses some other utility programs available in the Ada
environment.

@menu
* Using Other Utility Programs with GNAT::
* The GNAT STANDARD Utility Program::
* The External Symbol Naming Scheme of GNAT::
* Ada Mode for Glide::
* Converting Ada Files to html with gnathtml::
* Installing gnathtml::
* LSE::
* Profiling::
@end menu

@node Using Other Utility Programs with GNAT
@section Using Other Utility Programs with GNAT

@noindent
The object files generated by GNAT are in standard system format and in
particular the debugging information uses this format. This means
programs generated by GNAT can be used with existing utilities that
depend on these formats.


@node The GNAT STANDARD Utility Program
@section The @code{GNAT STANDARD} Utility Program

@noindent
Many of the definitions in package Standard are implementation-dependent.
However, the source of this package does not exist as an Ada source
file, so these values cannot be determined by inspecting the source.
They can be determined by examining in detail the coding of
@file{CSTAND.ADB} which creates the image of Standard in the compiler,
but this is awkward and requires a great deal of internal knowledge
about the system.

The @code{GNAT STANDARD} utility is designed to deal with this situation.
It is an Ada program that dynamically determines the
values of all the relevant parameters in Standard, and prints them
out in the form of an Ada source listing for Standard, displaying all
the values of interest. This output is generated to
@file{SYS$OUTPUT}.

To determine the value of any parameter in package Standard, simply
run @code{GNAT STANDARD} with no qualifiers or arguments, and examine
the output. This is preferable to consulting documentation, because
you know that the values you are getting are the actual ones provided
by the executing system.

@node The External Symbol Naming Scheme of GNAT
@section The External Symbol Naming Scheme of GNAT

@noindent
In order to interpret the output from GNAT, when using tools that are
originally intended for use with other languages, it is useful to
understand the conventions used to generate link names from the Ada
entity names.

All link names are in all lowercase letters. With the exception of library
procedure names, the mechanism used is simply to use the full expanded
Ada name with dots replaced by double underscores. For example, suppose
we have the following package spec:

@smallexample
@group
@cartouche
@b{package} QRS @b{is}
   MN : Integer;
@b{end} QRS;
@end cartouche
@end group
@end smallexample

@noindent
The variable @code{MN} has a full expanded Ada name of @code{QRS.MN}, so
the corresponding link name is @code{qrs__mn}.
@findex Export
Of course if a @code{pragma Export} is used this may be overridden:

@smallexample
@group
@cartouche
@b{package} Exports @b{is}
   Var1 : Integer;
   @b{pragma} Export (Var1, C, External_Name => "var1_name");
   Var2 : Integer;
   @b{pragma} Export (Var2, C, Link_Name => "var2_link_name");
@b{end} Exports;
@end cartouche
@end group
@end smallexample

@noindent
In this case, the link name for @var{Var1} is whatever link name the
C compiler would assign for the C function @var{var1_name}. This typically
would be either @var{var1_name} or @var{_var1_name}, depending on operating
system conventions, but other possibilities exist. The link name for
@var{Var2} is @var{var2_link_name}, and this is not operating system
dependent.

@findex _main
One exception occurs for library level procedures. A potential ambiguity
arises between the required name @code{_main} for the C main program,
and the name we would otherwise assign to an Ada library level procedure
called @code{Main} (which might well not be the main program).

To avoid this ambiguity, we attach the prefix @code{_ada_} to such
names. So if we have a library level procedure such as

@smallexample
@group
@cartouche
@b{procedure} Hello (S : String);
@end cartouche
@end group
@end smallexample

@noindent
the external name of this procedure will be @var{_ada_hello}.

@node Ada Mode for Glide
@section Ada Mode for @code{Glide}

@noindent
The Glide mode for programming in Ada (both, Ada83 and Ada95) helps the
user in understanding existing code and facilitates writing new code. It
furthermore provides some utility functions for easier integration of
standard EMACS features when programming in Ada.

@subsection General Features:

@itemize @bullet
@item
Full Integrated Development Environment :

@itemize @bullet
@item
support of 'project files' for the configuration (directories,
compilation options,...)

@item
compiling and stepping through error messages.

@item
running and debugging your applications within Glide.
@end itemize

@item
easy to use for beginners by pull-down menus,

@item
user configurable by many user-option variables.
@end itemize

@subsection Ada Mode Features That Help Understanding Code:

@itemize @bullet
@item
functions for easy and quick stepping through Ada code,

@item
getting cross reference information for identifiers (e.g. find the
defining place by a keystroke),

@item
displaying an index menu of types and subprograms and move point to
the chosen one,

@item
automatic color highlighting of the various entities in Ada code.
@end itemize

@subsection Glide Support for Writing Ada Code:

@itemize @bullet
@item
switching between spec and body files with possible
autogeneration of body files,

@item
automatic formating of subprograms parameter lists.

@item
automatic smart indentation according to Ada syntax,

@item
automatic completion of identifiers,

@item
automatic casing of identifiers, keywords, and attributes,

@item
insertion of statement templates,

@item
filling comment paragraphs like filling normal text,
@end itemize

For more information, please refer to the online Glide documentation
available in the Glide --> Help Menu.

@node Converting Ada Files to html with gnathtml
@section Converting Ada Files to html with @code{gnathtml}

@noindent
This @code{Perl} script allows Ada source files to be browsed using
standard Web browsers. For installation procedure, see the section
@xref{Installing gnathtml}.

Ada reserved keywords are highlighted in a bold font and Ada comments in
a blue font. Unless your program was compiled with the GNAT COMPILE @option{/XREF=SUPPRESS}
qualifier to suppress the generation of cross-referencing information, user
defined variables and types will appear in a different color; you will
be able to click on any identifier and go to its declaration.

The command line is as follow:
@smallexample
$ perl gnathtml.pl [qualifiers] ada-files
@end smallexample

You can pass it as many Ada files as you want. @code{gnathtml} will generate
an html file for every ada file, and a global file called @file{index.htm}.
This file is an index of every identifier defined in the files.

The available qualifiers are the following ones :

@table @code
@item -83
@cindex @code{-83} (@code{gnathtml})
Only the subset on the Ada 83 keywords will be highlighted, not the full
Ada 95 keywords set.

@item -cc @var{color}
This option allows you to change the color used for comments. The default
value is green. The color argument can be any name accepted by html.

@item -d
@cindex @code{-d} (@code{gnathtml})
If the ada files depend on some other files (using for instance the
@code{with} command, the latter will also be converted to html.
Only the files in the user project will be converted to html, not the files
in the run-time library itself.

@item -D
This command is the same as -d above, but @code{gnathtml} will also look
for files in the run-time library, and generate html files for them.

@item -f
@cindex @code{-f} (@code{gnathtml})
By default, gnathtml will generate html links only for global entities
('with'ed units, global variables and types,...). If you specify the
@code{-f} on the command line, then links will be generated for local
entities too.

@item -l @var{number}
@cindex @code{-l} (@code{gnathtml})
If this qualifier is provided and @var{number} is not 0, then @code{gnathtml}
will number the html files every @var{number} line.

@item -I @var{dir}
@cindex @code{-I} (@code{gnathtml})
Specify a directory to search for library files (@file{.ALI} files) and
source files. You can provide several -I qualifiers on the command line,
and the directories will be parsed in the order of the command line.

@item -o @var{dir}
@cindex @code{-o} (@code{gnathtml})
Specify the output directory for html files. By default, gnathtml will
saved the generated html files in a subdirectory named @file{html/}.

@item -p @var{file}
@cindex @code{-p} (@code{gnathtml})
If you are using EMACS and the most recent EMACS Ada mode, which provides
a full Integrated Development Environment for compiling, checking,
running and debugging applications, you may be using @file{.adp} files
to give the directories where EMACS can find sources and object files.

Using this qualifier, you can tell gnathtml to use these files. This allows
you to get an html version of your application, even if it is spread
over multiple directories.

@item -sc @var{color}
@cindex @code{-sc} (@code{gnathtml})
This option allows you to change the color used for symbol definitions.
The default value is red. The color argument can be any name accepted by html.

@item -t @var{file}
@cindex @code{-t} (@code{gnathtml})
This qualifier provides the name of a file. This file contains a list of
file names to be converted, and the effect is exactly as though they had
appeared explicitly on the command line. This
is the recommended way to work around the command line length limit on some
systems.

@end table

@node Installing gnathtml
@section Installing @code{gnathtml}

@noindent
@code{Perl} needs to be installed on your machine to run this script.
@code{Perl} is freely available for almost every architecture and
Operating System via the Internet.

On Unix systems, you  may want to modify  the  first line of  the script
@code{gnathtml},  to explicitly  tell  the Operating  system  where Perl
is. The syntax of this line is :
@smallexample
#!full_path_name_to_perl
@end smallexample

@noindent
Alternatively, you may run the script using the following command line:

@smallexample
$ perl gnathtml.pl [qualifiers] files
@end smallexample

@node LSE
@section LSE
@findex LSE

@noindent
The GNAT distribution provides an Ada 95 template for the Digital Language
Sensitive Editor (LSE), a component of DECset. In order to
access it, invoke LSE with the qualifier /ENVIRONMENT=GNU:[LIB]ADA95.ENV.

@node Profiling
@section Profiling
@findex PCA

@noindent
GNAT supports The Digital Performance Coverage Analyzer (PCA), a component
of DECset. To use it proceed as outlined under "HELP PCA", except for running
the collection phase with the /DEBUG qualifier.

@smallexample
$ GNAT MAKE /DEBUG <PROGRAM_NAME>
$ DEFINE LIB$DEBUG PCA$COLLECTOR
$ RUN/DEBUG <PROGRAM_NAME>
@end smallexample
@noindent

@node Running and Debugging Ada Programs
@chapter Running and Debugging Ada Programs
@cindex Debugging

@noindent
This chapter discusses how to debug Ada programs. An incorrect Ada program
may be handled in three ways by the GNAT compiler:

@enumerate
@item
The illegality may be a violation of the static semantics of Ada. In
that case GNAT diagnoses the constructs in the program that are illegal.
It is then a straightforward matter for the user to modify those parts of
the program.

@item
The illegality may be a violation of the dynamic semantics of Ada. In
that case the program compiles and executes, but may generate incorrect
results, or may terminate abnormally with some exception.

@item
When presented with a program that contains convoluted errors, GNAT
itself may terminate abnormally without providing full diagnostics on
the incorrect user program.
@end enumerate

@menu
* The GNAT Debugger GDB::
* Running GDB::
* Introduction to GDB Commands::
* Using Ada Expressions::
* Calling User-Defined Subprograms::
* Using the Next Command in a Function::
* Ada Exceptions::
* Ada Tasks::
* Debugging Generic Units::
* GNAT Abnormal Termination or Failure to Terminate::
* Naming Conventions for GNAT Source Files::
* Getting Internal Debugging Information::
* Stack Traceback::
@end menu

@cindex Debugger
@findex GDB

@node The GNAT Debugger GDB
@section The GNAT Debugger GDB

@noindent
@code{GDB} is a general purpose, platform-independent debugger that
can be used to debug mixed-language programs compiled with @code{GCC},
and in particular is capable of debugging Ada programs compiled with
GNAT. The latest versions of @code{GDB} are Ada-aware and can handle
complex Ada data structures.

The manual @cite{Debugging with GDB}
, located in the GNU:[DOCS] directory,
contains full details on the usage of @code{GDB}, including a section on
its usage on programs. This manual should be consulted for full
details. The section that follows is a brief introduction to the
philosophy and use of @code{GDB}.

When GNAT programs are compiled, the compiler optionally writes debugging
information into the generated object file, including information on
line numbers, and on declared types and variables. This information is
separate from the generated code. It makes the object files considerably
larger, but it does not add to the size of the actual executable that
will be loaded into memory, and has no impact on run-time performance. The
generation of debug information is triggered by the use of the
/DEBUG qualifier in the GNAT COMPILE or GNAT MAKE command used to carry out
the compilations. It is important to emphasize that the use of these
options does not change the generated code.

The debugging information is written in standard system formats that
are used by many tools, including debuggers and profilers. The format
of the information is typically designed to describe C types and
semantics, but GNAT implements a translation scheme which allows full
details about Ada types and variables to be encoded into these
standard C formats. Details of this encoding scheme may be found in
the file EXP_DBUG.ADS in the GNAT source distribution. However, the
details of this encoding are, in general, of no interest to a user,
since @code{GDB} automatically performs the necessary decoding.

When a program is bound and linked, the debugging information is
collected from the object files, and stored in the executable image of
the program. Again, this process significantly increases the size of
the generated executable file, but it does not increase the size of
the executable program itself. Furthermore, if this program is run in
the normal manner, it runs exactly as if the debug information were
not present, and takes no more actual memory.

However, if the program is run under control of @code{GDB}, the
debugger is activated.  The image of the program is loaded, at which
point it is ready to run.  If a run command is given, then the program
will run exactly as it would have if @code{GDB} were not present. This
is a crucial part of the @code{GDB} design philosophy.  @code{GDB} is
entirely non-intrusive until a breakpoint is encountered.  If no
breakpoint is ever hit, the program will run exactly as it would if no
debugger were present. When a breakpoint is hit, @code{GDB} accesses
the debugging information and can respond to user commands to inspect
variables, and more generally to report on the state of execution.

@node Running GDB
@section Running GDB

@noindent
The debugger can be launched directly and simply from @code{glide} or
through its graphical interface: @code{gvd}. It can also be used
directly in text mode. Here is described the basic use of @code{GDB}
in text mode. All the commands described below can be used in the
@code{gvd} console window eventhough there is usually other more
graphical ways to achieve the same goals.


@noindent
The command to run @code{GDB} in text mode is

@smallexample
$ $ GDB PROGRAM
@end smallexample

@noindent
where @code{PROGRAM} is the name of the executable file. This
activates the debugger and results in a prompt for debugger commands.
The simplest command is simply @code{run}, which causes the program to run
exactly as if the debugger were not present. The following section
describes some of the additional commands that can be given to @code{GDB}.


@node Introduction to GDB Commands
@section Introduction to GDB Commands

@noindent
@code{GDB} contains a large repertoire of commands. The manual
@cite{Debugging with GDB}
, located in the GNU:[DOCS] directory,
includes extensive documentation on the use
of these commands, together with examples of their use. Furthermore,
the command @var{help} invoked from within @code{GDB} activates a simple help
facility which summarizes the available commands and their options.
In this section we summarize a few of the most commonly
used commands to give an idea of what @code{GDB} is about. You should create
a simple program with debugging information and experiment with the use of
these @code{GDB} commands on the program as you read through the
following section.

@table @code
@item set args @var{arguments}
The @var{arguments} list above is a list of arguments to be passed to
the program on a subsequent run command, just as though the arguments
had been entered on a normal invocation of the program. The @code{set args}
command is not needed if the program does not require arguments.

@item run
The @code{run} command causes execution of the program to start from
the beginning. If the program is already running, that is to say if
you are currently positioned at a breakpoint, then a prompt will ask
for confirmation that you want to abandon the current execution and
restart.

@item breakpoint @var{location}
The breakpoint command sets a breakpoint, that is to say a point at which
execution will halt and @code{GDB} will await further
commands. @var{location} is
either a line number within a file, given in the format @code{file:linenumber},
or it is the name of a subprogram. If you request that a breakpoint be set on
a subprogram that is overloaded, a prompt will ask you to specify on which of
those subprograms you want to breakpoint. You can also
specify that all of them should be breakpointed. If the program is run
and execution encounters the breakpoint, then the program
stops and @code{GDB} signals that the breakpoint was encountered by
printing the line of code before which the program is halted.

@item breakpoint exception @var{name}
A special form of the breakpoint command which breakpoints whenever
exception @var{name} is raised.
If @var{name} is omitted,
then a breakpoint will occur when any exception is raised.

@item print @var{expression}
This will print the value of the given expression. Most simple
Ada expression formats are properly handled by @code{GDB}, so the expression
can contain function calls, variables, operators, and attribute references.

@item continue
Continues execution following a breakpoint, until the next breakpoint or the
termination of the program.

@item step
Executes a single line after a breakpoint. If the next statement is a subprogram
call, execution continues into (the first statement of) the
called subprogram.

@item next
Executes a single line. If this line is a subprogram call, executes and
returns from the call.

@item list
Lists a few lines around the current source location. In practice, it
is usually more convenient to have a separate edit window open with the
relevant source file displayed. Successive applications of this command
print subsequent lines. The command can be given an argument which is a
line number, in which case it displays a few lines around the specified one.

@item backtrace
Displays a backtrace of the call chain. This command is typically
used after a breakpoint has occurred, to examine the sequence of calls that
leads to the current breakpoint. The display includes one line for each
activation record (frame) corresponding to an active subprogram.

@item up
At a breakpoint, @code{GDB} can display the values of variables local
to the current frame. The command @code{up} can be used to
examine the contents of other active frames, by moving the focus up
the stack, that is to say from callee to caller, one frame at a time.

@item down
Moves the focus of @code{GDB} down from the frame currently being
examined to the frame of its callee (the reverse of the previous command),

@item frame @var{n}
Inspect the frame with the given number. The value 0 denotes the frame
of the current breakpoint, that is to say the top of the call stack.

@end table

The above list is a very short introduction to the commands that
@code{GDB} provides. Important additional capabilities, including conditional
breakpoints, the ability to execute command sequences on a breakpoint,
the ability to debug at the machine instruction level and many other
features are described in detail in @cite{Debugging with GDB}.
Note that most commands can be abbreviated
(for example, c for continue, bt for backtrace).

@node Using Ada Expressions
@section Using Ada Expressions
@cindex Ada expressions

@noindent
@code{GDB} supports a fairly large subset of Ada expression syntax, with some
extensions. The philosophy behind the design of this subset is

@itemize @bullet
@item
That @code{GDB} should provide basic literals and access to operations for
arithmetic, dereferencing, field selection, indexing, and subprogram calls,
leaving more sophisticated computations to subprograms written into the
program (which therefore may be called from @code{GDB}).

@item
That type safety and strict adherence to Ada language restrictions
are not particularly important to the @code{GDB} user.

@item
That brevity is important to the @code{GDB} user.
@end itemize

Thus, for brevity, the debugger acts as if there were
implicit @code{with} and @code{use} clauses in effect for all user-written
packages, thus making it unnecessary to fully qualify most names with
their packages, regardless of context. Where this causes ambiguity,
@code{GDB} asks the user's intent.

For details on the supported Ada syntax, see @cite{Debugging with GDB}.

@node Calling User-Defined Subprograms
@section Calling User-Defined Subprograms

@noindent
An important capability of @code{GDB} is the ability to call user-defined
subprograms while debugging. This is achieved simply by entering
a subprogram call statement in the form:

@smallexample
call subprogram-name (parameters)
@end smallexample

@noindent
The keyword @code{call} can be omitted in the normal case where the
@code{subprogram-name} does not coincide with any of the predefined
@code{GDB} commands.

The effect is to invoke the given subprogram, passing it the
list of parameters that is supplied. The parameters can be expressions and
can include variables from the program being debugged. The
subprogram must be defined
at the library level within your program, and @code{GDB} will call the
subprogram within the environment of your program execution (which
means that the subprogram is free to access or even modify variables
within your program).

The most important use of this facility is in allowing the inclusion of
debugging routines that are tailored to particular data structures
in your program. Such debugging routines can be written to provide a suitably
high-level description of an abstract type, rather than a low-level dump
of its physical layout. After all, the standard
@code{GDB print} command only knows the physical layout of your
types, not their abstract meaning. Debugging routines can provide information
at the desired semantic level and are thus enormously useful.

For example, when debugging GNAT itself, it is crucial to have access to
the contents of the tree nodes used to represent the program internally.
But tree nodes are represented simply by an integer value (which in turn
is an index into a table of nodes).
Using the @code{print} command on a tree node would simply print this integer
value, which is not very useful. But the PN routine (defined in file
TREEPR.ADB in the GNAT sources) takes a tree node as input, and displays
a useful high level representation of the tree node, which includes the
syntactic category of the node, its position in the source, the integers
that denote descendant nodes and parent node, as well as varied
semantic information. To study this example in more detail, you might want to
look at the body of the PN procedure in the stated file.

@node Using the Next Command in a Function
@section Using the Next Command in a Function

@noindent
When you use the @code{next} command in a function, the current source
location will advance to the next statement as usual. A special case
arises in the case of a @code{return} statement.

Part of the code for a return statement is the "epilog" of the function.
This is the code that returns to the caller. There is only one copy of
this epilog code, and it is typically associated with the last return
statement in the function if there is more than one return. In some
implementations, this epilog is associated with the first statement
of the function.

The result is that if you use the @code{next} command from a return
statement that is not the last return statement of the function you
may see a strange apparent jump to the last return statement or to
the start of the function. You should simply ignore this odd jump.
The value returned is always that from the first return statement
that was stepped through.

@node Ada Exceptions
@section Breaking on Ada Exceptions
@cindex Exceptions

@noindent
You can set breakpoints that trip when your program raises
selected exceptions.

@table @code
@item break exception
Set a breakpoint that trips whenever (any task in the) program raises
any exception.

@item break exception @var{name}
Set a breakpoint that trips whenever (any task in the) program raises
the exception @var{name}.

@item break exception unhandled
Set a breakpoint that trips whenever (any task in the) program raises an
exception for which there is no handler.

@item info exceptions
@itemx info exceptions @var{regexp}
The @code{info exceptions} command permits the user to examine all defined
exceptions within Ada programs. With a regular expression, @var{regexp}, as
argument, prints out only those exceptions whose name matches @var{regexp}.
@end table

@node Ada Tasks
@section Ada Tasks
@cindex Tasks

@noindent
@code{GDB} allows the following task-related commands:

@table @code
@item info tasks
This command shows a list of current Ada tasks, as in the following example:

@smallexample
@iftex
@leftskip=0cm
@end iftex
(GDB) info tasks
  ID       TID P-ID   Thread Pri State                 Name
   1   8088000   0   807e000  15 Child Activation Wait main_task
   2   80a4000   1   80ae000  15 Accept/Select Wait    b
   3   809a800   1   80a4800  15 Child Activation Wait a
*  4   80ae800   3   80b8000  15 Running               c
@end smallexample

@noindent
In this listing, the asterisk before the first task indicates it to be the
currently running task. The first column lists the task ID that is used
to refer to tasks in the following commands.

@item break @var{linespec} task @var{taskid}
@itemx break @var{linespec} task @var{taskid} if @dots{}
@cindex Breakpoints and tasks
These commands are like the @code{break @dots{} thread @dots{}}.
@var{linespec} specifies source lines.

Use the qualifier @samp{task @var{taskid}} with a breakpoint command
to specify that you only want @code{GDB} to stop the program when a
particular Ada task reaches this breakpoint. @var{taskid} is one of the
numeric task identifiers assigned by @code{GDB}, shown in the first
column of the @samp{info tasks} display.

If you do not specify @samp{task @var{taskid}} when you set a
breakpoint, the breakpoint applies to @emph{all} tasks of your
program.

You can use the @code{task} qualifier on conditional breakpoints as
well; in this case, place @samp{task @var{taskid}} before the
breakpoint condition (before the @code{if}).

@item task @var{taskno}
@cindex Task switching

This command allows to qualifier to the task referred by @var{taskno}. In
particular, This allows to browse the backtrace of the specified
task. It is advised to qualifier back to the original task before
continuing execution otherwise the scheduling of the program may be
perturbated.
@end table

@noindent
For more detailed information on the tasking support, see @cite{Debugging with GDB}.

@node Debugging Generic Units
@section Debugging Generic Units
@cindex Debugging Generic Units
@cindex Generics

@noindent
GNAT always uses code expansion for generic instantiation. This means that
each time an instantiation occurs, a complete copy of the original code is
made, with appropriate substitutions of formals by actuals.

It is not possible to refer to the original generic entities in
@code{GDB}, but it is always possible to debug a particular instance of
a generic, by using the appropriate expanded names. For example, if we have

@smallexample
@group
@cartouche
@b{procedure} g @b{is}

   @b{generic package} k @b{is}
      @b{procedure} kp (v1 : @b{in out} integer);
   @b{end} k;

   @b{package body} k @b{is}
      @b{procedure} kp (v1 : @b{in out} integer) @b{is}
      @b{begin}
         v1 := v1 + 1;
      @b{end} kp;
   @b{end} k;

   @b{package} k1 @b{is new} k;
   @b{package} k2 @b{is new} k;

   var : integer := 1;

@b{begin}
   k1.kp (var);
   k2.kp (var);
   k1.kp (var);
   k2.kp (var);
@b{end};
@end cartouche
@end group
@end smallexample

@noindent
Then to break on a call to procedure kp in the k2 instance, simply
use the command:

@smallexample
(GDB) break g.k2.kp
@end smallexample

@noindent
When the breakpoint occurs, you can step through the code of the
instance in the normal manner and examine the values of local variables, as for
other units.

@node GNAT Abnormal Termination or Failure to Terminate
@section GNAT Abnormal Termination or Failure to Terminate
@cindex GNAT Abnormal Termination or Failure to Terminate

@noindent
When presented with programs that contain serious errors in syntax
or semantics,
GNAT may on rare occasions  experience problems in operation, such
as aborting with a
segmentation fault or illegal memory access, raising an internal
exception, terminating abnormally, or failing to terminate at all.
In such cases, you can activate
various features of GNAT that can help you pinpoint the construct in your
program that is the likely source of the problem.

The following strategies are presented in increasing order of
difficulty, corresponding to your experience in using GNAT and your
familiarity with compiler internals.

@enumerate
@item
Run @code{GNAT COMPILE} with the @option{/REPORT_ERRORS=FULL}. This first
qualifier causes all errors on a given line to be reported. In its absence,
only the first error on a line is displayed.

The @option{/REPORT_ERRORS=IMMEDIATE} qualifier causes errors to be displayed as soon as they
are encountered, rather than after compilation is terminated. If GNAT
terminates prematurely or goes into an infinite loop, the last error
message displayed may help to pinpoint the culprit.

@item
Run @code{GNAT COMPILE} with the @code{/VERBOSE} qualifier. In this mode,
@code{GNAT COMPILE} produces ongoing information about the progress of the
compilation and provides the name of each procedure as code is
generated. This qualifier allows you to find which Ada procedure was being
compiled when it encountered a code generation problem.

@item
@cindex @option{/TRACE_UNITS} qualifier
Run @code{GNAT COMPILE} with the @option{/TRACE_UNITS} qualifier. This is a GNAT specific
qualifier that does for the front-end what @code{VERBOSE} does for the back end.
The system prints the name of each unit, either a compilation unit or
nested unit, as it is being analyzed.
@item
Finally, you can start
@code{GDB} directly on the @code{GNAT1} executable. @code{GNAT1} is the
front-end of GNAT, and can be run independently (normally it is just
called from @code{GNAT COMPILE}). You can use @code{GDB} on @code{GNAT1} as you
would on a C program (but @pxref{The GNAT Debugger GDB} for caveats). The
@code{where} command is the first line of attack; the variable
@code{lineno} (seen by @code{print lineno}), used by the second phase of
@code{GNAT1} and by the @code{GNAT COMPILE} backend, indicates the source line at
which the execution stopped, and @code{input_file name} indicates the name of
the source file.
@end enumerate

@node Naming Conventions for GNAT Source Files
@section Naming Conventions for GNAT Source Files

@noindent
In order to examine the workings of the GNAT system, the following
brief description of its organization may be helpful:

@itemize @bullet
@item
Files with prefix @file{SC} contain the lexical scanner.

@item
All files prefixed with @file{PAR} are components of the parser. The
numbers correspond to chapters of the Ada 95 Reference Manual. For example,
parsing of select statements can be found in @file{PAR-CH9.ADB}.

@item
All files prefixed with @file{SEM} perform semantic analysis. The
numbers correspond to chapters of the Ada standard. For example, all
issues involving context clauses can be found in @file{SEM_CH10.ADB}. In
addition, some features of the language require sufficient special processing
to justify their own semantic files: sem_aggr for aggregates, sem_disp for
dynamic dispatching, etc.

@item
All files prefixed with @file{EXP} perform normalization and
expansion of the intermediate representation (abstract syntax tree, or AST).
these files use the same numbering scheme as the parser and semantics files.
For example, the construction of record initialization procedures is done in
@file{EXP_CH3.ADB}.

@item
The files prefixed with @file{BIND} implement the binder, which
verifies the consistency of the compilation, determines an order of
elaboration, and generates the bind file.

@item
The files @file{ATREE.ADS} and @file{ATREE.ADB} detail the low-level
data structures used by the front-end.

@item
The files @file{SINFO.ADS} and @file{SINFO.ADB} detail the structure of
the abstract syntax tree as produced by the parser.

@item
The files @file{EINFO.ADS} and @file{EINFO.ADB} detail the attributes of
all entities, computed during semantic analysis.

@item
Library management issues are dealt with in files with prefix
@file{LIB}.

@item
@findex Ada
@cindex Annex A
Ada files with the prefix @file{A-} are children of @code{Ada}, as
defined in Annex A.

@item
@findex Interfaces
@cindex Annex B
Files with prefix @file{I-} are children of @code{Interfaces}, as
defined in Annex B.

@item
@findex System
Files with prefix @file{S-} are children of @code{System}. This includes
both language-defined children and GNAT run-time routines.

@item
@findex GNAT
Files with prefix @file{G-} are children of @code{GNAT}. These are useful
general-purpose packages, fully documented in their specifications. All
the other @file{.C} files are modifications of common @code{GNAT COMPILE} files.
@end itemize

@node Getting Internal Debugging Information
@section Getting Internal Debugging Information

@noindent
Most compilers have internal debugging qualifiers and modes. GNAT
does also, except GNAT internal debugging qualifiers and modes are not
secret. A summary and full description of all the compiler and binder
debug flags are in the file @file{DEBUG.ADB}. You must obtain the
sources of the compiler to see the full detailed effects of these flags.

The qualifiers that print the source of the program (reconstructed from
the internal tree) are of general interest for user programs, as are the
options to print
the full internal tree, and the entity table (the symbol table
information). The reconstructed source provides a readable version of the
program after the front-end has completed analysis and  expansion, and is useful
when studying the performance of specific constructs. For example, constraint
checks are indicated, complex aggregates are replaced with loops and
assignments, and tasking primitives are replaced with run-time calls.

@node Stack Traceback
@section Stack Traceback
@cindex traceback
@cindex stack traceback
@cindex stack unwinding

@noindent
Traceback is a mechanism to display the sequence of subprogram calls that
leads to a specified execution point in a program. Often (but not always)
the execution point is an instruction at which an exception has been raised.
This mechanism is also known as @i{stack unwinding} because it obtains
its information by scanning the run-time stack and recovering the activation
records of all active subprograms. Stack unwinding is one of the most
important tools for program debugging.

@noindent
The first entry stored in traceback corresponds to the deepest calling level,
that is to say the subprogram currently executing the instruction
from which we want to obtain the traceback.

@noindent
Note that there is no runtime performance penalty when stack traceback
is enabled and no exception are raised during program execution.

@menu
* Non-Symbolic Traceback::
* Symbolic Traceback::
@end menu

@node Non-Symbolic Traceback
@subsection Non-Symbolic Traceback
@cindex traceback, non-symbolic

@noindent
Note: this feature is not supported on all platforms. See
@file{GNAT.Traceback spec in G-TRACEB.ADS} for a complete list of supported
platforms.

@menu
* Tracebacks From an Unhandled Exception::
* Tracebacks From Exception Occurrences (non-symbolic)::
* Tracebacks From Anywhere in a Program (non-symbolic)::
@end menu

@node Tracebacks From an Unhandled Exception
@subsubsection Tracebacks From an Unhandled Exception

@noindent
A runtime non-symbolic traceback is a list of addresses of call instructions.
To enable this feature you must use the @code{-E}
@code{GNAT BIND}'s option. With this option a stack traceback is stored as part
of exception information. It is possible to retrieve this information using the
standard @code{Ada.Exception.Exception_Information} routine.

@noindent
Let's have a look at a simple example:

@smallexample
@cartouche
@group
procedure STB is

   procedure P1 is
   begin
      raise Constraint_Error;
   end P1;

   procedure P2 is
   begin
      P1;
   end P2;

begin
   P2;
end STB;
@end group
@end cartouche
@end smallexample

@smallexample
$ GNAT MAKE stb /BINDER_QUALIFIERS -E
$ stb

Execution terminated by unhandled exception
Exception name: CONSTRAINT_ERROR
Message: STB.ADB:5
Call stack traceback locations:
0x401373 0x40138b 0x40139c 0x401335 0x4011c4 0x4011f1 0x77e892a4
@end smallexample

@noindent
As we see the traceback lists a sequence of addresses for the unhandled
exception @code{CONSTAINT_ERROR} raised in procedure P1. It is easy to
guess that this exception come from procedure P1. To translate these
addresses into the source lines where the calls appear, the
@code{addr2line} tool, described below, is invaluable. The use of this tool
requires the program to be compiled with debug information.

@smallexample
$ GNAT MAKE -g stb /BINDER_QUALIFIERS -E
$ stb

Execution terminated by unhandled exception
Exception name: CONSTRAINT_ERROR
Message: STB.ADB:5
Call stack traceback locations:
0x401373 0x40138b 0x40139c 0x401335 0x4011c4 0x4011f1 0x77e892a4

$ addr2line --exe=stb 0x401373 0x40138b 0x40139c 0x401335 0x4011c4
   0x4011f1 0x77e892a4

00401373 at d:/stb/STB.ADB:5
0040138B at d:/stb/STB.ADB:10
0040139C at d:/stb/STB.ADB:14
00401335 at d:/stb/B~STB.ADB:104
004011C4 at /build/.../CRT1.C:200
004011F1 at /build/.../CRT1.C:222
77E892A4 in ?? at ??:0
@end smallexample

@noindent
@code{addr2line} has a number of other useful options:

@table @code
@item --functions
to get the function name corresponding to any location

@item --demangle=gnat
to use the @b{gnat} decoding mode for the function names. Note that
for binutils version 2.9.x the option is simply @code{--demangle}.
@end table

@smallexample
$ addr2line --exe=stb --functions --demangle=gnat 0x401373 0x40138b
   0x40139c 0x401335 0x4011c4 0x4011f1

00401373 in stb.p1 at d:/stb/STB.ADB:5
0040138B in stb.p2 at d:/stb/STB.ADB:10
0040139C in stb at d:/stb/STB.ADB:14
00401335 in main at d:/stb/B~STB.ADB:104
004011C4 in <__mingw_CRTStartup> at /build/.../CRT1.C:200
004011F1 in <mainCRTStartup> at /build/.../CRT1.C:222
@end smallexample

@noindent
From this traceback we can see that the exception was raised in
@file{STB.ADB} at line 5, which was reached from a procedure call in
@file{STB.ADB} at line 10, and so on. The @file{B~STD.ADB} is the binder file,
which contains the call to the main program.
@pxref{Running GNAT BIND}. The remaining entries are assorted runtime routines,
and the output will vary from platform to platform.

@noindent
It is also possible to use @code{GDB} with these traceback addresses to debug
the program. For example, we can break at a given code location, as reported
in the stack traceback:

@smallexample
$ GDB -nw stb

(GDB) break *0x401373
Breakpoint 1 at 0x401373: file STB.ADB, line 5.
@end smallexample

@noindent
It is important to note that the stack traceback addresses
do not change when debug information is included. This is particularly useful
because it makes it possible to release software without debug information (to
minimize object size), get a field report that includes a stack traceback
whenever an internal bug occurs, and then be able to retrieve the sequence
of calls with the same program compiled with debug information.

@node Tracebacks From Exception Occurrences (non-symbolic)
@subsubsection Tracebacks From Exception Occurrences

@noindent
Non-symbolic tracebacks are obtained by using the @code{-E} binder argument.
The stack traceback is attached to the exception information string, and can
be retrieved in an exception handler within the Ada program, by means of the
Ada95 facilities defined in @code{Ada.Exceptions}. Here is a simple example:

@smallexample
@cartouche
@group
with Ada.Text_IO;
with Ada.Exceptions;

procedure STB is

   use Ada;
   use Ada.Exceptions;

   procedure P1 is
      K : Positive := 1;
   begin
      K := K - 1;
   exception
      when E : others =>
         Text_IO.Put_Line (Exception_Information (E));
   end P1;

   procedure P2 is
   begin
      P1;
   end P2;

begin
   P2;
end STB;
@end group
@end cartouche
@end smallexample

@noindent
This program will output:

@smallexample
$ stb

Exception name: CONSTRAINT_ERROR
Message: STB.ADB:12
Call stack traceback locations:
0x4015e4 0x401633 0x401644 0x401461 0x4011c4 0x4011f1 0x77e892a4
@end smallexample

@node Tracebacks From Anywhere in a Program (non-symbolic)
@subsubsection Tracebacks From Anywhere in a Program

@noindent
It is also possible to retrieve a stack traceback from anywhere in a
program. For this you need to
use the @code{GNAT.Traceback} API. This package includes a procedure called
@code{Call_Chain} that computes a complete stack traceback, as well as useful
display procedures described below. It is not necessary to use the
@code{-E GNAT BIND} option in this case, because the stack traceback mechanism
is invoked explicitly.

@noindent
In the following example we compute a traceback at a specific location in
the program, and we display it using @code{GNAT.Debug_Utilities.Image} to
convert addresses to strings:

@smallexample
@cartouche
@group
with Ada.Text_IO;
with GNAT.Traceback;
with GNAT.Debug_Utilities;

procedure STB is

   use Ada;
   use GNAT;
   use GNAT.Traceback;

   procedure P1 is
      TB  : Tracebacks_Array (1 .. 10);
      --  We are asking for a maximum of 10 stack frames.
      Len : Natural;
      --  Len will receive the actual number of stack frames returned.
   begin
      Call_Chain (TB, Len);

      Text_IO.Put ("In STB.P1 : ");

      for K in 1 .. Len loop
         Text_IO.Put (Debug_Utilities.Image (TB (K)));
         Text_IO.Put (' ');
      end loop;

      Text_IO.New_Line;
   end P1;

   procedure P2 is
   begin
      P1;
   end P2;

begin
   P2;
end STB;
@end group
@end cartouche
@end smallexample

@smallexample
$ GNAT MAKE stb
$ stb

In STB.P1 : 16#0040_F1E4# 16#0040_14F2# 16#0040_170B# 16#0040_171C#
16#0040_1461# 16#0040_11C4# 16#0040_11F1# 16#77E8_92A4#
@end smallexample

@node Symbolic Traceback
@subsection Symbolic Traceback
@cindex traceback, symbolic

@noindent
A symbolic traceback is a stack traceback in which procedure names are
associated with each code location.

@noindent
Note that this feature is not supported on all platforms. See
@file{GNAT.Traceback.Symbolic spec in G-TRASYM.ADS} for a complete
list of currently supported platforms.

@noindent
Note that the symbolic traceback requires that the program be compiled
with debug information. If it is not compiled with debug information
only the non-symbolic information will be valid.

@menu
* Tracebacks From Exception Occurrences (symbolic)::
* Tracebacks From Anywhere in a Program (symbolic)::
@end menu

@node Tracebacks From Exception Occurrences (symbolic)
@subsubsection Tracebacks From Exception Occurrences

@smallexample
@cartouche
@group
with Ada.Text_IO;
with GNAT.Traceback.Symbolic;

procedure STB is

   procedure P1 is
   begin
      raise Constraint_Error;
   end P1;

   procedure P2 is
   begin
      P1;
   end P2;

   procedure P3 is
   begin
      P2;
   end P3;

begin
   P3;
exception
   when E : others =>
      Ada.Text_IO.Put_Line (GNAT.Traceback.Symbolic.Symbolic_Traceback (E));
end STB;
@end group
@end cartouche
@end smallexample

@smallexample
$ GNAT MAKE -g stb /BINDER_QUALIFIERS -E /LINKER_QUALIFIERS -lgnat -laddr2line -lintl
$ stb

0040149F in stb.p1 at STB.ADB:8
004014B7 in stb.p2 at STB.ADB:13
004014CF in stb.p3 at STB.ADB:18
004015DD in ada.stb at STB.ADB:22
00401461 in main at B~STB.ADB:168
004011C4 in __mingw_CRTStartup at CRT1.C:200
004011F1 in mainCRTStartup at CRT1.C:222
77E892A4 in ?? at ??:0
@end smallexample

@noindent
The exact sequence of linker options may vary from platform to platform.
The above @code{/LINKER_QUALIFIERS} section is for Windows platforms. By contrast,
under Unix there is no need for the @code{/LINKER_QUALIFIERS} section.
Differences across platforms are due to details of linker implementation.

@node Tracebacks From Anywhere in a Program (symbolic)
@subsubsection Tracebacks From Anywhere in a Program

@noindent
It is possible to get a symbolic stack traceback
from anywhere in a program, just as for non-symbolic tracebacks.
The first step is to obtain a non-symbolic
traceback, and then call @code{Symbolic_Traceback} to compute the symbolic
information. Here is an example:

@smallexample
@cartouche
@group
with Ada.Text_IO;
with GNAT.Traceback;
with GNAT.Traceback.Symbolic;

procedure STB is

   use Ada;
   use GNAT.Traceback;
   use GNAT.Traceback.Symbolic;

   procedure P1 is
      TB  : Tracebacks_Array (1 .. 10);
      --  We are asking for a maximum of 10 stack frames.
      Len : Natural;
      --  Len will receive the actual number of stack frames returned.
   begin
      Call_Chain (TB, Len);
      Text_IO.Put_Line (Symbolic_Traceback (TB (1 .. Len)));
   end P1;

   procedure P2 is
   begin
      P1;
   end P2;

begin
   P2;
end STB;
@end group
@end cartouche
@end smallexample

@node Compatibility with DEC Ada
@chapter Compatibility with DEC Ada
@cindex Compatibility

@noindent
This section of the manual compares DEC Ada for OpenVMS Alpha and GNAT
OpenVMS Alpha. GNAT achieves a high level of compatibility
with DEC Ada, and it should generally be straightforward to port code
from the DEC Ada environment to GNAT. However, there are a few language
and implementation differences of which the user must be aware. These
differences are discussed in this section. In
addition, the operating environment and command structure for the
compiler are different, and these differences are also discussed.

Note that this discussion addresses specifically the implementation
of Ada 83 for DIGITAL OpenVMS Alpha Systems. In cases where the implementation
of DEC Ada differs between OpenVMS Alpha Systems and OpenVMS VAX Systems, GNAT
always follows the Alpha implementation.

@menu
* Ada 95 Compatibility::
* Differences in the Definition of Package System::
* Language-Related Features::
* The Package STANDARD::
* The Package SYSTEM::
* Tasking and Task-Related Features::
* Implementation of Tasks in DEC Ada for OpenVMS Alpha Systems::
* Pragmas and Pragma-Related Features::
* Library of Predefined Units::
* Bindings::
* Main Program Definition::
* Implementation-Defined Attributes::
* Compiler and Run-Time Interfacing::
* Program Compilation and Library Management::
* Input-Output::
* Implementation Limits::
* Tools::
@end menu

@node Ada 95 Compatibility
@section Ada 95 Compatibility

@noindent
GNAT is an Ada 95 compiler, and DEC Ada is an Ada 83
compiler. Ada 95 is almost completely upwards compatible
with Ada 83, and therefore Ada 83 programs will compile
and run under GNAT with
no changes or only minor changes. The Ada 95 Reference
Manual (ANSI/ISO/IEC-8652:1995) provides details on specific
incompatibilities.

GNAT provides the qualifier /83 on the GNAT COMPILE command,
as well as the pragma ADA_83, to force the compiler to
operate in Ada 83 mode. This mode does not guarantee complete
conformance to Ada 83, but in practice is sufficient to
eliminate most sources of incompatibilities.
In particular, it eliminates the recognition of the
additional Ada 95 keywords, so that their use as identifiers
in Ada83 program is legal, and handles the cases of packages
with optional bodies, and generics that instantiate unconstrained
types without the use of @code{(<>)}.

@node Differences in the Definition of Package System
@section Differences in the Definition of Package System

@noindent
Both the Ada 95 and Ada 83 reference manuals permit a compiler to add
implementation-dependent declarations to package System. In normal mode,
GNAT does not take advantage of this permission, and the version of System
provided by GNAT exactly matches that in the Ada 95 Reference Manual.

However, DEC Ada adds an extensive set of declarations to package System,
as fully documented in the DEC Ada manuals. To minimize changes required
for programs that make use of these extensions, GNAT provides the pragma
Extend_System for extending the definition of package System. By using:

@smallexample
@group
@cartouche
@b{pragma} Extend_System (Aux_DEC);
@end cartouche
@end group
@end smallexample

@noindent
The set of definitions in System is extended to include those in package
@code{System.Aux_DEC}.
These definitions are incorporated directly into package
System, as though they had been declared there in the first place. For a
list of the declarations added, see the specification of this package,
which can be found in the file @code{S-AUXDEC.ADS} in the GNAT library.
The pragma Extend_System is a configuration pragma, which means that
it can be placed in the file @file{GNAT.ADC}, so that it will automatically
apply to all subsequent compilations. See the section on Configuration
Pragmas for further details.

An alternative approach that avoids the use of the non-standard
Extend_System pragma is to add a context clause to the unit that
references these facilities:

@smallexample
@group
@cartouche
@b{with} System.Aux_DEC;
@b{use}  System.Aux_DEC;
@end cartouche
@end group
@end smallexample

@noindent
The effect is not quite semantically identical to incorporating the declarations
directly into package @code{System},
but most programs will not notice a difference
unless they use prefix notation (e.g. @code{System.Integer_8})
to reference the
entities directly in package @code{System}.
For units containing such references,
the prefixes must either be removed, or the pragma @code{Extend_System}
must be used.

@node Language-Related Features
@section Language-Related Features

@noindent
The following sections highlight differences in types,
representations of types, operations, alignment, and
related topics.

@menu
* Integer Types and Representations::
* Floating-Point Types and Representations::
* Pragmas Float_Representation and Long_Float::
* Fixed-Point Types and Representations::
* Record and Array Component Alignment::
* Address Clauses::
* Other Representation Clauses::
@end menu

@node Integer Types and Representations
@subsection Integer Types and Representations

@noindent
The set of predefined integer types is identical in DEC Ada and GNAT.
Furthermore the representation of these integer types is also identical,
including the capability of size clauses forcing biased representation.

In addition,
DEC Ada for OpenVMS Alpha systems has defined the
following additional integer types in package System:

@itemize @bullet

@item
INTEGER_8

@item
INTEGER_16

@item
INTEGER_32

@item
INTEGER_64

@item
LARGEST_INTEGER
@end itemize

@noindent
When using GNAT, the first four of these types may be obtained from the
standard Ada 95 package @code{Interfaces}.
Alternatively, by use of the pragma
@code{Extend_System}, identical
declarations can be referenced directly in package @code{System}.
On both GNAT and DEC Ada, the maximum integer size is 64 bits.

@node Floating-Point Types and Representations
@subsection Floating-Point Types and Representations
@cindex Floating-Point types

@noindent
The set of predefined floating-point types is identical in DEC Ada and GNAT.
Furthermore the representation of these floating-point
types is also identical. One important difference is that the default
representation for DEC Ada is VAX_Float, but the default representation
for GNAT is IEEE.

Specific types may be declared to be VAX_Float or IEEE, using the pragma
@code{Float_Representation} as described in the DEC Ada documentation.
For example, the declarations:

@smallexample
@group
@cartouche
@b{type} F_Float @b{is digits} 6;
@b{pragma} Float_Representation (VAX_Float, F_Float);
@end cartouche
@end group
@end smallexample

@noindent
declare a type F_Float that will be represented in VAX_Float format.
This set of declarations actually appears in System.Aux_DEC, which provides
the full set of additional floating-point declarations provided in
the DEC Ada version of package
System. This and similar declarations may be accessed in a user program by using
pragma @code{Extend_System}. The use of this
pragma, and the related pragma @code{Long_Float} is described in further
detail in the following section.

@node Pragmas Float_Representation and Long_Float
@subsection Pragmas Float_Representation and Long_Float

@noindent
DEC Ada provides the pragma @code{Float_Representation}, which
acts as a program library qualifier to allow control over
the internal representation chosen for the predefined
floating-point types declared in the package @code{Standard}.
The format of this pragma is as follows:

@smallexample
@group
@cartouche
@b{pragma} @code{Float_Representation}(VAX_Float | IEEE_Float);
@end cartouche
@end group
@end smallexample

@noindent
This pragma controls the representation of floating-point
types as follows:

@itemize @bullet
@item
@code{VAX_Float} specifies that floating-point
types are represented by default with the VAX hardware types
F-floating, D-floating, G-floating. Note that the H-floating
type is available only on DIGITAL Vax systems, and is not available
in either DEC Ada or GNAT for Alpha systems.

@item
@code{IEEE_Float} specifies that floating-point
types are represented by default with the IEEE single and
double floating-point types.
@end itemize

@noindent
GNAT provides an identical implementation of the pragma
@code{Float_Representation}, except that it functions as a
configuration pragma, as defined by Ada 95. Note that the
notion of configuration pragma corresponds closely to the
DEC Ada notion of a program library qualifier.

When no pragma is used in GNAT, the default is IEEE_Float, which is different
from DEC Ada 83, where the default is VAX_Float. In addition, the
predefined libraries in GNAT are built using IEEE_Float, so it is not
advisable to change the format of numbers passed to standard library
routines, and if necessary explicit type conversions may be needed.

The use of IEEE_Float is recommended in GNAT since it is more efficient,
and (given that it conforms to an international standard) potentially more
portable. The situation in which VAX_Float may be useful is in interfacing
to existing code and data that expects the use of VAX_Float. There are
two possibilities here. If the requirement for the use of VAX_Float is
localized, then the best approach is to use the predefined VAX_Float
types in package @code{System}, as extended by
@code{Extend_System}. For example, use @code{System.F_Float}
to specify the 32-bit @code{F-Float} format.

Alternatively, if an entire program depends heavily on the use of
the @code{VAX_Float} and in particular assumes that the types in
package @code{Standard} are in @code{Vax_Float} format, then it
may be desirable to reconfigure GNAT to assume Vax_Float by default.
This is done by using the GNAT LIBRARY command to rebuild the library, and
then using the general form of the @code{Float_Representation}
pragma to ensure that this default format is used throughout.
The form of the GNAT LIBRARY command is:

@smallexample
GNAT LIBRARY /CONFIG=@i{file} /CREATE=@i{directory}
@end smallexample

@noindent
where @i{file} contains the new configuration pragmas
and @i{directory} is the directory to be created to contain
the new library.

@noindent
On OpenVMS systems, DEC Ada provides the pragma @code{Long_Float}
to allow control over the internal representation chosen
for the predefined type @code{Long_Float} and for floating-point
type declarations with digits specified in the range 7 .. 15.
The format of this pragma is as follows:

@smallexample
@cartouche
@b{pragma} Long_Float (D_FLOAT | G_FLOAT);
@end cartouche
@end smallexample

@node Fixed-Point Types and Representations
@subsection Fixed-Point Types and Representations

@noindent
On DEC Ada for OpenVMS Alpha systems, rounding is
away from zero for both positive and negative numbers.
Therefore, +0.5 rounds to 1 and -0.5 rounds to -1.

On GNAT for OpenVMS Alpha, the results of operations
on fixed-point types are in accordance with the Ada 95
rules. In particular, results of operations on decimal
fixed-point types are truncated.

@node Record and Array Component Alignment
@subsection Record and Array Component Alignment

@noindent
On DEC Ada for OpenVMS Alpha, all non composite components
are aligned on natural boundaries. For example, 1-byte
components are aligned on byte boundaries, 2-byte
components on 2-byte boundaries, 4-byte components on 4-byte
byte boundaries, and so on. The OpenVMS Alpha hardware
runs more efficiently with naturally aligned data.

ON GNAT for OpenVMS Alpha, alignment rules are compatible
with DEC Ada for OpenVMS Alpha.

@node Address Clauses
@subsection Address Clauses

@noindent
In DEC Ada and GNAT, address clauses are supported for
objects and imported subprograms.
The predefined type @code{System.Address} is a private type
in both compilers, with the same representation (it is simply
a machine pointer). Addition, subtraction, and comparison
operations are available in the standard Ada 95 package
@code{System.Storage_Elements}, or in package @code{System}
if it is extended to include @code{System.Aux_DEC} using a
pragma @code{Extend_System} as previously described.

Note that code that with's both this extended package @code{System}
and the package @code{System.Storage_Elements} should not @code{use}
both packages, or ambiguities will result. In general it is better
not to mix these two sets of facilities. The Ada 95 package was
designed specifically to provide the kind of features that DEC Ada
adds directly to package @code{System}.

GNAT is compatible with DEC Ada in its handling of address
clauses, except for some limitations in
the form of address clauses for composite objects with
initialization. Such address clauses are easily replaced
by the use of an explicitly-defined constant as described
in the Ada 95 Reference Manual (13.1(22)). For example, the sequence
of declarations:

@smallexample
@group
@cartouche
X, Y : Integer := Init_Func;
Q : String (X .. Y) := "abc";
...
@b{for} Q'Address @b{use} Compute_Address;
@end cartouche
@end group
@end smallexample

@noindent
will be rejected by GNAT, since the address cannot be computed at the time
that Q is declared. To achieve the intended effect, write instead:

@smallexample
@group
@cartouche
X, Y : Integer := Init_Func;
Q_Address : @b{constant} Address := Compute_Address;
Q : String (X .. Y) := "abc";
...
@b{for} Q'Address @b{use} Q_Address;
@end cartouche
@end group
@end smallexample

@noindent
which will be accepted by GNAT (and other Ada 95 compilers), and is also
backwards compatible with Ada 83. A fuller description of the restrictions
on address specifications is found in the GNAT Reference Manual.

@node Other Representation Clauses
@subsection Other Representation Clauses

@noindent
GNAT supports in a compatible manner all the representation
clauses supported by DEC Ada. In addition, it
supports representation clause forms that are new in Ada 95
including COMPONENT_SIZE and SIZE clauses for objects.

@node The Package STANDARD
@section The Package STANDARD

@noindent
The package STANDARD, as implemented by DEC Ada, is fully
described in the Reference Manual for the Ada Programming
Language (ANSI/MIL-STD-1815A-1983) and in the DEC Ada
Language Reference Manual. As implemented by GNAT, the
package STANDARD is described in the Ada 95 Reference
Manual.

In addition, DEC Ada supports the Latin-1 character set in
the type CHARACTER. GNAT supports the Latin-1 character set
in the type CHARACTER and also Unicode (ISO 10646 BMP) in
the type WIDE_CHARACTER.

The floating-point types supported by GNAT are those
supported by DEC Ada, but defaults are different, and are controlled by
pragmas. See @pxref{Floating-Point Types and Representations} for details.

@node The Package SYSTEM
@section The Package SYSTEM

@noindent
DEC Ada provides a system-specific version of the package
SYSTEM for each platform on which the language ships.
For the complete specification of the package SYSTEM, see
Appendix F of the DEC Ada Language Reference Manual.

On DEC Ada, the package SYSTEM includes the following conversion functions:
@itemize @bullet
@item TO_ADDRESS(INTEGER)

@item  TO_ADDRESS(UNSIGNED_LONGWORD)

@item  TO_ADDRESS(universal_integer)

@item  TO_INTEGER(ADDRESS)

@item  TO_UNSIGNED_LONGWORD(ADDRESS)

@item  Function IMPORT_VALUE return UNSIGNED_LONGWORD and the
                 functions IMPORT_ADDRESS and IMPORT_LARGEST_VALUE
@end itemize

@noindent
By default, GNAT supplies a version of SYSTEM that matches
the definition given in the Ada 95 Reference Manual.
This
is a subset of the DIGITAL system definitions, which is as
close as possible to the original definitions. The only difference
is that the definition of SYSTEM_NAME is different:

@smallexample
@group
@cartouche
@b{type} Name @b{is} (SYSTEM_NAME_GNAT);
System_Name : @b{constant} Name := SYSTEM_NAME_GNAT;
@end cartouche
@end group
@end smallexample

@noindent
Also, GNAT adds the new Ada 95 declarations for
BIT_ORDER and DEFAULT_BIT_ORDER.

However, the use of the following pragma causes GNAT
to extend the definition of package SYSTEM so that it
encompasses the full set of DIGITAL-specific extensions,
including the functions listed above:

@smallexample
@cartouche
@b{pragma} Extend_System (Aux_DEC);
@end cartouche
@end smallexample

@noindent
The pragma Extend_System is a configuration pragma that
is most conveniently placed in the @file{GNAT.ADC} file. See the
GNAT Reference Manual for further details.

DEC Ada does not allow the recompilation of the package
SYSTEM. Instead DEC Ada provides several pragmas (SYSTEM_
NAME, STORAGE_UNIT, and MEMORY_SIZE) to modify values in
the package SYSTEM. On OpenVMS Alpha systems, the pragma
SYSTEM_NAME takes the enumeration literal OPENVMS_AXP as
its single argument.

GNAT does permit the recompilation of package SYSTEM using
a special qualifier (/STYLE=GNAT) and this qualifier can be used if
it is necessary to change constants in SYSTEM. GNAT does
not permit the specification of SYSTEM_NAME, STORAGE_UNIT
or MEMORY_SIZE by any other means.

On GNAT systems, the pragma SYSTEM_NAME takes the
enumeration literal SYSTEM_NAME_GNAT.

The definitions provided by the use of

@smallexample
pragma Extend_System (AUX_Dec);
@end smallexample

@noindent
are virtually identical to those provided by the DEC Ada 83 package
System. One important difference is that the name of the TO_ADDRESS
function for type UNSIGNED_LONGWORD is changed to TO_ADDRESS_LONG.
See the GNAT Reference manual for a discussion of why this change was
necessary.

@noindent
The version of TO_ADDRESS taking a universal integer argument is in fact
an extension to Ada 83 not strictly compatible with the reference manual.
In GNAT, we are constrained to be exactly compatible with the standard,
and this means we cannot provide this capability. In DEC Ada 83, the
point of this definition is to deal with a call like:

@smallexample
TO_ADDRESS (16#12777#);
@end smallexample

@noindent
Normally, according to the Ada 83 standard, one would expect this to be
ambiguous, since it matches both the INTEGER and UNSIGNED_LONGWORD forms
of TO_ADDRESS. However, in DEC Ada 83, there is no ambiguity, since the
definition using universal_integer takes precedence.

In GNAT, since the version with universal_integer cannot be supplied, it is
not possible to be 100% compatible. Since there are many programs using
numeric constants for the argument to TO_ADDRESS, the decision in GNAT was
to change the name of the function in the UNSIGNED_LONGWORD case, so the
declarations provided in the GNAT version of AUX_Dec are:

@smallexample
function To_Address (X : Integer) return Address;
pragma Pure_Function (To_Address);

function To_Address_Long (X : Unsigned_Longword) return Address;
pragma Pure_Function (To_Address_Long);
@end smallexample

@noindent
This means that programs using TO_ADDRESS for UNSIGNED_LONGWORD must
change the name to TO_ADDRESS_LONG.

@node Tasking and Task-Related Features
@section Tasking and Task-Related Features

@noindent
The concepts relevant to a comparison of tasking on GNAT
and on DEC Ada for OpenVMS Alpha systems are discussed in
the following sections.

For detailed information on concepts related to tasking in
DEC Ada, see the DEC Ada Language Reference Manual and the
relevant run-time reference manual.

@node Implementation of Tasks in DEC Ada for OpenVMS Alpha Systems
@section Implementation of Tasks in DEC Ada for OpenVMS Alpha Systems

@noindent
On OpenVMS Alpha systems, each Ada task (except a passive
task) is implemented as a single stream of execution
that is created and managed by the kernel. On these
systems, DEC Ada tasking support is based on DECthreads,
an implementation of the POSIX standard for threads.

Although tasks are implemented as threads, all tasks in
an Ada program are part of the same process. As a result,
resources such as open files and virtual memory can be
shared easily among tasks. Having all tasks in one process
allows better integration with the programming environment
(the shell and the debugger, for example).

Also, on OpenVMS Alpha systems, DEC Ada tasks and foreign
code that calls DECthreads routines can be used together.
The interaction between Ada tasks and DECthreads routines
can have some benefits. For example when on OpenVMS Alpha,
DEC Ada can call C code that is already threaded.
GNAT on OpenVMS Alpha uses the facilities of DECthreads,
and Ada tasks are mapped to threads.

@menu
* Assigning Task IDs::
* Task IDs and Delays::
* Task-Related Pragmas::
* Scheduling and Task Priority::
* The Task Stack::
* External Interrupts::
@end menu

@node Assigning Task IDs
@subsection Assigning Task IDs

@noindent
The DEC Ada Run-Time Library always assigns %TASK 1 to
the environment task that executes the main program. On
OpenVMS Alpha systems, %TASK 0 is often used for tasks
that have been created but are not yet activated.

On OpenVMS Alpha systems, task IDs are assigned at
activation. On GNAT systems, task IDs are also assigned at
task creation but do not have the same form or values as
task ID values in DEC Ada. There is no null task, and the
environment task does not have a specific task ID value.

@node Task IDs and Delays
@subsection Task IDs and Delays

@noindent
On OpenVMS Alpha systems, tasking delays are implemented
using Timer System Services. The Task ID is used for the
identification of the timer request (the REQIDT parameter).
If Timers are used in the application take care not to use
0 for the identification, because cancelling such a timer
will cancel all timers and may lead to unpredictable results.

@node Task-Related Pragmas
@subsection Task-Related Pragmas

@noindent
Ada supplies the pragma TASK_STORAGE, which allows
specification of the size of the guard area for a task
stack. (The guard area forms an area of memory that has no
read or write access and thus helps in the detection of
stack overflow.) On OpenVMS Alpha systems, if the pragma
TASK_STORAGE specifies a value of zero, a minimal guard
area is created. In the absence of a pragma TASK_STORAGE, a default guard
area is created.

GNAT supplies the following task-related pragmas:

@itemize @bullet
@item  TASK_INFO

              This pragma appears within a task definition and
              applies to the task in which it appears. The argument
              must be of type SYSTEM.TASK_INFO.TASK_INFO_TYPE.

@item  TASK_STORAGE

              GNAT implements pragma TASK_STORAGE in the same way as
              DEC Ada.
              Both DEC Ada and GNAT supply the pragmas PASSIVE,
              SUPPRESS, and VOLATILE.
@end itemize
@node Scheduling and Task Priority
@subsection Scheduling and Task Priority

@noindent
DEC Ada implements the Ada language requirement that
when two tasks are eligible for execution and they have
different priorities, the lower priority task does not
execute while the higher priority task is waiting. The DEC
Ada Run-Time Library keeps a task running until either the
task is suspended or a higher priority task becomes ready.

On OpenVMS Alpha systems, the default strategy is round-
robin with preemption. Tasks of equal priority take turns
at the processor. A task is run for a certain period of
time and then placed at the rear of the ready queue for
its priority level.

DEC Ada provides the implementation-defined pragma TIME_SLICE,
which can be used to enable or disable round-robin
scheduling of tasks with the same priority.
See the relevant DEC Ada run-time reference manual for
information on using the pragmas to control DEC Ada task
scheduling.

GNAT follows the scheduling rules of Annex D (real-time
Annex) of the Ada 95 Reference Manual. In general, this
scheduling strategy is fully compatible with DEC Ada
although it provides some additional constraints (as
fully documented in Annex D).
GNAT implements time slicing control in a manner compatible with
DEC Ada 83, by means of the pragma Time_Slice, whose semantics are identical
to the DEC Ada 83 pragma of the same name.
Note that it is not possible to mix GNAT tasking and
DEC Ada 83 tasking in the same program, since the two run times are
not compatible.

@node The Task Stack
@subsection The Task Stack

@noindent
In DEC Ada, a task stack is allocated each time a
non passive task is activated. As soon as the task is
terminated, the storage for the task stack is deallocated.
If you specify a size of zero (bytes) with T'STORAGE_SIZE,
a default stack size is used. Also, regardless of the size
specified, some additional space is allocated for task
management purposes. On OpenVMS Alpha systems, at least
one page is allocated.

GNAT handles task stacks in a similar manner. According to
the Ada 95 rules, it provides the pragma STORAGE_SIZE as
an alternative method for controlling the task stack size.
The specification of the attribute T'STORAGE_SIZE is also
supported in a manner compatible with DEC Ada.

@node External Interrupts
@subsection External Interrupts

@noindent
On DEC Ada, external interrupts can be associated with task entries.
GNAT is compatible with DEC Ada in its handling of external interrupts.

@node Pragmas and Pragma-Related Features
@section Pragmas and Pragma-Related Features

@noindent
Both DEC Ada and GNAT supply all language-defined pragmas
as specified by the Ada 83 standard. GNAT also supplies all
language-defined pragmas specified in the Ada 95 Reference Manual.
In addition, GNAT implements the implementation-defined pragmas
from DEC Ada 83.

@itemize @bullet
@item  AST_ENTRY

@item  COMMON_OBJECT

@item  COMPONENT_ALIGNMENT

@item  EXPORT_EXCEPTION

@item  EXPORT_FUNCTION

@item  EXPORT_OBJECT

@item  EXPORT_PROCEDURE

@item  EXPORT_VALUED_PROCEDURE

@item  FLOAT_REPRESENTATION

@item  IDENT

@item  IMPORT_EXCEPTION

@item  IMPORT_FUNCTION

@item  IMPORT_OBJECT

@item  IMPORT_PROCEDURE

@item  IMPORT_VALUED_PROCEDURE

@item  INLINE_GENERIC

@item  INTERFACE_NAME

@item  LONG_FLOAT

@item  MAIN_STORAGE

@item  PASSIVE

@item  PSET_OBJECT

@item  SHARE_GENERIC

@item  SUPPRESS_ALL

@item  TASK_STORAGE

@item  TIME_SLICE

@item  TITLE
@end itemize

@noindent
These pragmas are all fully implemented, with the exception of @code{Title},
@code{Passive}, and @code{Share_Generic}, which are
recognized, but which have no
effect in GNAT. The effect of @code{Passive} may be obtained by the
use of protected objects in Ada 95. In GNAT, all generics are inlined.

Unlike DEC Ada, the GNAT 'EXPORT_@i{subprogram}' pragmas require
a separate subprogram specification which must appear before the
subprogram body.

GNAT also supplies a number of implementation-defined pragmas as follows:
@itemize @bullet
@item  C_PASS_BY_COPY

@item  EXTEND_SYSTEM

@item  SOURCE_FILE_NAME

@item  UNSUPPRESS

@item  WARNINGS

@item  ABORT_DEFER

@item  ADA_83

@item  ADA_95

@item  ANNOTATE

@item  ASSERT

@item  CPP_CLASS

@item  CPP_CONSTRUCTOR

@item  CPP_DESTRUCTOR

@item  CPP_VIRTUAL

@item  CP_VTABLE

@item  DEBUG

@item  LINKER_ALIAS

@item  LINKER_SECTION

@item  MACHINE_ATTRIBUTE

@item  NO_RETURN

@item  PURE_FUNCTION

@item  SOURCE_REFERENCE

@item  TASK_INFO

@item  UNCHECKED_UNION

@item  UNIMPLEMENTED_UNIT

@item  WEAK_EXTERNAL
@end itemize

@noindent
For full details on these GNAT implementation-defined pragmas, see
the GNAT Reference Manual.

@menu
* Restrictions on the Pragma INLINE::
* Restrictions on the Pragma INTERFACE::
* Restrictions on the Pragma SYSTEM_NAME::
@end menu

@node Restrictions on the Pragma INLINE
@subsection Restrictions on the Pragma INLINE

@noindent
DEC Ada applies the following restrictions to the pragma INLINE:
@itemize @bullet
@item  Parameters cannot be a task type.

@item  Function results cannot be task types, unconstrained
array types, or unconstrained types with discriminants.

@item  Bodies cannot declare the following:
@itemize @bullet
@item  Subprogram body or stub (imported subprogram is allowed)

@item  Tasks

@item  Generic declarations

@item  Instantiations

@item  Exceptions

@item  Access types (types derived from access types allowed)

@item  Array or record types

@item  Dependent tasks

@item  Direct recursive calls of subprogram or containing
subprogram, directly or via a renaming

@end itemize
@end itemize

@noindent
In GNAT, the only restriction on pragma INLINE is that the
body must occur before the call if both are in the same
unit, and the size must be appropriately small. There are
no other specific restrictions which cause subprograms to
be incapable of being inlined.

@node  Restrictions on the Pragma INTERFACE
@subsection  Restrictions on the Pragma INTERFACE

@noindent
The following lists and describes the restrictions on the
pragma INTERFACE on DEC Ada and GNAT:
@itemize @bullet
@item  Languages accepted: Ada, Bliss, C, Fortran, Default.
Default is the default on OpenVMS Alpha systems.

@item  Parameter passing: Language specifies default
mechanisms but can be overridden with an EXPORT pragma.

@itemize @bullet
@item  Ada: Use internal Ada rules.

@item  Bliss, C: Parameters must be mode @code{in}; cannot be
record or task type. Result cannot be a string, an
array, or a record.

@item  Fortran: Parameters cannot be a task. Result cannot
be a string, an array, or a record.
@end itemize
@end itemize

@noindent
GNAT is entirely upwards compatible with DEC Ada, and in addition allows
record parameters for all languages.

@node  Restrictions on the Pragma SYSTEM_NAME
@subsection  Restrictions on the Pragma SYSTEM_NAME

@noindent
For DEC Ada for OpenVMS Alpha, the enumeration literal
for the type NAME is OPENVMS_AXP. In GNAT, the enumeration
literal for the type NAME is SYSTEM_NAME_GNAT.

@node  Library of Predefined Units
@section  Library of Predefined Units

@noindent
A library of predefined units is provided as part of the
DEC Ada and GNAT implementations. DEC Ada does not provide
the package MACHINE_CODE but instead recommends importing
assembler code.

The GNAT versions of the DEC Ada Run-Time Library (ADA$PREDEFINED:)
units are taken from the OpenVMS Alpha version, not the OpenVMS VAX
version. During GNAT installation, the DEC Ada Predefined
Library units are copied into the GNU:[LIB.OPENVMS7_x.2_8_x.DECLIB]
(aka DECLIB) directory and patched to remove Ada 95 incompatibilities
and to make them interoperable with GNAT, @pxref{Changes to DECLIB}
for details.

The GNAT RTL is contained in
the GNU:[LIB.OPENVMS7_x.2_8_x.ADALIB] (aka ADALIB) directory and
the default search path is set up to find DECLIB units in preference
to ADALIB units with the same name (TEXT_IO, SEQUENTIAL_IO, and DIRECT_IO,
for example).

However, it is possible to change the default so that the
reverse is true, or even to mix them using child package
notation. The DEC Ada 83 units are available as DEC.xxx where xxx
is the package name, and the Ada units are available in the
standard manner defined for Ada 95, that is to say as Ada.xxx. To
change the default, set ADA_INCLUDE_PATH and ADA_OBJECTS_PATH
appropriately. For example, to change the default to use the Ada95
versions do:

@smallexample
$ DEFINE ADA_INCLUDE_PATH GNU:[LIB.OPENVMS7_1.2_8_1.ADAINCLUDE],-
                          GNU:[LIB.OPENVMS7_1.2_8_1.DECLIB]
$ DEFINE ADA_OBJECTS_PATH GNU:[LIB.OPENVMS7_1.2_8_1.ADALIB],-
                          GNU:[LIB.OPENVMS7_1.2_8_1.DECLIB]
@end smallexample

@menu
* Changes to DECLIB::
@end menu

@node Changes to DECLIB
@subsection Changes to DECLIB

@noindent
The changes made to the DEC Ada predefined library for GNAT and Ada 95
compatibility are minor and include the following:

@itemize @bullet
@item  Adjusting the location of pragmas and record representation
clauses to obey Ada 95 rules

@item  Adding the proper notation to generic formal parameters
that take unconstrained types in instantiation

@item  Adding pragma ELABORATE_BODY to package specifications
that have package bodies not otherwise allowed

@item  Occurrences of the identifier "PROTECTED" are renamed to "PROTECTD".
Currently these are found only in the STARLET package spec.
@end itemize

@noindent
None of the above changes is visible to users.

@node Bindings
@section Bindings

@noindent
On OpenVMS Alpha, DEC Ada provides the following strongly-typed bindings:
@itemize @bullet

@item  Command Language Interpreter (CLI interface)

@item  DECtalk Run-Time Library (DTK interface)

@item  Librarian utility routines (LBR interface)

@item  General Purpose Run-Time Library (LIB interface)

@item  Math Run-Time Library (MTH interface)

@item  National Character Set Run-Time Library (NCS interface)

@item  Compiled Code Support Run-Time Library (OTS interface)

@item  Parallel Processing Run-Time Library (PPL interface)

@item  Screen Management Run-Time Library (SMG interface)

@item  Sort Run-Time Library (SOR interface)

@item  String Run-Time Library (STR interface)

@item STARLET System Library
@findex Starlet

@item  X Window System Version 11R4 and 11R5 (X, XLIB interface)

@item  X Windows Toolkit (XT interface)

@item  X/Motif Version 1.1.3 and 1.2 (XM interface)
@end itemize

@noindent
GNAT provides implementations of these DEC bindings in the DECLIB directory.

The X/Motif bindings used to build DECLIB are whatever versions are in the
DEC Ada ADA$PREDEFINED directory with extension .ADC. The build script will
automatically add a pragma Linker_Options to packages Xm, Xt, and X_Lib
causing the default X/Motif shareable image libraries to be linked in. This
is done via options files named xm.opt, xt.opt, and x_lib.opt (also located
in the DECLIB directory).

It may be necessary to edit these options files to update or correct the
library names if, for example, the newer X/Motif bindings from ADA$EXAMPLES
had been (previous to installing GNAT) copied and renamed to superseded the
default ADA$PREDEFINED versions.

@menu
* Shared Libraries and Options Files::
* Interfaces to C::
@end menu

@node Shared Libraries and Options Files
@subsection Shared Libraries and Options Files

@noindent
When using the DEC Ada
predefined X and Motif bindings, the linking with their shareable images is
done automatically by GNAT LINK. When using other X and Motif bindings, it
is necessary to add the corresponding shareable images to the command line for
GNAT LINK. When linking with shared libraries, or with .OPT files, it is
also necessary to add them to the command line for GNAT LINK.

A shared library to be used with GNAT is built in the same way as other
libraries under VMS. The VMS Link command can be used in standard fashion.

@node Interfaces to C
@subsection Interfaces to C

@noindent
DEC Ada
provides the following Ada types and operations:

@itemize @bullet
@item C types package (C_TYPES)

@item C strings (C_TYPES.NULL_TERMINATED)

@item Other_types (SHORT_INT)
@end itemize

@noindent
Interfacing to C with GNAT, one can use the above approach
described for DEC Ada or the facilities of Annex B of
the Ada 95 Reference Manual (packages INTERFACES.C,
INTERFACES.C.STRINGS and INTERFACES.C.POINTERS). For more
information, see the section "Interfacing to C" in the
GNAT Reference Manual.

The @option{/UPPERCASE_EXTERNALS} qualifier forces default and explicit
@code{External_Name} parameters in pragmas Import and Export
to be uppercased for compatibility with the default behavior
of DEC C. The qualifier has no effect on @code{Link_Name} parameters.

@node Main Program Definition
@section Main Program Definition

@noindent
The following section discusses differences in the
definition of main programs on DEC Ada and GNAT.
On DEC Ada, main programs are defined to meet the
following conditions:
@itemize @bullet
@item  Procedure with no formal parameters (returns 0 upon
       normal completion)

@item  Procedure with no formal parameters (returns 42 when
       unhandled exceptions are raised)

@item  Function with no formal parameters whose returned value
       is of a discrete type

@item  Procedure with one OUT formal of a discrete type for
       which a specification of pragma EXPORT_VALUED_PROCEDURE is given.

@end itemize

@noindent
When declared with the pragma EXPORT_VALUED_PROCEDURE,
a main function or main procedure returns a discrete
value whose size is less than 64 bits (32 on VAX systems),
the value is zero- or sign-extended as appropriate.
On GNAT, main programs are defined as follows:
@itemize @bullet
@item  Must be a non-generic, parameter-less subprogram that
is either a procedure or function returning an Ada
STANDARD.INTEGER (the predefined type)

@item  Cannot be a generic subprogram or an instantiation of a
generic subprogram
@end itemize

@node Implementation-Defined Attributes
@section Implementation-Defined Attributes

@noindent
GNAT provides all DEC Ada implementation-defined
attributes.

@node Compiler and Run-Time Interfacing
@section Compiler and Run-Time Interfacing

@noindent
DEC Ada provides the following ways to pass options to the linker (ACS LINK):
@itemize @bullet
@item  /WAIT and /SUBMIT qualifiers

@item  /COMMAND qualifier

@item  /[NO]MAP qualifier

@item  /OUTPUT=file-spec

@item  /[NO]DEBUG and /[NO]TRACEBACK qualifiers
@end itemize

@noindent
To pass options to the linker, GNAT provides the following
qualifiers:

@itemize @bullet
@item   /EXECUTABLE=exec-name

@item   /VERBOSE qualifier

@item   /[NO]DEBUG and /[NO]TRACEBACK qualifiers
@end itemize

@noindent
For more information on these qualifiers, see the section
"Qualifiers for GNAT LINK" in the corresponding section of this Guide.
In DEC Ada, the command-line qualifier /OPTIMIZE is available
to control optimization. DEC Ada also supplies the
following pragmas:
@itemize @bullet
@item  OPTIMIZE

@item  INLINE

@item  INLINE_GENERIC

@item  SUPPRESS_ALL

@item  PASSIVE
@end itemize

@noindent
In GNAT, optimization is controlled strictly by command
line parameters, as described in the corresponding section of this guide.
The DIGITAL pragmas for control of optimization are
recognized but ignored.

Note that in GNAT, the default is optimization off, whereas in DEC Ada 83,
the default is that optimization is turned on.

@node Program Compilation and Library Management
@section Program Compilation and Library Management

@noindent
DEC Ada and GNAT provide a comparable set of commands to
build programs. DEC Ada also provides a program library,
which is a concept that does not exist on GNAT. Instead,
GNAT provides directories of sources that are compiled as
needed.

The following table summarizes
the DEC Ada commands and provides
equivalent GNAT commands. In this table, some GNAT
equivalents reflect the fact that GNAT does not use the
concept of a program library. Instead, it uses a model
in which collections of source and object files are used
in a manner consistent with other languages like C and
Fortran. Therefore, standard system file commands are used
to manipulate these elements. Those GNAT commands are marked with
an asterisk in the table that follows.
Note that, unlike DEC Ada,  none of the GNAT commands accepts wild cards.

@need 1500
@multitable @columnfractions .31 .30 .39

@item @strong{DEC_Ada_Command}
@tab @strong{GNAT_Equivalent}
@tab @strong{Description}

@item  ADA
@tab     GNAT COMPILE
@tab     Invokes the compiler to compile one or more Ada source files.

@item  ACS ATTACH
@tab     No equivalent
@tab     Qualifiers control of terminal from current process running the program
                                                library manager.

@item   ACS CHECK
@tab      GNAT MAKE /DEPENDENCY_LIST
@tab      Forms the execution closure of one
          or more compiled units and checks completeness and currency.

@item   ACS COMPILE
@tab      GNAT MAKE /ACTIONS=COMPILE
@tab      Forms the execution closure of one or
          more specified units, checks completeness and currency,
          identifies units that have revised source files, compiles same,
          and recompiles units that are or will become obsolete.
          Also completes incomplete generic instantiations.

@item   ACS COPY FOREIGN
@tab      Copy (*)
@tab      Copies a foreign object file into the program library as a
          library unit body.

@item   ACS COPY UNIT
@tab      Copy (*)
@tab      Copies a compiled unit from one program library to another.

@item   ACS CREATE LIBRARY
@tab      Create /directory (*)
@tab      Creates a program library.

@item   ACS CREATE SUBLIBRARY
@tab      Create /directory (*)
@tab      Creates a program sublibrary.

@item   ACS DELETE LIBRARY
@tab
@tab       Deletes a program library and its contents.

@item   ACS DELETE SUBLIBRARY
@tab
@tab      Deletes a program sublibrary and its contents.

@item   ACS DELETE UNIT
@tab      Delete @i{file} (*)
@tab      On OpenVMS systems, deletes one or more compiled units from
          the current program library.

@item   ACS DIRECTORY
@tab       Directory (*)
@tab       On OpenVMS systems, lists units contained in the current
           program library.

@item   ACS ENTER FOREIGN
@tab      Copy (*)
@tab      Allows the import of a foreign body as an Ada library
          specification and enters a reference to a pointer.

@item   ACS ENTER UNIT
@tab      Copy (*)
@tab      Enters a reference (pointer) from the current program library to
          a unit compiled into another program library.

@item   ACS EXIT
@tab      No equivalent
@tab      Exits from the program library manager.

@item   ACS EXPORT
@tab      Copy (*)
@tab      Creates an object file that contains system-specific object code
          for one or more units. With GNAT, object files can simply be copied
          into the desired directory.

@item   ACS EXTRACT SOURCE
@tab      Copy (*)
@tab      Allows access to the copied source file for each Ada compilation unit

@item   ACS HELP
@tab      HELP GNAT
@tab      Provides online help.

@item    ACS LINK
@tab       GNAT LINK
@tab       Links an object file containing Ada units into an executable
           file.

@item    ACS LOAD
@tab       Copy (*)
@tab       Loads (partially compiles) Ada units into the program library.
           Allows loading a program from a collection of files into a library
           without knowing the relationship among units.

@item    ACS MERGE
@tab      Copy (*)
@tab      Merges into the current program library, one or more units from
          another library where they were modified.

@item    ACS RECOMPILE
@tab       GNAT MAKE /ACTIONS=COMPILE
@tab       Recompiles from   external or copied source files any obsolete
           unit in the closure. Also, completes any incomplete generic
           instantiations.

@item    ACS REENTER
@tab       GNAT MAKE
@tab       Reenters current references to units compiled after last entered
           with the ACS ENTER UNIT command.

@item    ACS SET LIBRARY
@tab       Set default (*)
@tab       Defines a program library to be the compilation context as well
           as the target library for compiler output and commands in general.

@item    ACS SET PRAGMA
@tab       Edit GNAT.ADC (*)
@tab       Redefines specified  values of the library characteristics
            LONG_ FLOAT, MEMORY_SIZE, SYSTEM_NAME, and @code{Float_Representation}.

@item    ACS SET SOURCE
@tab       define @* ADA_INCLUDE_PATH @i{path} (*)
@tab       Defines the source file search list for the ACS COMPILE  command.

@item    ACS SHOW LIBRARY
@tab       Directory (*)
@tab        Lists information about one or more program libraries.

@item    ACS SHOW PROGRAM
@tab       No equivalent
@tab       Lists information about the execution closure of one or
           more units in the program library.

@item    ACS SHOW SOURCE
@tab       Show logical @* ADA_INCLUDE_PATH
@tab       Shows the source file search used when compiling units.

@item    ACS SHOW VERSION
@tab       Compile with VERBOSE option
@tab       Displays the version number of the compiler and program library
           manager used.

@item    ACS SPAWN
@tab        No equivalent
@tab        Creates a subprocess of the current process (same as DCL SPAWN
            command).

@item    ACS VERIFY
@tab       No equivalent
@tab       Performs a series of consistency checks on a program library to
           determine whether the library structure and library files are in
           valid_form.

@end multitable

@noindent

@node Input-Output
@section Input-Output

@noindent
On OpenVMS Alpha systems, DEC Ada uses OpenVMS Record
Management Services (RMS) to perform operations on
external files.

@noindent
DEC Ada and GNAT predefine an identical set of input-
output packages. To make the use of the
generic TEXT_IO operations more convenient, DEC Ada
provides predefined library packages that instantiate the
integer and floating-point operations for the predefined
integer and floating-point types as shown in the following table.

@table @code

@item   Package_Name
      Instantiation

@item   INTEGER_TEXT_IO
      INTEGER_IO(INTEGER)

@item   SHORT_INTEGER_TEXT_IO
     INTEGER_IO(SHORT_INTEGER)

@item   SHORT_SHORT_INTEGER_TEXT_IO
     INTEGER_IO(SHORT_SHORT_ INTEGER)

@item   FLOAT_TEXT_IO
      FLOAT_IO(FLOAT)

@item   LONG_FLOAT_TEXT_IO
      FLOAT_IO(LONG_FLOAT)
@end table

@noindent
The DEC Ada predefined packages and their operations
are implemented using OpenVMS Alpha files and input-
output facilities. DEC Ada supports asynchronous input-
output on OpenVMS Alpha. Familiarity with the following is
recommended:
@itemize @bullet
@item  RMS file organizations and access methods

@item  OpenVMS file specifications and directories

@item  OpenVMS File Definition Language (FDL)
@end itemize

@noindent
GNAT provides I/O facilities that are completely
compatible with DEC Ada. The distribution includes the
standard DEC Ada versions of all I/O packages, operating
in a manner compatible with DEC Ada. In particular, the
following packages are by default the DEC Ada (Ada 83)
versions of these packages rather than the renamings
suggested in annex J of the Ada 95 Reference Manual:
@itemize @bullet
@item  TEXT_IO

@item  SEQUENTIAL_IO

@item  DIRECT_IO
@end itemize

@noindent
The use of the standard Ada 95 syntax for child packages (for
example, ADA.TEXT_IO) retrieves the Ada 95 versions of these
packages, as defined in the Ada 95 Reference Manual.
GNAT provides DIGITAL-compatible predefined instantiations
of the TEXT_IO packages, and also
provides the standard predefined instantiations required
by the Ada 95 Reference Manual.

For further information on how GNAT interfaces to the file
system or how I/O is implemented in programs written in
mixed languages, see the chapter "Implementation of the
Standard I/O" in the GNAT Reference Manual.
This chapter covers the following:
@itemize @bullet
@item  Standard I/O packages

@item  FORM strings

@item  DIRECT_IO

@item  SEQUENTIAL_IO

@item  TEXT_IO

@item  Stream pointer positioning

@item  Reading and writing non-regular files

@item  GET_IMMEDIATE

@item  Treating TEXT_IO files as streams

@item  Shared files

@item  Open modes
@end itemize

@node Implementation Limits
@section Implementation Limits

@noindent
The following table lists implementation limits for DEC Ada and GNAT systems.
@multitable @columnfractions .60 .20 .20
@item  Compilation Parameter
@tab   DEC Ada
@tab   GNAT

@item  In a subprogram or entry  declaration, maximum number of
       formal parameters that are of an unconstrained record type
@tab   32
@tab   No set limit

@item  Maximum identifier length (number of characters)
@tab   255
@tab   255

@item  Maximum number of characters in a source line
@tab   255
@tab   255

@item  Maximum collection size   (number of bytes)
@tab   2**31-1
@tab   2**31-1

@item  Maximum number of discriminants for a record type
@tab   245
@tab   No set limit

@item  Maximum number of formal parameters in an entry or
       subprogram declaration
@tab   246
@tab    No set limit

@item  Maximum number of dimensions in an array type
@tab   255
@tab   No set limit

@item  Maximum number of library  units and subunits in a compilation.
@tab   4095
@tab   No set limit

@item  Maximum number of library units and subunits in an execution.
@tab   16383
@tab   No set limit

@item  Maximum number of objects declared with the pragma COMMON_OBJECT
       or PSECT_OBJECT
@tab   32757
@tab   No set limit

@item  Maximum number of enumeration literals in an enumeration type
       definition
@tab   65535
@tab   No set limit

@item  Maximum number of lines in a source file
@tab   65534
@tab   No set limit

@item  Maximum number of bits in any object
@tab   2**31-1
@tab   2**31-1

@item  Maximum size of the static portion of a stack frame (approximate)
@tab   2**31-1
@tab   2**31-1
@end multitable

@node  Tools
@section Tools


@node Inline Assembler
@chapter Inline Assembler

@noindent
If you need to write low-level software that interacts directly with the hardware, Ada provides two ways to incorporate assembly language code into your program.  First, you can import and invoke external routines written in assembly language, an Ada feature fully supported by GNAT.  However, for small sections of code it may be simpler or more efficient to include assembly language statements directly in your Ada source program, using the facilities of the implementation-defined package @code{System.Machine_Code}, which incorporates the GNAT COMPILE Inline Assembler.  The Inline Assembler approach offers a number of advantages, including the following:

@itemize @bullet
@item No need to use non-Ada tools
@item Consistent interface over different targets
@item Automatic usage of the proper calling conventions
@item Access to Ada constants and variables
@item Definition of intrinsic routines
@item Possibility of inlining a subprogram comprising assembler code
@item Code optimizer can take Inline Assembler code into account
@end itemize

This chapter presents a series of examples to show you how to use the Inline Assembler.  Although it focuses on the Intel x86, the general approach applies also to other processors.  It is assumed that you are familiar with Ada and with assembly language programming.

@menu
* Basic Assembler Syntax::
* A Simple Example of Inline Assembler::
* Output Variables in Inline Assembler::
* Input Variables in Inline Assembler::
* Inlining Inline Assembler Code::
* Other Asm Functionality::
* A Complete Example::
@end menu

@c ---------------------------------------------------------------------------
@node Basic Assembler Syntax
@section Basic Assembler Syntax

@noindent
The assembler used by GNAT and GNAT COMPILE is based not on the Intel assembly language, but rather on a
language that descends from the AT&T Unix assembler @emph{as} (and which is often
referred to as ``AT&T syntax'').
The following table summarizes the main features of @emph{as} syntax and points out the differences from the Intel conventions.
See the GNAT COMPILE @emph{as} and @emph{gas} (an @emph{as} macro
pre-processor) documentation for further information.

@table @asis
@item Register names
GNAT COMPILE / @emph{as}: Prefix with ``%''; for example @code{%eax}
@*
Intel: No extra punctuation; for example @code{eax}

@item Immediate operand
GNAT COMPILE / @emph{as}: Prefix with ``$''; for example @code{$4}
@*
Intel: No extra punctuation; for example @code{4}

@item Address
GNAT COMPILE / @emph{as}: Prefix with ``$''; for example @code{$loc}
@*
Intel: No extra punctuation; for example @code{loc}

@item Memory contents
GNAT COMPILE / @emph{as}: No extra punctuation; for example @code{loc}
@*
Intel: Square brackets; for example @code{[loc]}

@item Register contents
GNAT COMPILE / @emph{as}: Parentheses; for example @code{(%eax)}
@*
Intel: Square brackets; for example @code{[eax]}

@item Hexadecimal numbers
GNAT COMPILE / @emph{as}: Leading ``0x'' (C language syntax); for example @code{0xA0}
@*
Intel: Trailing ``h''; for example @code{A0h}

@item Operand size
GNAT COMPILE / @emph{as}: Explicit in op code; for example @code{movw} to move a 16-bit word
@*
Intel: Implicit, deduced by assembler; for example @code{mov}

@item Instruction repetition
GNAT COMPILE / @emph{as}: Split into two lines; for example
@*
@code{rep}
@*
@code{stosl}
@*
Intel: Keep on one line; for example @code{rep stosl}

@item Order of operands
GNAT COMPILE / @emph{as}: Source first; for example @code{movw $4, %eax}
@*
Intel: Destination first; for example @code{mov eax, 4}
@end table

@c ---------------------------------------------------------------------------
@node A Simple Example of Inline Assembler
@section A Simple Example of Inline Assembler

@noindent
The following example will generate a single assembly language statement, @code{nop}, which does nothing.  Despite its lack of run-time effect, the example will be useful in illustrating the basics of the Inline Assembler facility.

@smallexample
@group
with System.Machine_Code; use System.Machine_Code;
procedure Nothing is
begin
   Asm ("nop");
end Nothing;
@end group
@end smallexample

@code{Asm} is a procedure declared in package @code{System.Machine_Code}; here it takes one parameter, a @emph{template string} that must be a static expression and that will form the generated instruction.
@code{Asm} may be regarded as a compile-time procedure that parses the template string and additional parameters (none here), from which it generates a sequence of assembly language instructions.

The examples in this chapter will illustrate several of the forms for invoking @code{Asm}; a complete specification of the syntax is found in the @cite{GNAT Reference Manual}.

Under the standard GNAT conventions, the @code{Nothing} procedure should be in a file named @file{NOTHING.ADB}.  You can build the executable in the usual way:
@smallexample
GNAT MAKE nothing
@end smallexample
However, the interesting aspect of this example is not its run-time behavior but rather the
generated assembly code.  To see this output, invoke the compiler as follows:
@smallexample
   GNAT COMPILE -S -fomit-frame-pointer /CHECKS=SUPPRESS_ALL @file{NOTHING.ADB}
@end smallexample
where the options are:

@table @code
@item -c
compile only (no bind or link)
@item -S
generate assembler listing
@item -fomit-frame-pointer
do not set up separate stack frames
@item /CHECKS=SUPPRESS_ALL
do not add runtime checks
@end table

This gives a human-readable assembler version of the code. The resulting
file will have the same name as the Ada source file, but with a @code{.s} extension.
In our example, the file @file{nothing.s} has the following contents:

@smallexample
@group
.file "NOTHING.ADB"
gcc2_compiled.:
___gnu_compiled_ada:
.text
   .align 4
.globl __ada_nothing
__ada_nothing:
#APP
   nop
#NO_APP
   jmp L1
   .align 2,0x90
L1:
   ret
@end group
@end smallexample

The assembly code you included is clearly indicated by
the compiler, between the @code{#APP} and @code{#NO_APP}
delimiters. The character before the 'APP' and 'NOAPP'
can differ on different targets. For example, Linux uses '#APP' while
on NT you will see '/APP'.

If you make a mistake in your assembler code (such as using the
wrong size modifier, or using a wrong operand for the instruction) GNAT
will report this error in a temporary file, which will be deleted when
the compilation is finished.  Generating an assembler file will help
in such cases, since you can assemble this file separately using the
@emph{as} assembler that comes with GNAT COMPILE.

Assembling the file using the command

@smallexample
as @file{nothing.s}
@end smallexample
@noindent
will give you error messages whose lines correspond to the assembler
input file, so you can easily find and correct any mistakes you made.
If there are no errors, @emph{as} will generate an object file @file{nothing.out}.

@c ---------------------------------------------------------------------------
@node Output Variables in Inline Assembler
@section Output Variables in Inline Assembler

@noindent
The examples in this section, showing how to access the processor flags, illustrate how to specify the destination operands for assembly language statements.

@smallexample
@group
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Get_Flags is
   Flags : Unsigned_32;
   use ASCII;
begin
   Asm ("pushfl"          & LF & HT & -- push flags on stack
        "popl %%eax"      & LF & HT & -- load eax with flags
        "movl %%eax, %0",             -- store flags in variable
        Outputs => Unsigned_32'Asm_Output ("=g", Flags));
   Put_Line ("Flags register:" & Flags'Img);
end Get_Flags;
@end group
@end smallexample

In order to have a nicely aligned assembly listing, we have separated
multiple assembler statements in the Asm template string with linefeed (ASCII.LF)
and horizontal tab (ASCII.HT) characters.  The resulting section of the
assembly output file is:

@smallexample
@group
#APP
   pushfl
   popl %eax
   movl %eax, -40(%ebp)
#NO_APP
@end group
@end smallexample

It would have been legal to write the Asm invocation as:

@smallexample
Asm ("pushfl popl %%eax movl %%eax, %0")
@end smallexample

but in the generated assembler file, this would come out as:

@smallexample
#APP
   pushfl popl %eax movl %eax, -40(%ebp)
#NO_APP
@end smallexample

which is not so convenient for the human reader.

We use Ada comments
at the end of each line to explain what the assembler instructions
actually do.  This is a useful convention.

When writing Inline Assembler instructions, you need to precede each register and variable name with a percent sign.  Since the assembler already requires a percent sign at the beginning of a register name, you need two consecutive percent signs for such names in the Asm template string, thus @code{%%eax}.  In the generated assembly code, one of the percent signs will be stripped off.

Names such as @code{%0}, @code{%1}, @code{%2}, etc., denote input or output variables: operands you later define using @code{Input} or @code{Output} parameters to @code{Asm}.
An output variable is illustrated in
the third statement in the Asm template string:
@smallexample
movl %%eax, %0
@end smallexample
The intent is to store the contents of the eax register in a variable that can be accessed in Ada.  Simply writing @code{movl %%eax, Flags} would not necessarily work, since the compiler might optimize by using a register to hold Flags, and the expansion of the @code{movl} instruction would not be aware of this optimization.  The solution is not to store the result directly but rather to advise the compiler to choose the correct operand form; that is the purpose of the @code{%0} output variable.

Information about the output variable is supplied in the @code{Outputs} parameter to @code{Asm}:
@smallexample
Outputs => Unsigned_32'Asm_Output ("=g", Flags));
@end smallexample

The output is defined by the @code{Asm_Output} attribute of the target type; the general format is
@smallexample
Type'Asm_Output (constraint_string, variable_name)
@end smallexample

The constraint string directs the compiler how
to store/access the associated variable.  In the example
@smallexample
Unsigned_32'Asm_Output ("=m", Flags);
@end smallexample
the @code{"m"} (memory) constraint tells the compiler that the variable
@code{Flags} should be stored in a memory variable, thus preventing
the optimizer from keeping it in a register.  In contrast,
@smallexample
Unsigned_32'Asm_Output ("=r", Flags);
@end smallexample
uses the @code{"r"} (register) constraint, telling the compiler to
store the variable in a register.

If the constraint is preceded by the equal character (@strong{=}), it tells the
compiler that the variable will be used to store data into it.

In the @code{Get_Flags} example, we used the "g" (global) constraint, allowing the optimizer
to choose whatever it deems best.

There are a fairly large number of constraints, but the ones that are most useful (for the Intel x86 processor) are the following:

@table @code
@item =
output constraint
@item g
global (i.e. can be stored anywhere)
@item m
in memory
@item I
a constant
@item a
use eax
@item b
use ebx
@item c
use ecx
@item d
use edx
@item S
use esi
@item D
use edi
@item r
use one of eax, ebx, ecx or edx
@item q
use one of eax, ebx, ecx, edx, esi or edi
@end table

The full set of constraints is described in the GNAT COMPILE and @emph{as} documentation; note that it is possible to combine certain constraints in one constraint string.

You specify the association of an output variable with an assembler operand through the @code{%}@emph{n} notation, where @emph{n} is a non-negative integer.  Thus in
@smallexample
@group
Asm ("pushfl"          & LF & HT & -- push flags on stack
     "popl %%eax"      & LF & HT & -- load eax with flags
     "movl %%eax, %0",             -- store flags in variable
     Outputs => Unsigned_32'Asm_Output ("=g", Flags));
@end group
@end smallexample
@noindent
@code{%0} will be replaced in the expanded code by the appropriate operand,
whatever
the compiler decided for the @code{Flags} variable.

In general, you may have any number of output variables:
@itemize @bullet
@item
Count the operands starting at 0; thus @code{%0}, @code{%1}, etc.
@item
Specify the @code{Outputs} parameter as a parenthesized comma-separated list of @code{Asm_Output} attributes
@end itemize

For example:
@smallexample
@group
Asm ("movl %%eax, %0" & LF & HT &
     "movl %%ebx, %1" & LF & HT &
     "movl %%ecx, %2",
     Outputs => (Unsigned_32'Asm_Output ("=g", Var_A),   --  %0 = Var_A
                 Unsigned_32'Asm_Output ("=g", Var_B),   --  %1 = Var_B
                 Unsigned_32'Asm_Output ("=g", Var_C))); --  %2 = Var_C
@end group
@end smallexample
@noindent
where @code{Var_A}, @code{Var_B}, and @code{Var_C} are variables in the Ada program.

As a variation on the @code{Get_Flags} example, we can use the constraints string to direct the compiler to store the eax register into the @code{Flags} variable, instead of including the store instruction explicitly in the @code{Asm} template string:

@smallexample
@group
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Get_Flags_2 is
   Flags : Unsigned_32;
   use ASCII;
begin
   Asm ("pushfl"      & LF & HT & -- push flags on stack
        "popl %%eax",             -- save flags in eax
        Outputs => Unsigned_32'Asm_Output ("=a", Flags));
   Put_Line ("Flags register:" & Flags'Img);
end Get_Flags_2;
@end group
@end smallexample

@noindent
The @code{"a"} constraint tells the compiler that the @code{Flags}
variable will come from the eax register. Here is the resulting code:

@smallexample
@group
#APP
   pushfl
   popl %eax
#NO_APP
   movl %eax,-40(%ebp)
@end group
@end smallexample

@noindent
The compiler generated the store of eax into Flags after
expanding the assembler code.

Actually, there was no need to pop the flags into the eax register; more simply, we could just pop the flags directly into the program variable:

@smallexample
@group
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Get_Flags_3 is
   Flags : Unsigned_32;
   use ASCII;
begin
   Asm ("pushfl"  & LF & HT & -- push flags on stack
        "pop %0",             -- save flags in Flags
        Outputs => Unsigned_32'Asm_Output ("=g", Flags));
   Put_Line ("Flags register:" & Flags'Img);
end Get_Flags_3;
@end group
@end smallexample

@c ---------------------------------------------------------------------------
@node Input Variables in Inline Assembler
@section Input Variables in Inline Assembler

@noindent
The example in this section illustrates how to specify the source operands for assembly language statements.  The program simply increments its input value by 1:

@smallexample
@group
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Increment is

   function Incr (Value : Unsigned_32) return Unsigned_32 is
      Result : Unsigned_32;
   begin
      Asm ("incl %0",
           Inputs  => Unsigned_32'Asm_Input ("a", Value),
           Outputs => Unsigned_32'Asm_Output ("=a", Result));
      return Result;
   end Incr;

   Value : Unsigned_32;

begin
   Value := 5;
   Put_Line ("Value before is" & Value'Img);
   Value := Incr (Value);
   Put_Line ("Value after is" & Value'Img);
end Increment;
@end group
@end smallexample

The @code{Outputs} parameter to @code{Asm} specifies
that the result will be in the eax register and that it is to be stored in the @code{Result}
variable.

The @code{Inputs} parameter looks much like the @code{Outputs} parameter, but with an
@code{Asm_Input} attribute. The
@code{"="} constraint, indicating an output value, is not present.

You can have multiple input variables, in the same way that you can have more
than one output variable.

The parameter count (%0, %1) etc, now starts at the first input
statement, and continues with the output statements.
When both parameters use the same variable, the
compiler will treat them as the same %n operand, which is the case here.

Just as the @code{Outputs} parameter causes the register to be stored into the
target variable after execution of the assembler statements, so does the
@code{Inputs} parameter cause its variable to be loaded into the register before execution
of the
assembler statements.

Thus the effect of the @code{Asm} invocation is:
@enumerate
@item load the 32-bit value of @code{Value} into eax
@item execute the @code{incl %eax} instruction
@item store the contents of eax into the @code{Result} variable
@end enumerate

The resulting assembler file (with @code{/OPTIMIZE=ALL} optimization) contains:
@smallexample
@group
_increment__incr.1:
   subl $4,%esp
   movl 8(%esp),%eax
#APP
   incl %eax
#NO_APP
   movl %eax,%edx
   movl %ecx,(%esp)
   addl $4,%esp
   ret
@end group
@end smallexample

@c ---------------------------------------------------------------------------
@node Inlining Inline Assembler Code
@section Inlining Inline Assembler Code

@noindent
For a short subprogram such as the @code{Incr} function in the previous section, the overhead of the call and return (creating / deleting the stack frame)
can be significant, compared to the amount of code in the subprogram body.
A solution is to apply Ada's @code{Inline} pragma to the subprogram,
which directs the compiler to expand invocations of the subprogram at the point(s)
of call, instead of setting up a stack frame for out-of-line calls.
Here is the resulting program:

@smallexample
@group
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Increment_2 is

   function Incr (Value : Unsigned_32) return Unsigned_32 is
      Result : Unsigned_32;
   begin
      Asm ("incl %0",
           Inputs  => Unsigned_32'Asm_Input ("a", Value),
           Outputs => Unsigned_32'Asm_Output ("=a", Result));
      return Result;
   end Incr;
   pragma Inline (Increment);

   Value : Unsigned_32;

begin
   Value := 5;
   Put_Line ("Value before is" & Value'Img);
   Value := Increment (Value);
   Put_Line ("Value after is" & Value'Img);
end Increment_2;
@end group
@end smallexample

Compile the program with both optimization (@code{/OPTIMIZE=ALL}) and inlining
enabled (@option{-gnatpn} instead of @option{/CHECKS=SUPPRESS_ALL}).

The @code{Incr} function is still compiled as usual, but at the
point in @code{Increment} where our function used to be called:

@smallexample
@group
pushl %edi
call _increment__incr.1
@end group
@end smallexample

@noindent
the code for the function body directly appears:

@smallexample
@group
movl %esi,%eax
#APP
   incl %eax
#NO_APP
   movl %eax,%edx
@end group
@end smallexample

@noindent
thus saving the overhead of stack frame setup and an out-of-line call.

@c ---------------------------------------------------------------------------
@node Other Asm Functionality
@section Other @code{Asm} Functionality

@noindent
This section describes two important parameters to the @code{Asm} procedure: @code{Clobber}, which identifies register usage; and @code{Volatile}, which inhibits unwanted optimizations.

@menu
* The Clobber Parameter::
* The Volatile Parameter::
@end menu

@c ---------------------------------------------------------------------------
@node The Clobber Parameter
@subsection The @code{Clobber} Parameter

@noindent
One of the dangers of intermixing assembly language and a compiled language such as Ada is
that the compiler needs to be aware of which registers are being used by the assembly code.
In some cases, such as the earlier examples, the constraint string is sufficient to
indicate register usage (e.g. "a" for the eax register).  But more generally, the
compiler needs an explicit identification of the registers that are used by the Inline
Assembly statements.

Using a register that the compiler doesn't know about
could be a side effect of an instruction (like @code{mull}
storing its result in both eax and edx).
It can also arise from explicit register usage in your
assembly code; for example:
@smallexample
@group
Asm ("movl %0, %%ebx" & LF & HT &
     "movl %%ebx, %1",
     Inputs  => Unsigned_32'Asm_Input  ("g", Var_In),
     Outputs => Unsigned_32'Asm_Output ("=g", Var_Out));
@end group
@end smallexample
@noindent
where the compiler (since it does not analyze the @code{Asm} template string)
does not know you are using the ebx register.

In such cases you need to supply the @code{Clobber} parameter to @code{Asm},
to identify the registers that will be used by your assembly code:

@smallexample
@group
Asm ("movl %0, %%ebx" & LF & HT &
     "movl %%ebx, %1",
     Inputs  => Unsigned_32'Asm_Input  ("g", Var_In),
     Outputs => Unsigned_32'Asm_Output ("=g", Var_Out),
     Clobber => "ebx");
@end group
@end smallexample

The Clobber parameter is a static string expression specifying the
register(s) you are using.  Note that register names are @emph{not} prefixed by a percent sign.
Also, if more than one register is used then their names are separated by commas; e.g., @code{"eax, ebx"}

The @code{Clobber} parameter has several additional uses:
@enumerate
@item Use the "register" name @code{cc} to indicate that flags might have changed
@item Use the "register" name @code{memory} if you changed a memory location
@end enumerate

@c ---------------------------------------------------------------------------
@node The Volatile Parameter
@subsection The @code{Volatile} Parameter
@cindex Volatile parameter

@noindent
Compiler optimizations in the presence of Inline Assembler may sometimes have unwanted effects.
For example, when
an @code{Asm} invocation with an input variable is inside a loop, the compiler might move
the loading of the input variable outside the loop, regarding it as a
one-time initialization.

If this effect is not desired, you can disable such optimizations by setting the
@code{Volatile} parameter to @code{True}; for example:

@smallexample
@group
Asm ("movl %0, %%ebx" & LF & HT &
     "movl %%ebx, %1",
     Inputs   => Unsigned_32'Asm_Input  ("g", Var_In),
     Outputs  => Unsigned_32'Asm_Output ("=g", Var_Out),
     Clobber  => "ebx",
     Volatile => True);
@end group
@end smallexample

By default, @code{Volatile} is set to @code{False} unless there is no @code{Outputs}
parameter.

Although setting @code{Volatile} to @code{True} prevents unwanted optimizations,
it will also disable other optimizations that might be important for efficiency.
In general, you should set @code{Volatile} to @code{True} only if the compiler's
optimizations have created problems.

@c ---------------------------------------------------------------------------
@node A Complete Example
@section A Complete Example

@noindent
This section contains a complete program illustrating a realistic usage of GNAT's Inline Assembler
capabilities.  It comprises a main procedure @code{Check_CPU} and a package @code{Intel_CPU}.
The package declares a collection of functions that detect the properties of the 32-bit
x86 processor that is running the program.  The main procedure invokes these functions
and displays the information.

The Intel_CPU package could be enhanced by adding functions to
detect the type of x386 co-processor, the processor caching options and
special operations such as the SIMD extensions.

Although the Intel_CPU package has been written for 32-bit Intel
compatible CPUs, it is OS neutral. It has been tested on DOS,
Windows/NT and Linux.

@menu
* Check_CPU Procedure::
* Intel_CPU Package Specification::
* Intel_CPU Package Body::
@end menu

@c ---------------------------------------------------------------------------
@node Check_CPU Procedure
@subsection @code{Check_CPU} Procedure
@cindex Check_CPU procedure

@smallexample
---------------------------------------------------------------------
--                                                                 --
--  Uses the Intel_CPU package to identify the CPU the program is  --
--  running on, and some of the features it supports.              --
--                                                                 --
---------------------------------------------------------------------

with Intel_CPU;                     --  Intel CPU detection functions
with Ada.Text_IO;                   --  Standard text I/O
with Ada.Command_Line;              --  To set the exit status

procedure Check_CPU is

   Type_Found : Boolean := False;
   --  Flag to indicate that processor was identified

   Features   : Intel_CPU.Processor_Features;
   --  The processor features

   Signature  : Intel_CPU.Processor_Signature;
   --  The processor type signature

begin

   -----------------------------------
   --  Display the program banner.  --
   -----------------------------------

   Ada.Text_IO.Put_Line (Ada.Command_Line.Command_Name &
                         ": check Intel CPU version and features, v1.0");
   Ada.Text_IO.Put_Line ("distribute freely, but no warranty whatsoever");
   Ada.Text_IO.New_Line;

   -----------------------------------------------------------------------
   --  We can safely start with the assumption that we are on at least  --
   --  a x386 processor. If the CPUID instruction is present, then we   --
   --  have a later processor type.                                     --
   -----------------------------------------------------------------------

   if Intel_CPU.Has_CPUID = False then

      --  No CPUID instruction, so we assume this is indeed a x386
      --  processor. We can still check if it has a FP co-processor.
      if Intel_CPU.Has_FPU then
         Ada.Text_IO.Put_Line
           ("x386-type processor with a FP co-processor");
      else
         Ada.Text_IO.Put_Line
           ("x386-type processor without a FP co-processor");
      end if;  --  check for FPU

      --  Program done
      Ada.Command_Line.Set_Exit_Status (Ada.Command_Line.Success);
      return;

   end if;  --  check for CPUID

   -----------------------------------------------------------------------
   --  If CPUID is supported, check if this is a true Intel processor,  --
   --  if it is not, display a warning.                                 --
   -----------------------------------------------------------------------

   if Intel_CPU.Vendor_ID /= Intel_CPU.Intel_Processor then
      Ada.Text_IO.Put_Line ("*** This is a Intel compatible processor");
      Ada.Text_IO.Put_Line ("*** Some information may be incorrect");
   end if;  --  check if Intel

   ----------------------------------------------------------------------
   --  With the CPUID instruction present, we can assume at least a    --
   --  x486 processor. If the CPUID support level is < 1 then we have  --
   --  to leave it at that.                                            --
   ----------------------------------------------------------------------

   if Intel_CPU.CPUID_Level < 1 then

      --  Ok, this is a x486 processor. we still can get the Vendor ID
      Ada.Text_IO.Put_Line ("x486-type processor");
      Ada.Text_IO.Put_Line ("Vendor ID is " & Intel_CPU.Vendor_ID);

      --  We can also check if there is a FPU present
      if Intel_CPU.Has_FPU then
         Ada.Text_IO.Put_Line ("Floating-Point support");
      else
         Ada.Text_IO.Put_Line ("No Floating-Point support");
      end if;  --  check for FPU

      --  Program done
      Ada.Command_Line.Set_Exit_Status (Ada.Command_Line.Success);
      return;

   end if;  --  check CPUID level

   ---------------------------------------------------------------------
   --  With a CPUID level of 1 we can use the processor signature to  --
   --  determine it's exact type.                                     --
   ---------------------------------------------------------------------

   Signature := Intel_CPU.Signature;

   ----------------------------------------------------------------------
   --  Ok, now we go into a lot of messy comparisons to get the        --
   --  processor type. For clarity, no attememt to try to optimize the --
   --  comparisons has been made. Note that since Intel_CPU does not   --
   --  support getting cache info, we cannot distinguish between P5    --
   --  and Celeron types yet.                                          --
   ----------------------------------------------------------------------

   --  x486SL
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0100# and
     Signature.Model           = 2#0100# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("x486SL processor");
   end if;

   --  x486DX2 Write-Back
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0100# and
     Signature.Model           = 2#0111# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("Write-Back Enhanced x486DX2 processor");
   end if;

   --  x486DX4
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0100# and
     Signature.Model           = 2#1000# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("x486DX4 processor");
   end if;

   --  x486DX4 Overdrive
   if Signature.Processor_Type = 2#01#   and
     Signature.Family          = 2#0100# and
     Signature.Model           = 2#1000# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("x486DX4 OverDrive processor");
   end if;

   --  Pentium (60, 66)
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0101# and
     Signature.Model           = 2#0001# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("Pentium processor (60, 66)");
   end if;

   --  Pentium (75, 90, 100, 120, 133, 150, 166, 200)
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0101# and
     Signature.Model           = 2#0010# then
      Type_Found := True;
      Ada.Text_IO.Put_Line
        ("Pentium processor (75, 90, 100, 120, 133, 150, 166, 200)");
   end if;

   --  Pentium OverDrive (60, 66)
   if Signature.Processor_Type = 2#01#   and
     Signature.Family          = 2#0101# and
     Signature.Model           = 2#0001# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("Pentium OverDrive processor (60, 66)");
   end if;

   --  Pentium OverDrive (75, 90, 100, 120, 133, 150, 166, 200)
   if Signature.Processor_Type = 2#01#   and
     Signature.Family          = 2#0101# and
     Signature.Model           = 2#0010# then
      Type_Found := True;
      Ada.Text_IO.Put_Line
        ("Pentium OverDrive cpu (75, 90, 100, 120, 133, 150, 166, 200)");
   end if;

   --  Pentium OverDrive processor for x486 processor-based systems
   if Signature.Processor_Type = 2#01#   and
     Signature.Family          = 2#0101# and
     Signature.Model           = 2#0011# then
      Type_Found := True;
      Ada.Text_IO.Put_Line
        ("Pentium OverDrive processor for x486 processor-based systems");
   end if;

   --  Pentium processor with MMX technology (166, 200)
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0101# and
     Signature.Model           = 2#0100# then
      Type_Found := True;
      Ada.Text_IO.Put_Line
        ("Pentium processor with MMX technology (166, 200)");
   end if;

   --  Pentium OverDrive with MMX for Pentium (75, 90, 100, 120, 133)
   if Signature.Processor_Type = 2#01#   and
     Signature.Family          = 2#0101# and
     Signature.Model           = 2#0100# then
      Type_Found := True;
      Ada.Text_IO.Put_Line
        ("Pentium OverDrive processor with MMX " &
         "technology for Pentium processor (75, 90, 100, 120, 133)");
   end if;

   --  Pentium Pro processor
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0110# and
     Signature.Model           = 2#0001# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("Pentium Pro processor");
   end if;

   --  Pentium II processor, model 3
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0110# and
     Signature.Model           = 2#0011# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("Pentium II processor, model 3");
   end if;

   --  Pentium II processor, model 5 or Celeron processor
   if Signature.Processor_Type = 2#00#   and
     Signature.Family          = 2#0110# and
     Signature.Model           = 2#0101# then
      Type_Found := True;
      Ada.Text_IO.Put_Line
        ("Pentium II processor, model 5 or Celeron processor");
   end if;

   --  Pentium Pro OverDrive processor
   if Signature.Processor_Type = 2#01#   and
     Signature.Family          = 2#0110# and
     Signature.Model           = 2#0011# then
      Type_Found := True;
      Ada.Text_IO.Put_Line ("Pentium Pro OverDrive processor");
   end if;

   --  If no type recognized, we have an unknown. Display what
   --  we _do_ know
   if Type_Found = False then
      Ada.Text_IO.Put_Line ("Unknown processor");
   end if;

   -----------------------------------------
   --  Display processor stepping level.  --
   -----------------------------------------

   Ada.Text_IO.Put_Line ("Stepping level:" & Signature.Stepping'Img);

   ---------------------------------
   --  Display vendor ID string.  --
   ---------------------------------

   Ada.Text_IO.Put_Line ("Vendor ID: " & Intel_CPU.Vendor_ID);

   ------------------------------------
   --  Get the processors features.  --
   ------------------------------------

   Features := Intel_CPU.Features;

   -----------------------------
   --  Check for a FPU unit.  --
   -----------------------------

   if Features.FPU = True then
      Ada.Text_IO.Put_Line ("Floating-Point unit available");
   else
      Ada.Text_IO.Put_Line ("no Floating-Point unit");
   end if;  --  check for FPU

   --------------------------------
   --  List processor features.  --
   --------------------------------

   Ada.Text_IO.Put_Line ("Supported features: ");

   --  Virtual Mode Extension
   if Features.VME = True then
      Ada.Text_IO.Put_Line ("    VME    - Virtual Mode Extension");
   end if;

   --  Debugging Extension
   if Features.DE = True then
      Ada.Text_IO.Put_Line ("    DE     - Debugging Extension");
   end if;

   --  Page Size Extension
   if Features.PSE = True then
      Ada.Text_IO.Put_Line ("    PSE    - Page Size Extension");
   end if;

   --  Time Stamp Counter
   if Features.TSC = True then
      Ada.Text_IO.Put_Line ("    TSC    - Time Stamp Counter");
   end if;

   --  Model Specific Registers
   if Features.MSR = True then
      Ada.Text_IO.Put_Line ("    MSR    - Model Specific Registers");
   end if;

   --  Physical Address Extension
   if Features.PAE = True then
      Ada.Text_IO.Put_Line ("    PAE    - Physical Address Extension");
   end if;

   --  Machine Check Extension
   if Features.MCE = True then
      Ada.Text_IO.Put_Line ("    MCE    - Machine Check Extension");
   end if;

   --  CMPXCHG8 instruction supported
   if Features.CX8 = True then
      Ada.Text_IO.Put_Line ("    CX8    - CMPXCHG8 instruction");
   end if;

   --  on-chip APIC hardware support
   if Features.APIC = True then
      Ada.Text_IO.Put_Line ("    APIC   - on-chip APIC hardware support");
   end if;

   --  Fast System Call
   if Features.SEP = True then
      Ada.Text_IO.Put_Line ("    SEP    - Fast System Call");
   end if;

   --  Memory Type Range Registers
   if Features.MTRR = True then
      Ada.Text_IO.Put_Line ("    MTTR   - Memory Type Range Registers");
   end if;

   --  Page Global Enable
   if Features.PGE = True then
      Ada.Text_IO.Put_Line ("    PGE    - Page Global Enable");
   end if;

   --  Machine Check Architecture
   if Features.MCA = True then
      Ada.Text_IO.Put_Line ("    MCA    - Machine Check Architecture");
   end if;

   --  Conditional Move Instruction Supported
   if Features.CMOV = True then
      Ada.Text_IO.Put_Line
        ("    CMOV   - Conditional Move Instruction Supported");
   end if;

   --  Page Attribute Table
   if Features.PAT = True then
      Ada.Text_IO.Put_Line ("    PAT    - Page Attribute Table");
   end if;

   --  36-bit Page Size Extension
   if Features.PSE_36 = True then
      Ada.Text_IO.Put_Line ("    PSE_36 - 36-bit Page Size Extension");
   end if;

   --  MMX technology supported
   if Features.MMX = True then
      Ada.Text_IO.Put_Line ("    MMX    - MMX technology supported");
   end if;

   --  Fast FP Save and Restore
   if Features.FXSR = True then
      Ada.Text_IO.Put_Line ("    FXSR   - Fast FP Save and Restore");
   end if;

   ---------------------
   --  Program done.  --
   ---------------------

   Ada.Command_Line.Set_Exit_Status (Ada.Command_Line.Success);

exception

   when others =>
      Ada.Command_Line.Set_Exit_Status (Ada.Command_Line.Failure);
      raise;

end Check_CPU;
@end smallexample

@c ---------------------------------------------------------------------------
@node Intel_CPU Package Specification
@subsection @code{Intel_CPU} Package Specification
@cindex Intel_CPU package specification

@smallexample
-------------------------------------------------------------------------
--                                                                     --
--  file: INTEL_CPU.ADS                                                --
--                                                                     --
--           *********************************************             --
--           * WARNING: for 32-bit Intel processors only *             --
--           *********************************************             --
--                                                                     --
--  This package contains a number of subprograms that are useful in   --
--  determining the Intel x86 CPU (and the features it supports) on    --
--  which the program is running.                                      --
--                                                                     --
--  The package is based upon the information given in the Intel       --
--  Application Note AP-485: "Intel Processor Identification and the   --
--  CPUID Instruction" as of April 1998. This application note can be  --
--  found on www.intel.com.                                            --
--                                                                     --
--  It currently deals with 32-bit processors only, will not detect    --
--  features added after april 1998, and does not guarantee proper     --
--  results on Intel-compatible processors.                            --
--                                                                     --
--  Cache info and x386 fpu type detection are not supported.          --
--                                                                     --
--  This package does not use any privileged instructions, so should   --
--  work on any OS running on a 32-bit Intel processor.                --
--                                                                     --
-------------------------------------------------------------------------

with Interfaces;             use Interfaces;
--  for using unsigned types

with System.Machine_Code;    use System.Machine_Code;
--  for using inline assembler code

with Ada.Characters.Latin_1; use Ada.Characters.Latin_1;
--  for inserting control characters

package Intel_CPU is

   ----------------------
   --  Processor bits  --
   ----------------------

   subtype Num_Bits is Natural range 0 .. 31;
   --  the number of processor bits (32)

   --------------------------
   --  Processor register  --
   --------------------------

   --  define a processor register type for easy access to
   --  the individual bits

   type Processor_Register is array (Num_Bits) of Boolean;
   pragma Pack (Processor_Register);
   for Processor_Register'Size use 32;

   -------------------------
   --  Unsigned register  --
   -------------------------

   --  define a processor register type for easy access to
   --  the individual bytes

   type Unsigned_Register is
      record
         L1 : Unsigned_8;
         H1 : Unsigned_8;
         L2 : Unsigned_8;
         H2 : Unsigned_8;
      end record;

   for Unsigned_Register use
      record
         L1 at 0 range  0 ..  7;
         H1 at 0 range  8 .. 15;
         L2 at 0 range 16 .. 23;
         H2 at 0 range 24 .. 31;
      end record;

   for Unsigned_Register'Size use 32;

   ---------------------------------
   --  Intel processor vendor ID  --
   ---------------------------------

   Intel_Processor : constant String (1 .. 12) := "GenuineIntel";
   --  indicates an Intel manufactured processor

   ------------------------------------
   --  Processor signature register  --
   ------------------------------------

   --  a register type to hold the processor signature

   type Processor_Signature is
      record
         Stepping       : Natural range 0 .. 15;
         Model          : Natural range 0 .. 15;
         Family         : Natural range 0 .. 15;
         Processor_Type : Natural range 0 .. 3;
         Reserved       : Natural range 0 .. 262143;
      end record;

   for Processor_Signature use
      record
         Stepping       at 0 range  0 ..  3;
         Model          at 0 range  4 ..  7;
         Family         at 0 range  8 .. 11;
         Processor_Type at 0 range 12 .. 13;
         Reserved       at 0 range 14 .. 31;
      end record;

   for Processor_Signature'Size use 32;

   -----------------------------------
   --  Processor features register  --
   -----------------------------------

   --  a processor register to hold the processor feature flags

   type Processor_Features is
      record
         FPU    : Boolean;                --  floating point unit on chip
         VME    : Boolean;                --  virtual mode extension
         DE     : Boolean;                --  debugging extension
         PSE    : Boolean;                --  page size extension
         TSC    : Boolean;                --  time stamp counter
         MSR    : Boolean;                --  model specific registers
         PAE    : Boolean;                --  physical address extension
         MCE    : Boolean;                --  machine check extension
         CX8    : Boolean;                --  cmpxchg8 instruction
         APIC   : Boolean;                --  on-chip apic hardware
         Res_1  : Boolean;                --  reserved for extensions
         SEP    : Boolean;                --  fast system call
         MTRR   : Boolean;                --  memory type range registers
         PGE    : Boolean;                --  page global enable
         MCA    : Boolean;                --  machine check architecture
         CMOV   : Boolean;                --  conditional move supported
         PAT    : Boolean;                --  page attribute table
         PSE_36 : Boolean;                --  36-bit page size extension
         Res_2  : Natural range 0 .. 31;  --  reserved for extensions
         MMX    : Boolean;                --  MMX technology supported
         FXSR   : Boolean;                --  fast FP save and restore
         Res_3  : Natural range 0 .. 127; --  reserved for extensions
      end record;

   for Processor_Features use
      record
         FPU    at 0 range  0 ..  0;
         VME    at 0 range  1 ..  1;
         DE     at 0 range  2 ..  2;
         PSE    at 0 range  3 ..  3;
         TSC    at 0 range  4 ..  4;
         MSR    at 0 range  5 ..  5;
         PAE    at 0 range  6 ..  6;
         MCE    at 0 range  7 ..  7;
         CX8    at 0 range  8 ..  8;
         APIC   at 0 range  9 ..  9;
         Res_1  at 0 range 10 .. 10;
         SEP    at 0 range 11 .. 11;
         MTRR   at 0 range 12 .. 12;
         PGE    at 0 range 13 .. 13;
         MCA    at 0 range 14 .. 14;
         CMOV   at 0 range 15 .. 15;
         PAT    at 0 range 16 .. 16;
         PSE_36 at 0 range 17 .. 17;
         Res_2  at 0 range 18 .. 22;
         MMX    at 0 range 23 .. 23;
         FXSR   at 0 range 24 .. 24;
         Res_3  at 0 range 25 .. 31;
      end record;

   for Processor_Features'Size use 32;

   -------------------
   --  Subprograms  --
   -------------------

   function Has_FPU return Boolean;
   --  return True if a FPU is found
   --  use only if CPUID is not supported

   function Has_CPUID return Boolean;
   --  return True if the processor supports the CPUID instruction

   function CPUID_Level return Natural;
   --  return the CPUID support level (0, 1 or 2)
   --  can only be called if the CPUID instruction is supported

   function Vendor_ID return String;
   --  return the processor vendor identification string
   --  can only be called if the CPUID instruction is supported

   function Signature return Processor_Signature;
   --  return the processor signature
   --  can only be called if the CPUID instruction is supported

   function Features return Processor_Features;
   --  return the processors features
   --  can only be called if the CPUID instruction is supported

private

   ------------------------
   --  EFLAGS bit names  --
   ------------------------

   ID_Flag : constant Num_Bits := 21;
   --  ID flag bit

end Intel_CPU;
@end smallexample

@c ---------------------------------------------------------------------------
@node Intel_CPU Package Body
@subsection @code{Intel_CPU} Package Body
@cindex Intel_CPU package body

@smallexample
package body Intel_CPU is

   ---------------------------
   --  Detect FPU presence  --
   ---------------------------

   --  There is a FPU present if we can set values to the FPU Status
   --  and Control Words.

   function Has_FPU return Boolean is

      Register : Unsigned_16;
      --  processor register to store a word

   begin

      --  check if we can change the status word
      Asm (

           --  the assembler code
           "finit"              & LF & HT &    --  reset status word
           "movw $0x5A5A, %%ax" & LF & HT &    --  set value status word
           "fnstsw %0"          & LF & HT &    --  save status word
           "movw %%ax, %0",                    --  store status word

           --  output stored in Register
           --  register must be a memory location
           Outputs => Unsigned_16'Asm_output ("=m", Register),

           --  tell compiler that we used eax
           Clobber => "eax");

      --  if the status word is zero, there is no FPU
      if Register = 0 then
         return False;   --  no status word
      end if;  --  check status word value

      --  check if we can get the control word
      Asm (

           --  the assembler code
           "fnstcw %0",   --  save the control word

           --  output into Register
           --  register must be a memory location
           Outputs => Unsigned_16'Asm_output ("=m", Register));

      --  check the relevant bits
      if (Register and 16#103F#) /= 16#003F# then
         return False;   --  no control word
      end if;  --  check control word value

      --  FPU found
      return True;

   end Has_FPU;

   --------------------------------
   --  Detect CPUID instruction  --
   --------------------------------

   --  The processor supports the CPUID instruction if it is possible
   --  to change the value of ID flag bit in the EFLAGS register.

   function Has_CPUID return Boolean is

      Original_Flags, Modified_Flags : Processor_Register;
      --  EFLAG contents before and after changing the ID flag

   begin

      --  try flipping the ID flag in the EFLAGS register
      Asm (

           --  the assembler code
           "pushfl"               & LF & HT &     --  push EFLAGS on stack
           "pop %%eax"            & LF & HT &     --  pop EFLAGS into eax
           "movl %%eax, %0"       & LF & HT &     --  save EFLAGS content
           "xor $0x200000, %%eax" & LF & HT &     --  flip ID flag
           "push %%eax"           & LF & HT &     --  push EFLAGS on stack
           "popfl"                & LF & HT &     --  load EFLAGS register
           "pushfl"               & LF & HT &     --  push EFLAGS on stack
           "pop %1",                              --  save EFLAGS content

           --  output values, may be anything
           --  Original_Flags is %0
           --  Modified_Flags is %1
           Outputs =>
              (Processor_Register'Asm_output ("=g", Original_Flags),
               Processor_Register'Asm_output ("=g", Modified_Flags)),

           --  tell compiler eax is destroyed
           Clobber => "eax");

      --  check if CPUID is supported
      if Original_Flags(ID_Flag) /= Modified_Flags(ID_Flag) then
         return True;   --  ID flag was modified
      else
         return False;  --  ID flag unchanged
      end if;  --  check for CPUID

   end Has_CPUID;

   -------------------------------
   --  Get CPUID support level  --
   -------------------------------

   function CPUID_Level return Natural is

      Level : Unsigned_32;
      --  returned support level

   begin

      --  execute CPUID, storing the results in the Level register
      Asm (

           --  the assembler code
           "cpuid",    --  execute CPUID

           --  zero is stored in eax
           --  returning the support level in eax
           Inputs => Unsigned_32'Asm_input ("a", 0),

           --  eax is stored in Level
           Outputs => Unsigned_32'Asm_output ("=a", Level),

           --  tell compiler ebx, ecx and edx registers are destroyed
           Clobber => "ebx, ecx, edx");

      --  return the support level
      return Natural (Level);

   end CPUID_Level;

   --------------------------------
   --  Get CPU Vendor ID String  --
   --------------------------------

   --  The vendor ID string is returned in the ebx, ecx and edx register
   --  after executing the CPUID instruction with eax set to zero.
   --  In case of a true Intel processor the string returned is
   --  "GenuineIntel"

   function Vendor_ID return String is

      Ebx, Ecx, Edx : Unsigned_Register;
      --  registers containing the vendor ID string

      Vendor_ID : String (1 .. 12);
      -- the vendor ID string

   begin

      --  execute CPUID, storing the results in the processor registers
      Asm (

           --  the assembler code
           "cpuid",    --  execute CPUID

           --  zero stored in eax
           --  vendor ID string returned in ebx, ecx and edx
           Inputs => Unsigned_32'Asm_input ("a", 0),

           --  ebx is stored in Ebx
           --  ecx is stored in Ecx
           --  edx is stored in Edx
           Outputs => (Unsigned_Register'Asm_output ("=b", Ebx),
                       Unsigned_Register'Asm_output ("=c", Ecx),
                       Unsigned_Register'Asm_output ("=d", Edx)));

      --  now build the vendor ID string
      Vendor_ID( 1) := Character'Val (Ebx.L1);
      Vendor_ID( 2) := Character'Val (Ebx.H1);
      Vendor_ID( 3) := Character'Val (Ebx.L2);
      Vendor_ID( 4) := Character'Val (Ebx.H2);
      Vendor_ID( 5) := Character'Val (Edx.L1);
      Vendor_ID( 6) := Character'Val (Edx.H1);
      Vendor_ID( 7) := Character'Val (Edx.L2);
      Vendor_ID( 8) := Character'Val (Edx.H2);
      Vendor_ID( 9) := Character'Val (Ecx.L1);
      Vendor_ID(10) := Character'Val (Ecx.H1);
      Vendor_ID(11) := Character'Val (Ecx.L2);
      Vendor_ID(12) := Character'Val (Ecx.H2);

      --  return string
      return Vendor_ID;

   end Vendor_ID;

   -------------------------------
   --  Get processor signature  --
   -------------------------------

   function Signature return Processor_Signature is

      Result : Processor_Signature;
      --  processor signature returned

   begin

      --  execute CPUID, storing the results in the Result variable
      Asm (

           --  the assembler code
           "cpuid",    --  execute CPUID

           --  one is stored in eax
           --  processor signature returned in eax
           Inputs => Unsigned_32'Asm_input ("a", 1),

           --  eax is stored in Result
           Outputs => Processor_Signature'Asm_output ("=a", Result),

           --  tell compiler that ebx, ecx and edx are also destroyed
           Clobber => "ebx, ecx, edx");

      --  return processor signature
      return Result;

   end Signature;

   ------------------------------
   --  Get processor features  --
   ------------------------------

   function Features return Processor_Features is

      Result : Processor_Features;
      --  processor features returned

   begin

      --  execute CPUID, storing the results in the Result variable
      Asm (

           --  the assembler code
           "cpuid",    --  execute CPUID

           --  one stored in eax
           --  processor features returned in edx
           Inputs => Unsigned_32'Asm_input ("a", 1),

           --  edx is stored in Result
           Outputs => Processor_Features'Asm_output ("=d", Result),

           --  tell compiler that ebx and ecx are also destroyed
           Clobber => "ebx, ecx");

      --  return processor signature
      return Result;

   end Features;

end Intel_CPU;
@end smallexample
@c END OF INLINE ASSEMBLER CHAPTER
@c ===============================




@node Performance Considerations
@chapter Performance Considerations
@cindex Performance

@noindent
The GNAT system provides a number of options that allow a trade-off
between

@itemize @bullet
@item
performance of the generated code

@item
speed of compilation

@item
minimization of dependences and recompilation

@item
the degree of run-time checking.
@end itemize

@noindent
The defaults (if no options are selected) aim at improving the speed
of compilation and minimizing dependences, at the expense of performance
of the generated code:

@itemize @bullet
@item
no optimization

@item
no inlining of subprogram calls

@item
all run-time checks enabled except overflow and elaboration checks
@end itemize

@noindent
These options are suitable for most program development purposes. This
chapter describes how you can modify these choices, and also provides
some guidelines on debugging optimized code.

@menu
* Controlling Run-Time Checks::
* Optimization Levels::
* Debugging Optimized Code::
* Inlining of Subprograms::
* Coverage Analysis::
@end menu

@node Controlling Run-Time Checks
@section Controlling Run-Time Checks

@noindent
By default, GNAT generates all run-time checks, except arithmetic overflow
checking for integer operations and checks for access before elaboration on
subprogram calls. The latter are not required in default mode, because all
necessary checking is done at compile time.
@cindex @option{/CHECKS=SUPPRESS_ALL} (@code{GNAT COMPILE})
@cindex @option{/CHECKS=OVERFLOW} (@code{GNAT COMPILE})
Two gnat qualifiers, @option{/CHECKS=SUPPRESS_ALL} and @option{/CHECKS=OVERFLOW} allow this default to
be modified. @xref{Run-Time Checks}.

Our experience is that the default is suitable for most development
purposes.

We treat integer overflow specially because these
are quite expensive and in our experience are not as important as other
run-time checks in the development process. Note that division by zero
is not considered an overflow check, and divide by zero checks are
generated where required by default.

Elaboration checks are off by default, and also not needed by default, since
GNAT uses a static elaboration analysis approach that avoids the need for
run-time checking. This manual contains a full chapter discussing the issue
of elaboration checks, and if the default is not satisfactory for your use,
you should read this chapter.

For validity checks, the minimal checks required by the Ada Reference
Manual (for case statements and assignments to array elements) are on
by default. These can be suppressed by use of the @option{-gnatVn} qualifier.
Note that in Ada 83, there were no validity checks, so if the Ada 83 mode
is acceptable (or when comparing GNAT performance with an Ada 83 compiler),
it may be reasonable to routinely use @option{-gnatVn}. Validity checks
are also suppressed entirely if @option{/CHECKS=SUPPRESS_ALL} is used.

@cindex Overflow checks
@cindex Checks, overflow
@findex Suppress
@findex Unsuppress
@cindex pragma Suppress
@cindex pragma Unsuppress
Note that the setting of the qualifiers controls the default setting of
the checks. They may be modified using either @code{pragma Suppress} (to
remove checks) or @code{pragma Unsuppress} (to add back suppressed
checks) in the program source.

@node Optimization Levels
@section Optimization Levels
@cindex @code{/OPTIMIZE} (@code{GNAT COMPILE})

@noindent
The default is optimization off. This results in the fastest compile
times, but GNAT makes absolutely no attempt to optimize, and the
generated programs are considerably larger and slower than when
optimization is enabled. You can use the
@code{/OPTIMIZE}
on the @code{GNAT COMPILE} command line to control the optimization level:

@table @code
@item /OPTIMIZE=NONE
no optimization (the default)

@item /OPTIMIZE=SOME
medium level optimization

@item /OPTIMIZE=ALL
full optimization

@item /OPTIMIZE=INLINING
full optimization, and also attempt automatic inlining of small
subprograms within a unit (@pxref{Inlining of Subprograms}).
@end table

Higher optimization levels perform more global transformations on the
program and apply more expensive analysis algorithms in order to generate
faster and more compact code. The price in compilation time, and the
resulting improvement in execution time,
both depend on the particular application and the hardware environment.
You should experiment to find the best level for your application.

Note: Unlike some other compilation systems, @code{GNAT COMPILE} has
been tested extensively at all optimization levels. There are some bugs
which appear only with optimization turned on, but there have also been
bugs which show up only in @emph{unoptimized} code. Selecting a lower
level of optimization does not improve the reliability of the code
generator, which in practice is highly reliable at all optimization
levels.

Note regarding the use of @code{/OPTIMIZE=INLINING}: The use of this optimization level
is generally discouraged with GNAT, since it often results in larger
executables which run more slowly. See further discussion of this point
in @pxref{Inlining of Subprograms}.

@node Debugging Optimized Code
@section Debugging Optimized Code

@noindent
Since the compiler generates debugging tables for a compilation unit before
it performs optimizations, the optimizing transformations may invalidate some
of the debugging data.  You therefore need to anticipate certain
anomalous situations that may arise while debugging optimized code.  This
section describes the most common cases.

@enumerate
@item
@i{The "hopping Program Counter":}  Repeated 'step' or 'next' commands show the PC
bouncing back and forth in the code.  This may result from any of the following
optimizations:

@itemize @bullet
@item
@i{Common subexpression elimination:} using a single instance of code for a
quantity that the source computes several times.  As a result you
may not be able to stop on what looks like a statement.

@item
@i{Invariant code motion:} moving an expression that does not change within a
loop, to the beginning of the loop.

@item
@i{Instruction scheduling:} moving instructions so as to
overlap loads and stores (typically) with other code, or in
general to move computations of values closer to their uses. Often
this causes you to pass an assignment statement without the assignment
happening and then later bounce back to the statement when the
value is actually needed.  Placing a breakpoint on a line of code
and then stepping over it may, therefore, not always cause all the
expected side-effects.
@end itemize

@item
@i{The "big leap":} More commonly known as @i{cross-jumping}, in which two
identical pieces of code are merged and the program counter suddenly
jumps to a statement that is not supposed to be executed, simply because
it (and the code following) translates to the same thing as the code
that @emph{was} supposed to be executed.  This effect is typically seen in
sequences that end in a jump, such as a @code{goto}, a @code{return}, or
a @code{break} in a C @code{qualifier} statement.

@item
@i{The "roving variable":} The symptom is an unexpected value in a variable.
There are various reasons for this effect:

@itemize @bullet
@item
In a subprogram prologue, a parameter may not yet have been moved to its
"home".

@item
A variable may be dead, and its register re-used.  This is
probably the most common cause.

@item
As mentioned above, the assignment of a value to a variable may
have been moved.

@item
A variable may be eliminated entirely by value propagation or
other means.  In this case, GCC may incorrectly generate debugging
information for the variable
@end itemize

@noindent
In general, when an unexpected value appears for a local variable or parameter
you should first ascertain if that value was actually computed by
your program, as opposed to being incorrectly reported by the debugger.
Record fields or
array elements in an object designated by an access value
are generally less of a problem, once you have ascertained that the access value
is sensible.
Typically, this means checking variables in the preceding code and in the
calling subprogram to verify that the value observed is explainable from other
values (one must apply the procedure recursively to those
other values); or re-running the code and stopping a little earlier
(perhaps before the call) and stepping to better see how the variable obtained
the value in question; or continuing to step @emph{from} the point of the
strange value to see if code motion had simply moved the variable's
assignments later.
@end enumerate

@node Inlining of Subprograms
@section Inlining of Subprograms

@noindent
A call to a subprogram in the current unit is inlined if all the
following conditions are met:

@itemize @bullet
@item
The optimization level is at least @code{/OPTIMIZE=SOME}.

@item
The called subprogram is suitable for inlining: It must be small enough
and not contain nested subprograms or anything else that @code{GNAT COMPILE}
cannot support in inlined subprograms.

@item
The call occurs after the definition of the body of the subprogram.

@item
@cindex pragma Inline
@findex Inline
Either @code{pragma Inline} applies to the subprogram or it is
small and automatic inlining (optimization level @code{/OPTIMIZE=INLINING}) is
specified.
@end itemize

@noindent
Calls to subprograms in @code{with}'ed units are normally not inlined.
To achieve this level of inlining, the following conditions must all be
true:

@itemize @bullet
@item
The optimization level is at least @code{/OPTIMIZE=SOME}.

@item
The called subprogram is suitable for inlining: It must be small enough
and not contain nested subprograms or anything else @code{GNAT COMPILE} cannot
support in inlined subprograms.

@item
The call appears in a body (not in a package spec).

@item
There is a @code{pragma Inline} for the subprogram.

@item
@cindex @option{/INLINE=PRAGMA} (@code{GNAT COMPILE})
The @code{/INLINE} qualifier
is used in the @code{GNAT COMPILE} command line
@end itemize

Note that specifying the @option{/INLINE=PRAGMA} qualifier causes additional
compilation dependencies. Consider the following:

@smallexample
@group
@cartouche
@b{package} R @b{is}
   @b{procedure} Q;
   @b{pragma} Inline (Q);
@b{end} R;
@b{package body} R @b{is}
   ...
@b{end} R;

@b{with} R;
@b{procedure} Main @b{is}
@b{begin}
   ...
   R.Q;
@b{end} Main;
@end cartouche
@end group
@end smallexample

@noindent
With the default behavior (no @option{/INLINE=PRAGMA} qualifier specified), the
compilation of the @code{Main} procedure depends only on its own source,
@file{MAIN.ADB}, and the spec of the package in file @file{R.ADS}. This
means that editing the body of @code{R} does not require recompiling
@code{Main}.

On the other hand, the call @code{R.Q} is not inlined under these
circumstances. If the @option{/INLINE=PRAGMA} qualifier is present when @code{Main}
is compiled, the call will be inlined if the body of @code{Q} is small
enough, but now @code{Main} depends on the body of @code{R} in
@file{R.ADB} as well as on the spec. This means that if this body is edited,
the main program must be recompiled. Note that this extra dependency
occurs whether or not the call is in fact inlined by @code{GNAT COMPILE}.

The use of front end inlining with @option{-gnatN} generates similar
additional dependencies.

@cindex @code{/INLINE=SUPPRESS} (@code{GNAT COMPILE})
Note: The @code{/INLINE=SUPPRESS} qualifier
can be used to prevent
all inlining. This qualifier overrides all other conditions and ensures
that no inlining occurs. The extra dependences resulting from
@option{/INLINE=PRAGMA} will still be active, even if
this qualifier is used to suppress the resulting inlining actions.

Note regarding the use of @code{/OPTIMIZE=INLINING}: There is no difference in inlining
behavior between @code{/OPTIMIZE=ALL} and @code{/OPTIMIZE=INLINING} for subprograms with an explicit
pragma @code{Inline} assuming the use of @option{/INLINE=PRAGMA}
or @option{-gnatN} (the qualifiers that activate inlining). If you have used
pragma @code{Inline} in appropriate cases, then it is usually much better
to use @code{/OPTIMIZE=ALL} and @option{/INLINE=PRAGMA} and avoid the use of @code{/OPTIMIZE=INLINING} which
in this case only has the effect of inlining subprograms you did not
think should be inlined. We often find that the use of @code{/OPTIMIZE=INLINING} slows
down code by performing excessive inlining, leading to increased instruction
cache pressure from the increased code size. So the bottom line here is
that you should not automatically assume that @code{/OPTIMIZE=INLINING} is better than
@code{/OPTIMIZE=ALL}, and indeed you should use @code{/OPTIMIZE=INLINING} only if tests show that
it actually improves performance.

@node Coverage Analysis
@section Coverage Analysis

@noindent
GNAT supports the Digital Performance Coverage Analyzer (PCA), which allows
the user to determine the distribution of execution time across a program,
@pxref{Profiling} for details of usage.

@include fdl.texi
@c GNU Free Documentation License

@node Index,,GNU Free Documentation License, Top
@unnumbered Index

@printindex cp

@contents

@bye