* extend.texi: Improve documentation of volatile asms.

[official-gcc.git] / gcc / cpp.texi

\input texinfo
@setfilename cpp.info
@settitle The C Preprocessor

@ifinfo
@dircategory Programming
@direntry
* Cpp: (cpp).                  The GNU C preprocessor.
@end direntry
@end ifinfo

@c @smallbook
@c @cropmarks
@c @finalout
@setchapternewpage odd
@ifinfo
This file documents the GNU C Preprocessor.

Copyright 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end ifinfo

@titlepage
@c @finalout
@title The C Preprocessor
@subtitle Last revised January 2001
@subtitle for GCC version 3
@author Richard M. Stallman
@page
@vskip 2pc
This booklet is eventually intended to form the first chapter of a GNU 
C Language manual.

@vskip 0pt plus 1filll
@c man begin COPYRIGHT
Copyright @copyright{} 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2001
Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@c man end
@end titlepage
@page

@node Top, Global Actions,, (DIR)
@chapter The C Preprocessor
@c man begin DESCRIPTION

The C preprocessor is a @dfn{macro processor} that is used automatically
by the C compiler to transform your program before actual compilation.
It is called a macro processor because it allows you to define
@dfn{macros}, which are brief abbreviations for longer constructs.

The C preprocessor is intended only for macro processing of C, C++ and
Objective C source files.  For macro processing of other files, you are
strongly encouraged to use alternatives like M4, which will likely give
you better results and avoid many problems.  For example, normally the C
preprocessor does not preserve arbitrary whitespace verbatim, but
instead replaces each sequence with a single space.

For use on C-like source files, the C preprocessor provides four
separate facilities that you can use as you see fit:

@itemize @bullet
@item
Inclusion of header files.  These are files of declarations that can be
substituted into your program.

@item
Macro expansion.  You can define @dfn{macros}, which are abbreviations
for arbitrary fragments of C code, and then the C preprocessor will
replace the macros with their definitions throughout the program.

@item
Conditional compilation.  Using special preprocessing directives, you
can include or exclude parts of the program according to various
conditions.

@item
Line control.  If you use a program to combine or rearrange source files
into an intermediate file which is then compiled, you can use line
control to inform the compiler of where each source line originally came
from.
@end itemize

C preprocessors vary in some details.  This manual discusses the GNU C
preprocessor, which provides a small superset of the features of ISO
Standard C@.

In its default mode, the GNU C preprocessor does not do a few things
required by the standard.  These are features which are rarely, if ever,
used, and may cause surprising changes to the meaning of a program which
does not expect them.  To get strict ISO Standard C, you should use the
@samp{-std=c89} or @samp{-std=c99} options, depending on which version
of the standard you want.  To get all the mandatory diagnostics, you
must also use @samp{-pedantic}.  @xref{Invocation}.

@c man end

@menu
* Global Actions::       Actions made uniformly on all input files.
* Directives::           General syntax of preprocessing directives.
* Header Files::         How and why to use header files.
* Macros::               How and why to use macros.
* Conditionals::         How and why to use conditionals.
* Assertions::           How and why to use assertions.
* Line Control::         Use of line control when you combine source files.
* Other Directives::     Miscellaneous preprocessing directives.
* Output::               Format of output from the C preprocessor.
* Implementation::       Implementation limits and behavior.
* Unreliable Features::  Undefined behavior and deprecated features.
* Invocation::           How to invoke the preprocessor; command options.
* Concept Index::        Index of concepts and terms.
* Index::                Index of directives, predefined macros and options.
@end menu

@node Global Actions, Directives, Top, Top
@section Transformations Made Globally
@cindex ASCII NUL handling

Most C preprocessor features are inactive unless you give specific
directives to request their use.  (Preprocessing directives are lines
starting with a @samp{#} token, possibly preceded by whitespace;
@pxref{Directives}).  However, there are four transformations that the
preprocessor always makes on all the input it receives, even in the
absence of directives.  These are, in order:

@enumerate
@item
Trigraphs, if enabled, are replaced with the character they represent.

@item
Backslash-newline sequences are deleted, no matter where.  This
feature allows you to break long lines for cosmetic purposes without
changing their meaning.

Recently, the non-traditional preprocessor has relaxed its treatment of
escaped newlines.  Previously, the newline had to immediately follow a
backslash.  The current implementation allows whitespace in the form of
spaces, horizontal and vertical tabs, and form feeds between the
backslash and the subsequent newline.  The preprocessor issues a
warning, but treats it as a valid escaped newline and combines the two
lines to form a single logical line.  This works within comments and
tokens, as well as between tokens.  Comments are @emph{not} treated as
whitespace for the purposes of this relaxation, since they have not yet
been replaced with spaces.

@item
All comments are replaced with single spaces.

@item
Predefined macro names are replaced with their expansions
(@pxref{Predefined}).
@end enumerate

For end-of-line indicators, any of \n, \r\n, \n\r and \r are recognised,
and treated as ending a single line.  As a result, if you mix these in a
single file you might get incorrect line numbering, because the
preprocessor would interpret the two-character versions as ending just
one line.  Previous implementations would only handle UNIX-style \n
correctly, so DOS-style \r\n would need to be passed through a filter
first.

The first three transformations are done @emph{before} all other parsing
and before preprocessing directives are recognized.  Thus, for example,
you can split a line mechanically with backslash-newline anywhere
(except within trigraphs since they are replaced first; see below).

@example
/*
*/ # /*
*/ defi\
ne FO\
O 10\
20
@end example

@noindent
is equivalent into @samp{#define FOO 1020}.

There is no way to prevent a backslash at the end of a line from being
interpreted as a backslash-newline.  For example,

@example
"foo\\
bar"
@end example

is equivalent to @code{"foo\bar"}, not to @code{"foo\\bar"}.  To avoid
having to worry about this, do not use the deprecated GNU extension
which permits multi-line strings.  Instead, use string literal
concatenation:

@example
   "foo\\"
   "bar"
@end example

Your program will be more portable this way, too.

There are a few things to note about the above four transformations.

@itemize @bullet
@item
Comments and predefined macro names (or any macro names, for that
matter) are not recognized inside the argument of an @samp{#include}
directive, when it is delimited with quotes or with @samp{<} and
@samp{>}.

@item
Comments and predefined macro names are never recognized within a
character or string constant.

@item
ISO ``trigraphs'' are converted before backslash-newlines are deleted.
If you write what looks like a trigraph with a backslash-newline inside,
the backslash-newline is deleted as usual, but it is too late to
recognize the trigraph.

This is relevant only if you use the @samp{-trigraphs} option to enable
trigraph processing.  @xref{Invocation}.
@end itemize

The preprocessor handles null characters embedded in the input file
depending upon the context in which the null appears.  Note that here we
are referring not to the two-character escape sequence "\0", but to the
single character ASCII NUL.

There are three different contexts in which a null character may
appear:

@itemize @bullet
@item
Within comments.  Here, null characters are silently ignored.

@item
Within a string or character constant.  Here the preprocessor emits a
warning, but preserves the null character and passes it through to the
output file or compiler front-end.

@item
In any other context, the preprocessor issues a warning, and discards
the null character.  The preprocessor treats it like whitespace,
combining it with any surrounding whitespace to become a single
whitespace block.  Representing the null character by "^@@", this means
that code like

@example
#define X^@@1
@end example

is equivalent to

@example
#define X 1
@end example

and X is defined with replacement text "1".
@end itemize

@node Directives, Header Files, Global Actions, Top
@section Preprocessing Directives

@cindex preprocessing directives
@cindex directives
Most preprocessor features are active only if you use preprocessing
directives to request their use.

Preprocessing directives are lines in your program that start with
@samp{#}.  Whitespace is allowed before and after the @samp{#}.  The
@samp{#} is followed by an identifier that is the @dfn{directive name}.
For example, @samp{#define} is the directive that defines a macro.

Since the @samp{#} must be the first token on the line, it cannot come
from a macro expansion if you wish it to begin a directive.  Also, the
directive name is not macro expanded.  Thus, if @samp{foo} is defined as
a macro expanding to @samp{define}, that does not make @samp{#foo} a
valid preprocessing directive.

The set of valid directive names is fixed.  Programs cannot define new
preprocessing directives.

Some directive names require arguments; these make up the rest of the
directive line and must be separated from the directive name by
whitespace.  For example, @samp{#define} must be followed by a macro
name and the intended expansion of the macro.  @xref{Object-like
Macros}.

A preprocessing directive cannot cover more than one line.  It may be
logically extended with backslash-newline, but that has no effect on its
meaning.  Comments containing newlines can also divide the directive
into multiple lines, but a comment is replaced by a single space before
the directive is interpreted.

@node Header Files, Macros, Directives, Top
@section Header Files

@cindex header file
A header file is a file containing C declarations and macro definitions
(@pxref{Macros}) to be shared between several source files.  You request
the use of a header file in your program with the C preprocessing
directive @samp{#include}.

@menu
* Header Uses::         What header files are used for.
* Include Syntax::      How to write @samp{#include} directives.
* Include Operation::   What @samp{#include} does.
* Once-Only::           Preventing multiple inclusion of one header file.
* Inheritance::         Including one header file in another header file.
* System Headers::      Special treatment for some header files.
@end menu

@node Header Uses, Include Syntax, Header Files, Header Files
@subsection Uses of Header Files

Header files serve two kinds of purposes.

@itemize @bullet
@item
@cindex system header files
System header files declare the interfaces to parts of the operating
system.  You include them in your program to supply the definitions and
declarations you need to invoke system calls and libraries.

@item
Your own header files contain declarations for interfaces between the
source files of your program.  Each time you have a group of related
declarations and macro definitions all or most of which are needed in
several different source files, it is a good idea to create a header
file for them.
@end itemize

Including a header file produces the same results in C compilation as
copying the header file into each source file that needs it.  Such
copying would be time-consuming and error-prone.  With a header file,
the related declarations appear in only one place.  If they need to be
changed, they can be changed in one place, and programs that include the
header file will automatically use the new version when next recompiled.
The header file eliminates the labor of finding and changing all the
copies as well as the risk that a failure to find one copy will result
in inconsistencies within a program.

The usual convention is to give header files names that end with
@file{.h}.  Avoid unusual characters in header file names, as they
reduce portability.

@node Include Syntax, Include Operation, Header Uses, Header Files
@subsection The @samp{#include} Directive

@findex #include
Both user and system header files are included using the preprocessing
directive @samp{#include}.  It has three variants:

@table @code
@item #include <@var{file}>
This variant is used for system header files.  It searches for a file
named @var{file} in a list of directories specified by you, then in a
standard list of system directories.  You specify directories to search
for header files with the command option @samp{-I} (@pxref{Invocation}).
The option @samp{-nostdinc} inhibits searching the standard system
directories; in this case only the directories you specify are searched.

The first @samp{>} character terminates the file name.  The file name
may contain a @samp{<} character.

@item #include "@var{file}"
This variant is used for header files of your own program.  It searches
for a file named @var{file} first in the current directory, then in the
same directories used for system header files.  The current directory is
the directory of the current input file.  It is tried first because it
is presumed to be the location of the files that the current input file
refers to.  (If the @samp{-I-} option is used, the special treatment of
the current directory is inhibited. @xref{Invocation}.)

The first @samp{"} character terminates the file name.

In both these variants, the argument behaves like a string constant in
that comments are not recognized, and macro names are not expanded.
Thus, in @samp{#include <x/*y>} the @samp{/*} does not start a comment
and the directive specifies inclusion of a system header file named
@file{x/*y}.

However, in either variant, if backslashes occur within @var{file}, they
are considered ordinary text characters, not escape characters.  None of
the character escape sequences appropriate to string constants in C are
processed.  Thus, @samp{#include "x\n\\y"} specifies a filename
containing three backslashes.

@item #include @var{anything else}
@cindex computed @samp{#include}
This variant is called a @dfn{computed #include}.  Any @samp{#include}
directive whose argument does not fit the above two forms is a computed
include.  The text @var{anything else} is checked for macro calls, which
are expanded (@pxref{Macros}).  When this is done, the result must match
one of the above two variants --- in particular, the expansion must form
a string literal token, or a sequence of tokens surrounded by angle
braces. @xref{Implementation}.

This feature allows you to define a macro which controls the file name
to be used at a later point in the program.  One application of this is
to allow a site-specific configuration file for your program to specify
the names of the system include files to be used.  This can help in
porting the program to various operating systems in which the necessary
system header files are found in different places.
@end table

@node Include Operation, Once-Only, Include Syntax, Header Files
@subsection How @samp{#include} Works

The @samp{#include} directive works by directing the C preprocessor to
scan the specified file as input before continuing with the rest of the
current file.  The output from the preprocessor contains the output
already generated, followed by the output resulting from the included
file, followed by the output that comes from the text after the
@samp{#include} directive.  For example, given a header file
@file{header.h} as follows,

@example
char *test ();
@end example

@noindent
and a main program called @file{program.c} that uses the header file,
like this,

@example
int x;
#include "header.h"

main ()
@{
  printf (test ());
@}
@end example

@noindent
the output generated by the C preprocessor for @file{program.c} as input
would be

@example
int x;
char *test ();

main ()
@{
  printf (test ());
@}
@end example

Included files are not limited to declarations and macro definitions;
those are merely the typical uses.  Any fragment of a C program can be
included from another file.  The include file could even contain the
beginning of a statement that is concluded in the containing file, or
the end of a statement that was started in the including file.  However,
a comment or a string or character constant may not start in the
included file and finish in the including file.  An unterminated
comment, string constant or character constant in an included file is
considered to end (with an error message) at the end of the file.

It is possible for a header file to begin or end a syntactic unit such
as a function definition, but that would be very confusing, so don't do
it.

The line following the @samp{#include} directive is always treated as a
separate line by the C preprocessor, even if the included file lacks a
final newline.

@node Once-Only, Inheritance, Include Operation, Header Files
@subsection Once-Only Include Files
@cindex repeated inclusion
@cindex including just once

Very often, one header file includes another.  It can easily result that
a certain header file is included more than once.  This may lead to
errors, if the header file defines structure types or typedefs, and is
certainly wasteful.  Therefore, we often wish to prevent multiple
inclusion of a header file.

The standard way to do this is to enclose the entire real contents of the
file in a conditional, like this:

@example
#ifndef FILE_FOO_SEEN
#define FILE_FOO_SEEN

@var{the entire file}

#endif /* FILE_FOO_SEEN */
@end example

The macro @code{FILE_FOO_SEEN} indicates that the file has been included
once already.  In a user header file, the macro name should not begin
with @samp{_}.  In a system header file, this name should begin with
@samp{__} to avoid conflicts with user programs.  In any kind of header
file, the macro name should contain the name of the file and some
additional text, to avoid conflicts with other header files.

The GNU C preprocessor is programmed to notice when a header file uses
this particular construct and handle it efficiently.  If a header file
is contained entirely in a @samp{#ifndef} conditional, modulo whitespace
and comments, then it remembers that fact.  If a subsequent
@samp{#include} specifies the same file, and the macro in the
@samp{#ifndef} is already defined, then the directive is skipped without
processing the specified file at all.

@findex #import
In the Objective C language, there is a variant of @samp{#include}
called @samp{#import} which includes a file, but does so at most once.
If you use @samp{#import} @emph{instead of} @samp{#include}, then you
don't need the conditionals inside the header file to prevent multiple
execution of the contents.

@samp{#import} is obsolete because it is not a well designed feature.
It requires the users of a header file --- the applications programmers
--- to know that a certain header file should only be included once.  It
is much better for the header file's implementor to write the file so
that users don't need to know this.  Using @samp{#ifndef} accomplishes
this goal.

@node Inheritance, System Headers, Once-Only, Header Files
@subsection Inheritance and Header Files
@cindex inheritance
@cindex overriding a header file

@dfn{Inheritance} is what happens when one object or file derives some
of its contents by virtual copying from another object or file.  In
the case of C header files, inheritance means that one header file 
includes another header file and then replaces or adds something.

If the inheriting header file and the base header file have different
names, then inheritance is straightforward: simply write @samp{#include
"@var{base}"} in the inheriting file.

Sometimes it is necessary to give the inheriting file the same name as
the base file.  This is less straightforward.

For example, suppose an application program uses the system header
@file{sys/signal.h}, but the version of @file{/usr/include/sys/signal.h}
on a particular system doesn't do what the application program expects.
It might be convenient to define a ``local'' version, perhaps under the
name @file{/usr/local/include/sys/signal.h}, to override or add to the
one supplied by the system.

You can do this by compiling with the option @samp{-I.}, and writing a
file @file{sys/signal.h} that does what the application program expects.
Making this file include the standard @file{sys/signal.h} is not so easy
--- writing @samp{#include <sys/signal.h>} in that file doesn't work,
because it includes your own version of the file, not the standard
system version.  Used in that file itself, this leads to an infinite
recursion and a fatal error in compilation.

@samp{#include </usr/include/sys/signal.h>} would find the proper file,
but that is not clean, since it makes an assumption about where the
system header file is found.  This is bad for maintenance, since it
means that any change in where the system's header files are kept
requires a change somewhere else.

@findex #include_next
The clean way to solve this problem is to use 
@samp{#include_next}, which means, ``Include the @emph{next} file with
this name.''  This directive works like @samp{#include} except in
searching for the specified file: it starts searching the list of header
file directories @emph{after} the directory in which the current file
was found.

Suppose you specify @samp{-I /usr/local/include}, and the list of
directories to search also includes @file{/usr/include}; and suppose
both directories contain @file{sys/signal.h}.  Ordinary @samp{#include
<sys/signal.h>} finds the file under @file{/usr/local/include}.  If that
file contains @samp{#include_next <sys/signal.h>}, it starts searching
after that directory, and finds the file in @file{/usr/include}.

@samp{#include_next} is a GCC extension and should not be used in
programs intended to be portable to other compilers.

@node System Headers,, Inheritance, Header Files
@subsection System Headers
@cindex system header files

The header files declaring interfaces to the operating system and
runtime libraries often cannot be written in strictly conforming C.
Therefore, GNU C gives code found in @dfn{system headers} special
treatment.  Certain categories of warnings are suppressed, notably those
enabled by @samp{-pedantic}.

Normally, only the headers found in specific directories are considered
system headers.  The set of these directories is determined when GCC is
compiled.  There are, however, two ways to add to the set.

@findex -isystem
The @samp{-isystem} command line option adds its argument to the list of
directories to search for headers, just like @samp{-I}.  In addition,
any headers found in that directory will be considered system headers.
Note that unlike @samp{-I}, you must put a space between @samp{-isystem}
and its argument.

All directories named by @samp{-isystem} are searched @strong{after} all
directories named by @samp{-I}, no matter what their order was on the
command line.  If the same directory is named by both @samp{-I} and
@samp{-isystem}, @samp{-I} wins; it is as if the @samp{-isystem} option
had never been specified at all.

@findex #pragma GCC system_header
There is also a directive, @samp{#pragma GCC system_header}, which tells
GCC to consider the rest of the current include file a system header, no
matter where it was found.  Code that comes before the @samp{#pragma} in
the file will not be affected.

@samp{#pragma GCC system_header} has no effect in the primary source file.

@node Macros, Conditionals, Header Files, Top
@section Macros

A macro is a sort of abbreviation which you can define once and then
use later.  There are many complicated features associated with macros
in the C preprocessor.

@menu
* Object-like Macros::   Macros that always expand the same way.
* Function-like Macros:: Macros that accept arguments that are substituted
                         into the macro expansion.
* Macro Varargs::        Macros with variable number of arguments.
* Predefined::           Predefined macros that are always available.
* Stringification::      Macro arguments converted into string constants.
* Concatenation::        Building tokens from parts taken from macro arguments.
* Undefining::           Cancelling a macro's definition.
* Redefining::           Changing a macro's definition.
* Poisoning::            Ensuring a macro is never defined or used.
* Macro Pitfalls::       Macros can confuse the unwary.  Here we explain
                           several common problems and strange features.
@end menu

@node Object-like Macros, Function-like Macros, Macros, Macros
@subsection Object-like Macros
@cindex object-like macro
@cindex manifest constant

An @dfn{object-like macro} is a kind of abbreviation.  It is a name
which stands for a fragment of code.  Some people refer to these as
@dfn{manifest constants}.

Before you can use a macro, you must @dfn{define} it explicitly with the
@samp{#define} directive.  @samp{#define} is followed by the name of the
macro and then the token sequence it should be an abbreviation for,
which is variously referred to as the macro's @dfn{body},
@dfn{expansion} or @dfn{replacement list}.  For example,

@example
#define BUFFER_SIZE 1020
@end example

@noindent
defines a macro named @samp{BUFFER_SIZE} as an abbreviation for the
token @samp{1020}.  If somewhere after this @samp{#define} directive
there comes a C statement of the form

@example
foo = (char *) xmalloc (BUFFER_SIZE);
@end example

@noindent
then the C preprocessor will recognize and @dfn{expand} the macro
@samp{BUFFER_SIZE}, resulting in

@example
foo = (char *) xmalloc (1020);
@end example

The use of all upper case for macro names is a standard convention.
Programs are easier to read when it is possible to tell at a glance
which names are macros.

Normally, a macro definition can only span a single logical line, like
all C preprocessing directives.  Comments within a macro definition may
contain newlines, which make no difference since each comment is
replaced by a space regardless of its contents.

Apart from this, there is no restriction on what can go in a macro body
provided it decomposes into valid preprocessing tokens.  In particular,
parentheses need not balance, and the body need not resemble valid C
code.  (If it does not, you may get error messages from the C
compiler when you use the macro.)

The C preprocessor scans your program sequentially, so macro definitions
take effect at the place you write them.  Therefore, the following input
to the C preprocessor

@example
foo = X;
#define X 4
bar = X;
@end example

@noindent
produces as output

@example
foo = X;

bar = 4;
@end example

When the preprocessor expands a macro name, the macro's expansion
replaces the macro invocation, and the result is re-scanned for more
macros to expand.  For example, after

@example
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
@end example

@noindent
the name @samp{TABLESIZE} when used in the program would go through two
stages of expansion, resulting ultimately in @samp{1020}.

This is not the same as defining @samp{TABLESIZE} to be @samp{1020}.
The @samp{#define} for @samp{TABLESIZE} uses exactly the expansion you
specify --- in this case, @samp{BUFSIZE} --- and does not check to see
whether it too contains macro names.  Only when you @emph{use}
@samp{TABLESIZE} is the result of its expansion scanned for more macro
names.  @xref{Cascaded Macros}.

@node Function-like Macros, Macro Varargs, Object-like Macros, Macros
@subsection Macros with Arguments
@cindex macros with argument
@cindex arguments in macro definitions
@cindex function-like macro

An object-like macro is always replaced by exactly the same tokens each
time it is used.  Macros can be made more flexible by taking
@dfn{arguments}.  Arguments are fragments of code that you supply each
time the macro is used.  These fragments are included in the expansion
of the macro according to the directions in the macro definition.  A
macro that accepts arguments is called a @dfn{function-like macro}
because the syntax for using it looks like a function call.

@findex #define
To define a macro that uses arguments, you write a @samp{#define}
directive with a list of @dfn{parameters} in parentheses after the name
of the macro.  The parameters must be valid C identifiers, separated by
commas and optionally whitespace.  The @samp{(} must follow the macro
name immediately, with no space in between.  If you leave a space, you
instead define an object-like macro whose expansion begins with a
@samp{(}, and often leads to confusing errors at compile time.

As an example, here is a macro that computes the minimum of two numeric
values, as it is defined in many C programs:

@example
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
@end example

@noindent
(This is not the best way to define a ``minimum'' macro in GNU C@.
@xref{Side Effects}, for more information.)

To invoke a function-like macro, you write the name of the macro
followed by a list of @dfn{arguments} in parentheses, separated by
commas.  The invocation of the macro need not be restricted to a single
logical line - it can cross as many lines in the source file as you
wish.  The number of arguments you give must match the number of
parameters in the macro definition; empty arguments are fine.  Examples
of use of the macro @samp{min} include @samp{min (1, 2)} and @samp{min
(x + 28, *p)}.

The expansion text of the macro depends on the arguments you use.  Each
macro parameter is replaced throughout the macro expansion with the
tokens of the corresponding argument.  Leading and trailing argument
whitespace is dropped, and all whitespace between the tokens of an
argument is reduced to a single space.  Using the same macro @samp{min}
defined above, @samp{min (1, 2)} expands into

@example
((1) < (2) ? (1) : (2))
@end example

@noindent
where @samp{1} has been substituted for @samp{X} and @samp{2} for @samp{Y}.

Likewise, @samp{min (x + 28, *p)} expands into

@example
((x + 28) < (*p) ? (x + 28) : (*p))
@end example

Parentheses within each argument must balance; a comma within such
parentheses does not end the argument.  However, there is no requirement
for square brackets or braces to balance, and they do not prevent a
comma from separating arguments.  Thus,

@example
macro (array[x = y, x + 1])
@end example

@noindent
passes two arguments to @code{macro}: @samp{array[x = y} and @samp{x +
1]}.  If you want to supply @samp{array[x = y, x + 1]} as an argument,
you must write it as @samp{array[(x = y, x + 1)]}, which is equivalent C
code.

After the arguments have been substituted into the macro body, the
resulting expansion replaces the macro invocation, and re-scanned for
more macro calls.  Therefore even arguments can contain calls to other
macros, either with or without arguments, and even to the same macro.
For example, @samp{min (min (a, b), c)} expands into this text:

@example
((((a) < (b) ? (a) : (b))) < (c)
 ? (((a) < (b) ? (a) : (b)))
 : (c))
@end example

@noindent
(Line breaks shown here for clarity would not actually be generated.)

@cindex empty macro arguments
If a macro @code{foo} takes one argument, and you want to supply an
empty argument, simply supply no preprocessing tokens.  Since whitespace
does not form a preprocessing token, it is optional.  For example,
@samp{foo ()}, @samp{foo ( )} and @samp{bar (, arg2)}.

Previous GNU preprocessor implementations and documentation were
incorrect on this point, insisting that a function-like macro that takes
a single argument be passed a space if an empty argument was required.

If you use a macro name followed by something other than a @samp{(}
(after ignoring any whitespace that might follow), it does not form an
invocation of the macro, and the preprocessor does not change what you
have written.  Therefore, it is possible for the same identifier to be a
variable or function in your program as well as a macro, and you can
choose in each instance whether to refer to the macro (if an actual
argument list follows) or the variable or function (if an argument list
does not follow).  For example,

@example
#define foo(X) X
foo bar foo(baz)
@end example

expands to @samp{foo bar baz}.  Such dual use of one name could be
confusing and should be avoided except when the two meanings are
effectively synonymous: that is, when the name is both a macro and a
function and the two have similar effects.  You can think of the name
simply as a function; use of the name for purposes other than calling it
(such as, to take the address) will refer to the function, while calls
will expand the macro and generate better but equivalent code.

For example, you can use a function named @samp{min} in the same source
file that defines the macro.  If you write @samp{&min} with no argument
list, you refer to the function.  If you write @samp{min (x, bb)}, with
an argument list, the macro is expanded.  If you write @samp{(min) (a,
bb)}, where the name @samp{min} is not followed by an open-parenthesis,
the macro is not expanded, so you wind up with a call to the function
@samp{min}.

In the definition of a macro with arguments, the list of argument names
must follow the macro name immediately with no space in between.  If
there is a space after the macro name, the macro is defined as taking no
arguments, and all the rest of the line is taken to be the expansion.
The reason for this is that it is often useful to define a macro that
takes no arguments and whose definition begins with an identifier in
parentheses.  This rule makes it possible for you to do either this:

@example
#define FOO(x) - 1 / (x)
@end example

@noindent
(which defines @samp{FOO} to take an argument and expand into minus the
reciprocal of that argument) or this:

@example
#define BAR (x) - 1 / (x)
@end example

@noindent
(which defines @samp{BAR} to take no argument and always expand into
@samp{(x) - 1 / (x)}).

Note that the @emph{uses} of a macro with arguments can have spaces
before the left parenthesis; it's the @emph{definition} where it matters
whether there is a space.

@node Macro Varargs, Predefined, Function-like Macros, Macros
@subsection Macros with Variable Numbers of Arguments
@cindex variable number of arguments
@cindex macro with variable arguments
@cindex rest argument (in macro)

In the ISO C standard of 1999, a macro can be declared to accept a
variable number of arguments much as a function can.  The syntax for
defining the macro is similar to that of a function.  Here is an
example:

@example
#define eprintf(...) fprintf (stderr, __VA_ARGS__)
@end example

Here @samp{@dots{}} is a @dfn{variable argument}.  In the invocation of
such a macro, it represents the zero or more tokens until the closing
parenthesis that ends the invocation, including any commas.  This set of
tokens replaces the identifier @code{__VA_ARGS__} in the macro body
wherever it appears.  Thus, we have this expansion:

@example
eprintf ("%s:%d: ", input_file_name, line_number)
@expansion{}
fprintf (stderr, "%s:%d: " , input_file_name, line_number)
@end example

Within a @samp{#define} directive, ISO C mandates that the only place
the identifier @code{__VA_ARGS__} can appear is in the replacement list
of a variable-argument macro.  It may not be used as a macro name, macro
argument name, or within a different type of macro.  It may also be
forbidden in open text; the standard is ambiguous.  We recommend you
avoid using it except for its defined purpose.

If your macro is complicated, you may want a more descriptive name for
the variable argument than @code{__VA_ARGS__}.  GNU cpp permits this, as
an extension.  You may write an argument name immediately before the
@samp{@dots{}}; that name is used for the variable argument.  The
@code{eprintf} macro above could be written

@example
#define eprintf(args...) fprintf (stderr, args)
@end example

@noindent
using this extension.  You cannot use @code{__VA_ARGS__} and this
extension in the same macro.

We might instead have defined eprintf as follows:

@example
#define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__)
@end example

This formulation looks more descriptive, but cannot be used as flexibly.
There is no way to produce expanded output of

@example
fprintf (stderr, "success!\n")
@end example

@noindent
because, in standard C, you are not allowed to leave the variable
argument out entirely, and passing an empty argument for the variable
arguments will not do what you want.  Writing

@example
eprintf ("success!\n", )
@end example

@noindent
produces

@example
fprintf (stderr, "success!\n",)
@end example

@noindent
where the extra comma originates from the replacement list and not from
the arguments to eprintf.

There is another extension in the GNU C preprocessor which deals with
this difficulty.  First, you are allowed to leave the variable argument
out entirely:

@example
eprintf ("success!\n")
@end example

Second, the @samp{##} token paste operator has a special meaning when
placed between a comma and a variable argument.  If you write

@example
#define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__)
@end example

and the variable argument is left out when the @samp{eprintf} macro is
used, then the comma before the @samp{##} will be deleted.  This does
@emph{not} happen if you pass an empty argument, nor does it happen if
the token preceding @samp{##} is anything other than a comma.

Previous versions of the preprocessor implemented this extension much
more generally.  We have restricted it in order to minimize the
difference from the C standard.  @xref{Unreliable Features}.

@node Predefined, Stringification, Macro Varargs, Macros
@subsection Predefined Macros

@cindex predefined macros
Several object-like macros are predefined; you use them without
supplying their definitions.  They fall into two classes: standard
macros and system-specific macros.

@menu
* Standard Predefined::     Standard predefined macros.
* Nonstandard Predefined::  Nonstandard predefined macros.
@end menu

@node Standard Predefined, Nonstandard Predefined, Predefined, Predefined
@subsubsection Standard Predefined Macros
@cindex standard predefined macros

The standard predefined macros are available with the same meanings
regardless of the machine or operating system on which you are using GNU
C@.  Their names all start and end with double underscores.  Those
preceding @code{__GNUC__} in this table are standardized by ISO C; the
rest are GNU C extensions.

@table @code
@item __FILE__
@findex __FILE__
This macro expands to the name of the current input file, in the form of
a C string constant.  The precise name returned is the one that was
specified in @samp{#include} or as the input file name argument.  For
example, @samp{"/usr/local/include/myheader.h"} is a possible expansion
of this macro.

@item __LINE__
@findex __LINE__
This macro expands to the current input line number, in the form of a
decimal integer constant.  While we call it a predefined macro, it's
a pretty strange macro, since its ``definition'' changes with each
new line of source code.

This and @samp{__FILE__} are useful in generating an error message to
report an inconsistency detected by the program; the message can state
the source line at which the inconsistency was detected.  For example,

@smallexample
fprintf (stderr, "Internal error: "
                 "negative string length "
                 "%d at %s, line %d.",
         length, __FILE__, __LINE__);
@end smallexample

A @samp{#include} directive changes the expansions of @samp{__FILE__}
and @samp{__LINE__} to correspond to the included file.  At the end of
that file, when processing resumes on the input file that contained
the @samp{#include} directive, the expansions of @samp{__FILE__} and
@samp{__LINE__} revert to the values they had before the
@samp{#include} (but @samp{__LINE__} is then incremented by one as
processing moves to the line after the @samp{#include}).

The expansions of both @samp{__FILE__} and @samp{__LINE__} are altered
if a @samp{#line} directive is used.  @xref{Line Control}.

@item __DATE__
@findex __DATE__
This macro expands to a string constant that describes the date on
which the preprocessor is being run.  The string constant contains
eleven characters and looks like @w{@samp{"Feb  1 1996"}}.
@c After reformatting the above, check that the date remains `Feb  1 1996',
@c all on one line, with two spaces between the `Feb' and the `1'.

@item __TIME__
@findex __TIME__
This macro expands to a string constant that describes the time at
which the preprocessor is being run.  The string constant contains
eight characters and looks like @samp{"23:59:01"}.

@item __STDC__
@findex __STDC__
This macro expands to the constant 1, to signify that this is ISO
Standard C@.  (Whether that is actually true depends on what C compiler
will operate on the output from the preprocessor.)

On some hosts, system include files use a different convention, where
@samp{__STDC__} is normally 0, but is 1 if the user specifies strict
conformance to the C Standard.  The preprocessor follows the host
convention when processing system include files, but when processing
user files it follows the usual GNU C convention.

This macro is not defined if the @samp{-traditional} option is used.

@item __STDC_VERSION__
@findex __STDC_VERSION__
This macro expands to the C Standard's version number, a long integer
constant of the form @samp{@var{yyyy}@var{mm}L} where @var{yyyy} and
@var{mm} are the year and month of the Standard version.  This signifies
which version of the C Standard the preprocessor conforms to.  Like
@samp{__STDC__}, whether this version number is accurate for the entire
implementation depends on what C compiler will operate on the output
from the preprocessor.

This macro is not defined if the @samp{-traditional} option is used.

@item __GNUC__
@findex __GNUC__
This macro is defined if and only if this is GNU C@.  This macro is
defined only when the entire GNU C compiler is in use; if you invoke the
preprocessor directly, @samp{__GNUC__} is undefined.  The value
identifies the major version number of GNU CC (@samp{1} for GNU CC
version 1, which is now obsolete, and @samp{2} for version 2).

@item __GNUC_MINOR__
@findex __GNUC_MINOR__
The macro contains the minor version number of the compiler.  This can
be used to work around differences between different releases of the
compiler (for example, if GCC 2.6.x is known to support a feature, you
can test for @code{__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 6)}).

@item __GNUC_PATCHLEVEL__
@findex __GNUC_PATCHLEVEL__
This macro contains the patch level of the compiler.  This can be
used to work around differences between different patch level releases
of the compiler (for example, if GCC 2.6.2 is known to contain a bug,
whereas GCC 2.6.3 contains a fix, and you have code which can workaround
the problem depending on whether the bug is fixed or not, you can test for
@code{__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ > 6) || 
(__GNUC__ == 2 && __GNUC_MINOR__ == 6 && __GNUC_PATCHLEVEL__ >= 3)}).

@item __GNUG__
@findex __GNUG__
The GNU C compiler defines this when the compilation language is
C++; use @samp{__GNUG__} to distinguish between GNU C and GNU
C++.

@item __cplusplus 
@findex __cplusplus 
The ISO standard for C++ requires predefining this variable.  You can
use @samp{__cplusplus} to test whether a header is compiled by a C
compiler or a C++ compiler. The compiler currently uses a value of
@samp{1}, instead of the value @samp{199711L}, which would indicate full
conformance with the standard.

@item __STRICT_ANSI__
@findex __STRICT_ANSI__
GNU C defines this macro if and only if the @option{-ansi} switch, or a
@option{-std} switch specifying strict conformance to some version of ISO C,
was specified when GNU C was invoked.  Its definition is the null string.
This macro exists primarily to direct certain GNU header files not to
define certain traditional Unix constructs which are incompatible with
ISO C@.

@item __BASE_FILE__
@findex __BASE_FILE__
This macro expands to the name of the main input file, in the form
of a C string constant.  This is the source file that was specified
on the command line of the preprocessor or C compiler.

@item __INCLUDE_LEVEL__
@findex __INCLUDE_LEVEL_
This macro expands to a decimal integer constant that represents the
depth of nesting in include files.  The value of this macro is
incremented on every @samp{#include} directive and decremented at the
end of every included file.  It starts out at 0, it's value within the
base file specified on the command line.

@item __VERSION__
@findex __VERSION__
This macro expands to a string constant which describes the version
number of GNU C@.  The string is normally a sequence of decimal numbers
separated by periods, such as @samp{"2.6.0"}.

@item __OPTIMIZE__
@findex __OPTIMIZE__
GNU CC defines this macro in optimizing compilations.  It causes certain
GNU header files to define alternative macro definitions for some system
library functions.  You should not refer to or test the definition of
this macro unless you make very sure that programs will execute with the
same effect regardless.

@item __OPTIMIZE_SIZE__
@findex __OPTIMIZE_SIZE__
GNU CC defines this macro when optimizing for size with @samp{-Os}.  It
causes certain GNU header files to define alternative macro definitions
for some system library functions.  You should not refer to or test the
definition of this macro unless you make very sure that programs will
execute with the same effect regardless.

@item __CHAR_UNSIGNED__
@findex __CHAR_UNSIGNED__
GNU C defines this macro if and only if the data type @code{char} is
unsigned on the target machine.  It exists to cause the standard header
file @file{limits.h} to work correctly.  You should not refer to this
macro yourself; instead, refer to the standard macros defined in
@file{limits.h}.  The preprocessor uses this macro to determine whether
or not to sign-extend large character constants written in octal; see
@ref{#if Directive,,The @samp{#if} Directive}.

@item __REGISTER_PREFIX__
@findex __REGISTER_PREFIX__
This macro expands to a string (not a string constant) describing the
prefix applied to CPU registers in assembler code.  You can use it to
write assembler code that is usable in multiple environments.  For
example, in the @samp{m68k-aout} environment it expands to the null
string, but in the @samp{m68k-coff} environment it expands to the string
@samp{%}.

@item __USER_LABEL_PREFIX__
@findex __USER_LABEL_PREFIX__
Similar to @code{__REGISTER_PREFIX__}, but describes the prefix applied
to user generated labels in assembler code.  For example, in the
@samp{m68k-aout} environment it expands to the string @samp{_}, but in
the @samp{m68k-coff} environment it expands to the null string.  This
does not work with the @samp{-mno-underscores} option that the i386
OSF/rose and m88k targets provide nor with the @samp{-mcall*} options of
the rs6000 System V Release 4 target.
@end table

@node Nonstandard Predefined,, Standard Predefined, Predefined
@subsubsection Nonstandard Predefined Macros

The C preprocessor normally has several predefined macros that vary
between machines because their purpose is to indicate what type of
system and machine is in use.  This manual, being for all systems and
machines, cannot tell you exactly what their names are; instead, we
offer a list of some typical ones.  You can use @samp{cpp -dM} to see
the values of predefined macros; see @ref{Invocation}.

Some nonstandard predefined macros describe the operating system in use,
with more or less specificity.  For example,

@table @code
@item unix
@findex unix
@samp{unix} is normally predefined on all Unix systems.

@item BSD
@findex BSD
@samp{BSD} is predefined on recent versions of Berkeley Unix
(perhaps only in version 4.3).
@end table

Other nonstandard predefined macros describe the kind of CPU, with more or
less specificity.  For example,

@table @code
@item vax
@findex vax
@samp{vax} is predefined on Vax computers.

@item mc68000
@findex mc68000
@samp{mc68000} is predefined on most computers whose CPU is a Motorola
68000, 68010 or 68020.

@item m68k
@findex m68k
@samp{m68k} is also predefined on most computers whose CPU is a 68000,
68010 or 68020; however, some makers use @samp{mc68000} and some use
@samp{m68k}.  Some predefine both names.  What happens in GNU C
depends on the system you are using it on.

@item M68020
@findex M68020
@samp{M68020} has been observed to be predefined on some systems that
use 68020 CPUs --- in addition to @samp{mc68000} and @samp{m68k}, which
are less specific.

@item _AM29K
@findex _AM29K
@itemx _AM29000
@findex _AM29000
Both @samp{_AM29K} and @samp{_AM29000} are predefined for the AMD 29000
CPU family.

@item ns32000
@findex ns32000
@samp{ns32000} is predefined on computers which use the National
Semiconductor 32000 series CPU.
@end table

Yet other nonstandard predefined macros describe the manufacturer of
the system.  For example,

@table @code
@item sun
@findex sun
@samp{sun} is predefined on all models of Sun computers.

@item pyr
@findex pyr
@samp{pyr} is predefined on all models of Pyramid computers.

@item sequent
@findex sequent
@samp{sequent} is predefined on all models of Sequent computers.
@end table

These predefined symbols are not only nonstandard, they are contrary to the
ISO standard because their names do not start with underscores.
Therefore, the option @samp{-ansi} inhibits the definition of these
symbols.

This tends to make @samp{-ansi} useless, since many programs depend on
the customary nonstandard predefined symbols.  Even system header files
check them and will generate incorrect declarations if they do not find
the names that are expected.  You might think that the header files
supplied for the Uglix computer would not need to test what machine they
are running on, because they can simply assume it is the Uglix; but
often they do, and they do so using the customary names.  As a result,
very few C programs will compile with @samp{-ansi}.  We intend to avoid
such problems on the GNU system.

What, then, should you do in an ISO C program to test the type of machine
it will run on?

GNU C offers a parallel series of symbols for this purpose, whose names
are made from the customary ones by adding @samp{__} at the beginning
and end.  Thus, the symbol @code{__vax__} would be available on a Vax,
and so on.

The set of nonstandard predefined names in the GNU C preprocessor is
controlled (when @code{cpp} is itself compiled) by the macro
@samp{CPP_PREDEFINES}, which should be a string containing @samp{-D}
options, separated by spaces.  For example, on the Sun 3, we use the
following definition:

@example
#define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"
@end example

@noindent 
This macro is usually specified in @file{tm.h}.

@node Stringification, Concatenation, Predefined, Macros
@subsection Stringification

@cindex stringification
@dfn{Stringification} means turning a sequence of preprocessing tokens
into a string literal.  For example, stringifying @samp{foo (z)} results
in @samp{"foo (z)"}.

In the C preprocessor, stringification is possible when macro arguments
are substituted during macro expansion.  When a parameter appears
preceded by a @samp{#} token in the replacement list of a function-like
macro, it indicates that both tokens should be replaced with the
stringification of the corresponding argument during expansion.  The
same argument may be substituted in other places in the definition
without stringification if the argument name appears in those places
with no preceding @samp{#}.

Here is an example of a macro definition that uses stringification:

@smallexample
@group
#define WARN_IF(EXP) \
do @{ if (EXP) \
        fprintf (stderr, "Warning: " #EXP "\n"); @} \
while (0)
@end group
@end smallexample

@noindent
Here the argument for @samp{EXP} is substituted once, as-is, into the
@samp{if} statement, and once, stringified, into the argument to
@samp{fprintf}.  The @samp{do} and @samp{while (0)} are a kludge to make
it possible to write @samp{WARN_IF (@var{arg});}, which the resemblance
of @samp{WARN_IF} to a function would make C programmers want to do; see
@ref{Swallow Semicolon}.

The stringification feature is limited to transforming the tokens of a
macro argument into a string constant: there is no way to combine the
argument with surrounding text and stringify it all together.  The
example above shows how an equivalent result can be obtained in ISO
Standard C, using the fact that adjacent string constants are
concatenated by the C compiler to form a single string constant.  The
preprocessor stringifies the actual value of @samp{EXP} into a separate
string constant, resulting in text like

@smallexample
@group
do @{ if (x == 0) \
        fprintf (stderr, "Warning: " "x == 0" "\n"); @} \
while (0)
@end group
@end smallexample

@noindent
but the compiler then sees three consecutive string constants and
concatenates them into one, producing effectively

@smallexample
do @{ if (x == 0) \
        fprintf (stderr, "Warning: x == 0\n"); @} \
while (0)
@end smallexample

Stringification in C involves more than putting double-quote characters
around the fragment.  The preprocessor backslash-escapes the surrounding
quotes of string literals, and all backslashes within string and
character constants, in order to get a valid C string constant with the
proper contents.  Thus, stringifying @samp{p = "foo\n";} results in
@samp{"p = \"foo\\n\";"}.  However, backslashes that are not inside
string or character constants are not duplicated: @samp{\n} by itself
stringifies to @samp{"\n"}.

Whitespace (including comments) in the text being stringified is handled
according to precise rules.  All leading and trailing whitespace is
ignored.  Any sequence of whitespace in the middle of the text is
converted to a single space in the stringified result.

@node Concatenation, Undefining, Stringification, Macros
@subsection Concatenation
@cindex concatenation
@cindex @samp{##}
@dfn{Concatenation} means joining two strings into one.  In the context
of macro expansion, concatenation refers to joining two preprocessing
tokens to form one.  In particular, a token of a macro argument can be
concatenated with another argument's token or with fixed text to produce
a longer name.  The longer name might be the name of a function,
variable, type, or a C keyword; it might even be the name of another
macro, in which case it will be expanded.

When you define a function-like or object-like macro, you request
concatenation with the special operator @samp{##} in the macro's
replacement list.  When the macro is called, any arguments are
substituted without performing macro expansion, every @samp{##} operator
is deleted, and the two tokens on either side of it are concatenated to
form a single token.

Consider a C program that interprets named commands.  There probably needs
to be a table of commands, perhaps an array of structures declared as
follows:

@example
struct command
@{
  char *name;
  void (*function) ();
@};

struct command commands[] =
@{
  @{ "quit", quit_command@},
  @{ "help", help_command@},
  @dots{}
@};
@end example

It would be cleaner not to have to give each command name twice, once in
the string constant and once in the function name.  A macro which takes the
name of a command as an argument can make this unnecessary.  The string
constant can be created with stringification, and the function name by
concatenating the argument with @samp{_command}.  Here is how it is done:

@example
#define COMMAND(NAME)  @{ #NAME, NAME ## _command @}

struct command commands[] =
@{
  COMMAND (quit),
  COMMAND (help),
  @dots{}
@};
@end example

The usual case of concatenation is concatenating two names (or a name
and a number) into a longer name.  This isn't the only valid case.
It is also possible to concatenate two numbers (or a number and a name,
such as @samp{1.5} and @samp{e3}) into a number.  Also, multi-character
operators such as @samp{+=} can be formed by concatenation.  However,
two tokens that don't together form a valid token cannot be
concatenated.  For example, concatenation of @samp{x} on one side and
@samp{+} on the other is not meaningful because those two tokens do not
form a valid preprocessing token when concatenated.  UNDEFINED

Keep in mind that the C preprocessor converts comments to whitespace
before macros are even considered.  Therefore, you cannot create a
comment by concatenating @samp{/} and @samp{*}: the @samp{/*} sequence
that starts a comment is not a token, but rather the beginning of a
comment.  You can freely use comments next to @samp{##} in a macro
definition, or in arguments that will be concatenated, because the
comments will be converted to spaces at first sight, and concatenation
operates on tokens and so ignores whitespace.

@node Undefining, Redefining, Concatenation, Macros
@subsection Undefining Macros

@cindex undefining macros
To @dfn{undefine} a macro means to cancel its definition.  This is done
with the @samp{#undef} directive.  @samp{#undef} is followed by the macro
name to be undefined.

Like definition, undefinition occurs at a specific point in the source
file, and it applies starting from that point.  The name ceases to be a
macro name, and from that point on it is treated by the preprocessor as
if it had never been a macro name.

For example,

@example
#define FOO 4
x = FOO;
#undef FOO
x = FOO;
@end example

@noindent
expands into

@example
x = 4;

x = FOO;
@end example

@noindent
In this example, @samp{FOO} had better be a variable or function as well
as (temporarily) a macro, in order for the result of the expansion to be
valid C code.

The same form of @samp{#undef} directive will cancel definitions with
arguments or definitions that don't expect arguments.  The @samp{#undef}
directive has no effect when used on a name not currently defined as a
macro.

@node Redefining, Poisoning, Undefining, Macros
@subsection Redefining Macros

@cindex redefining macros
@dfn{Redefining} a macro means defining (with @samp{#define}) a name that
is already defined as a macro.

A redefinition is trivial if the new definition is transparently
identical to the old one.  You probably wouldn't deliberately write a
trivial redefinition, but they can happen automatically when a header
file is included more than once (@pxref{Header Files}), so they are
accepted silently and without effect.

Nontrivial redefinition is considered likely to be an error, so it
provokes a warning message from the preprocessor.  However, sometimes it
is useful to change the definition of a macro in mid-compilation.  You
can inhibit the warning by undefining the macro with @samp{#undef}
before the second definition.

In order for a redefinition to be trivial, the parameter names must
match and be in the same order, and the new replacement list must
exactly match the one already in effect, with two possible exceptions:

@itemize @bullet
@item
Whitespace may be added or deleted at the beginning or the end of the
replacement list.  In a sense this is vacuous, since strictly such
whitespace doesn't form part of the macro's expansion.

@item
Between tokens in the expansion, any two forms of whitespace are
considered equivalent.  In particular, whitespace may not be eliminated
entirely, nor may it be added where there previously wasn't any.
@end itemize

Recall that a comment counts as whitespace.

As a particular case of the above, you may not redefine an object-like
macro as a function-like macro, and vice-versa.

@node Poisoning, Macro Pitfalls, Redefining, Macros
@subsection Poisoning Macros
@cindex poisoning macros
@findex #pragma GCC poison

Sometimes, there is an identifier that you want to remove completely
from your program, and make sure that it never creeps back in.  To
enforce this, the @samp{#pragma GCC poison} directive can be used.
@samp{#pragma GCC poison} is followed by a list of identifiers to
poison, and takes effect for the rest of the source.  You cannot
@samp{#undef} a poisoned identifier or test to see if it's defined with
@samp{#ifdef}.

For example,

@example
#pragma GCC poison printf sprintf fprintf
sprintf(some_string, "hello");
@end example

@noindent
will produce an error.

Note, if the poisoned identifier appears through the result of macro
expansion it @emph{won't} cause an error.  So if you poison an
identifier you need not worry about system headers defining macros that
use it.

For example,

@example
#define strrchr rindex
#pragma GCC poison rindex
strrchr(some_string, 'h');
@end example

@noindent
will not produce an error.

@node Macro Pitfalls,, Poisoning, Macros
@subsection Pitfalls and Subtleties of Macros
@cindex problems with macros
@cindex pitfalls of macros

In this section we describe some special rules that apply to macros and
macro expansion, and point out certain cases in which the rules have
counterintuitive consequences that you must watch out for.

@menu
* Misnesting::        Macros can contain unmatched parentheses.
* Macro Parentheses:: Why apparently superfluous parentheses
                         may be necessary to avoid incorrect grouping.
* Swallow Semicolon:: Macros that look like functions
                         but expand into compound statements.
* Side Effects::      Unsafe macros that cause trouble when
                         arguments contain side effects.
* Self-Reference::    Macros whose definitions use the macros' own names.
* Argument Prescan::  Arguments are checked for macro calls before they
                         are substituted.
* Cascaded Macros::   Macros whose definitions use other macros.
* Newlines in Args::  Sometimes line numbers get confused.
@end menu

@node Misnesting, Macro Parentheses, Macro Pitfalls, Macro Pitfalls
@subsubsection Improperly Nested Constructs

Recall that when a macro is called with arguments, the arguments are
substituted into the macro body and the result is checked, together with
the rest of the input file, for more macro calls.

It is possible to piece together a macro call coming partially from the
macro body and partially from the arguments.  For example,

@example
#define double(x) (2*(x))
#define call_with_1(x) x(1)
@end example

@noindent
would expand @samp{call_with_1 (double)} into @samp{(2*(1))}.

Macro definitions do not have to have balanced parentheses.  By writing
an unbalanced open parenthesis in a macro body, it is possible to create
a macro call that begins inside the macro body but ends outside of it.
For example,

@example
#define strange(file) fprintf (file, "%s %d",
@dots{}
strange(stderr) p, 35)
@end example

@noindent
This bizarre example expands to @samp{fprintf (stderr, "%s %d", p, 35)}!

@node Macro Parentheses, Swallow Semicolon, Misnesting, Macro Pitfalls
@subsubsection Unintended Grouping of Arithmetic
@cindex parentheses in macro bodies

You may have noticed that in most of the macro definition examples shown
above, each occurrence of a macro argument name had parentheses around
it.  In addition, another pair of parentheses usually surround the
entire macro definition.  Here is why it is best to write macros that
way.

Suppose you define a macro as follows,

@example
#define ceil_div(x, y) (x + y - 1) / y
@end example

@noindent
whose purpose is to divide, rounding up.  (One use for this operation is
to compute how many @samp{int} objects are needed to hold a certain
number of @samp{char} objects.)  Then suppose it is used as follows:

@example
a = ceil_div (b & c, sizeof (int));
@end example

@noindent
This expands into

@example
a = (b & c + sizeof (int) - 1) / sizeof (int);
@end example

@noindent
which does not do what is intended.  The operator-precedence rules of
C make it equivalent to this:

@example
a = (b & (c + sizeof (int) - 1)) / sizeof (int);
@end example

@noindent
What we want is this:

@example
a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
@end example

@noindent
Defining the macro as

@example
#define ceil_div(x, y) ((x) + (y) - 1) / (y)
@end example

@noindent
provides the desired result.

Unintended grouping can result in another way.  Consider @samp{sizeof
ceil_div(1, 2)}.  That has the appearance of a C expression that would
compute the size of the type of @samp{ceil_div (1, 2)}, but in fact it
means something very different.  Here is what it expands to:

@example
sizeof ((1) + (2) - 1) / (2)
@end example

@noindent
This would take the size of an integer and divide it by two.  The
precedence rules have put the division outside the @samp{sizeof} when it
was intended to be inside.

Parentheses around the entire macro definition can prevent such
problems.  Here, then, is the recommended way to define @samp{ceil_div}:

@example
#define ceil_div(x, y) (((x) + (y) - 1) / (y))
@end example

@node Swallow Semicolon, Side Effects, Macro Parentheses, Macro Pitfalls
@subsubsection Swallowing the Semicolon

@cindex semicolons (after macro calls)
Often it is desirable to define a macro that expands into a compound
statement.  Consider, for example, the following macro, that advances a
pointer (the argument @samp{p} says where to find it) across whitespace
characters:

@example
#define SKIP_SPACES(p, limit)  \
@{ register char *lim = (limit); \
  while (p != lim) @{            \
    if (*p++ != ' ') @{          \
      p--; break; @}@}@}
@end example

@noindent
Here backslash-newline is used to split the macro definition, which must
be a single logical line, so that it resembles the way such C code would
be laid out if not part of a macro definition.

A call to this macro might be @samp{SKIP_SPACES (p, lim)}.  Strictly
speaking, the call expands to a compound statement, which is a complete
statement with no need for a semicolon to end it.  However, since it
looks like a function call, it minimizes confusion if you can use it
like a function call, writing a semicolon afterward, as in
@samp{SKIP_SPACES (p, lim);}

This can cause trouble before @samp{else} statements, because the
semicolon is actually a null statement.  Suppose you write

@example
if (*p != 0)
  SKIP_SPACES (p, lim);
else @dots{}
@end example

@noindent
The presence of two statements --- the compound statement and a null
statement --- in between the @samp{if} condition and the @samp{else}
makes invalid C code.

The definition of the macro @samp{SKIP_SPACES} can be altered to solve
this problem, using a @samp{do @dots{} while} statement.  Here is how:

@example
#define SKIP_SPACES(p, limit)     \
do @{ register char *lim = (limit); \
     while (p != lim) @{            \
       if (*p++ != ' ') @{          \
         p--; break; @}@}@}           \
while (0)
@end example

Now @samp{SKIP_SPACES (p, lim);} expands into

@example
do @{@dots{}@} while (0);
@end example

@noindent
which is one statement.

@node Side Effects, Self-Reference, Swallow Semicolon, Macro Pitfalls
@subsubsection Duplication of Side Effects

@cindex side effects (in macro arguments)
@cindex unsafe macros
Many C programs define a macro @samp{min}, for ``minimum'', like this:

@example
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
@end example

When you use this macro with an argument containing a side effect,
as shown here,

@example
next = min (x + y, foo (z));
@end example

@noindent
it expands as follows:

@example
next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
@end example

@noindent
where @samp{x + y} has been substituted for @samp{X} and @samp{foo (z)}
for @samp{Y}.

The function @samp{foo} is used only once in the statement as it appears
in the program, but the expression @samp{foo (z)} has been substituted
twice into the macro expansion.  As a result, @samp{foo} might be called
two times when the statement is executed.  If it has side effects or if
it takes a long time to compute, the results might not be what you
intended.  We say that @samp{min} is an @dfn{unsafe} macro.

The best solution to this problem is to define @samp{min} in a way that
computes the value of @samp{foo (z)} only once.  The C language offers
no standard way to do this, but it can be done with GNU C extensions as
follows:

@example
#define min(X, Y)                     \
(@{ typeof (X) __x = (X), __y = (Y);   \
   (__x < __y) ? __x : __y; @})
@end example

If you do not wish to use GNU C extensions, the only solution is to be
careful when @emph{using} the macro @samp{min}.  For example, you can
calculate the value of @samp{foo (z)}, save it in a variable, and use
that variable in @samp{min}:

@example
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
@dots{}
@{
  int tem = foo (z);
  next = min (x + y, tem);
@}
@end example

@noindent
(where we assume that @samp{foo} returns type @samp{int}).

@node Self-Reference, Argument Prescan, Side Effects, Macro Pitfalls
@subsubsection Self-Referential Macros

@cindex self-reference
A @dfn{self-referential} macro is one whose name appears in its
definition.  A special feature of ISO Standard C is that the
self-reference is not considered a macro call.  It is passed into the
preprocessor output unchanged.

Let's consider an example:

@example
#define foo (4 + foo)
@end example

@noindent
where @samp{foo} is also a variable in your program.

Following the ordinary rules, each reference to @samp{foo} will expand
into @samp{(4 + foo)}; then this will be rescanned and will expand into
@samp{(4 + (4 + foo))}; and so on until it causes a fatal error (memory
full) in the preprocessor.

However, the special rule about self-reference cuts this process short
after one step, at @samp{(4 + foo)}.  Therefore, this macro definition
has the possibly useful effect of causing the program to add 4 to the
value of @samp{foo} wherever @samp{foo} is referred to.

In most cases, it is a bad idea to take advantage of this feature.  A
person reading the program who sees that @samp{foo} is a variable will
not expect that it is a macro as well.  The reader will come across the
identifier @samp{foo} in the program and think its value should be that
of the variable @samp{foo}, whereas in fact the value is four greater.

The special rule for self-reference applies also to @dfn{indirect}
self-reference.  This is the case where a macro @var{x} expands to use a
macro @samp{y}, and the expansion of @samp{y} refers to the macro
@samp{x}.  The resulting reference to @samp{x} comes indirectly from the
expansion of @samp{x}, so it is a self-reference and is not further
expanded.  Thus, after

@example
#define x (4 + y)
#define y (2 * x)
@end example

@noindent
@samp{x} would expand into @samp{(4 + (2 * x))}.  Clear?

Suppose @samp{y} is used elsewhere, not from the definition of @samp{x}.
Then the use of @samp{x} in the expansion of @samp{y} is not a
self-reference because @samp{x} is not ``in progress''.  So it does
expand.  However, the expansion of @samp{x} contains a reference to
@samp{y}, and that is an indirect self-reference now because @samp{y} is
``in progress''.  The result is that @samp{y} expands to @samp{(2 * (4 +
y))}.

This behavior is specified by the ISO C standard, so you may need to
understand it.

@node Argument Prescan, Cascaded Macros, Self-Reference, Macro Pitfalls
@subsubsection Separate Expansion of Macro Arguments
@cindex expansion of arguments
@cindex macro argument expansion
@cindex prescan of macro arguments

We have explained that the expansion of a macro, including the substituted
arguments, is re-scanned for macro calls to be expanded.

What really happens is more subtle: first each argument is scanned
separately for macro calls.  Then the resulting tokens are substituted
into the macro body to produce the macro expansion, and the macro
expansion is scanned again for macros to expand.

The result is that the arguments are scanned @emph{twice} to expand
macro calls in them.

Most of the time, this has no effect.  If the argument contained any
macro calls, they are expanded during the first scan.  The result
therefore contains no macro calls, so the second scan does not change
it.  If the argument were substituted as given, with no prescan, the
single remaining scan would find the same macro calls and produce the
same results.

You might expect the double scan to change the results when a
self-referential macro is used in an argument of another macro
(@pxref{Self-Reference}): the self-referential macro would be expanded
once in the first scan, and a second time in the second scan.  However,
this is not what happens.  The self-references that do not expand in the
first scan are marked so that they will not expand in the second scan
either.

The prescan is not done when an argument is stringified or concatenated.
Thus,

@example
#define str(s) #s
#define foo 4
str (foo)
@end example

@noindent
expands to @samp{"foo"}.  Once more, prescan has been prevented from
having any noticeable effect.

More precisely, stringification and concatenation use the argument
tokens as given without initially scanning for macros.  The same
argument would be used in expanded form if it is substituted elsewhere
without stringification or concatenation.

@example
#define str(s) #s lose(s)
#define foo 4
str (foo)
@end example

expands to @samp{"foo" lose(4)}.

You might now ask, ``Why mention the prescan, if it makes no difference?
And why not skip it and make the preprocessor faster?''  The answer is
that the prescan does make a difference in three special cases:

@itemize @bullet
@item
Nested calls to a macro.

@item
Macros that call other macros that stringify or concatenate.

@item
Macros whose expansions contain unshielded commas.
@end itemize

We say that @dfn{nested} calls to a macro occur when a macro's argument
contains a call to that very macro.  For example, if @samp{f} is a macro
that expects one argument, @samp{f (f (1))} is a nested pair of calls to
@samp{f}.  The desired expansion is made by expanding @samp{f (1)} and
substituting that into the definition of @samp{f}.  The prescan causes
the expected result to happen.  Without the prescan, @samp{f (1)} itself
would be substituted as an argument, and the inner use of @samp{f} would
appear during the main scan as an indirect self-reference and would not
be expanded.  Here, the prescan cancels an undesirable side effect (in
the medical, not computational, sense of the term) of the special rule
for self-referential macros.

Prescan causes trouble in certain other cases of nested macro calls.
Here is an example:

@example
#define foo  a,b
#define bar(x) lose(x)
#define lose(x) (1 + (x))

bar(foo)
@end example

@noindent
We would like @samp{bar(foo)} to turn into @samp{(1 + (foo))}, which
would then turn into @samp{(1 + (a,b))}.  Instead, @samp{bar(foo)}
expands into @samp{lose(a,b)}, and you get an error because @code{lose}
requires a single argument.  In this case, the problem is easily solved
by the same parentheses that ought to be used to prevent misnesting of
arithmetic operations:

@example
#define foo (a,b)
#define bar(x) lose((x))
@end example

The problem is more serious when the operands of the macro are not
expressions; for example, when they are statements.  Then parentheses
are unacceptable because they would make for invalid C code:

@example
#define foo @{ int a, b; @dots{} @}
@end example

@noindent
In GNU C you can shield the commas using the @samp{(@{@dots{}@})}
construct which turns a compound statement into an expression:

@example
#define foo (@{ int a, b; @dots{} @})
@end example

Or you can rewrite the macro definition to avoid such commas:

@example
#define foo @{ int a; int b; @dots{} @}
@end example

There is also one case where prescan is useful.  It is possible to use
prescan to expand an argument and then stringify it --- if you use two
levels of macros.  Let's add a new macro @samp{xstr} to the example
shown above:

@example
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
xstr (foo)
@end example

This expands into @samp{"4"}, not @samp{"foo"}.  The reason for the
difference is that the argument of @samp{xstr} is expanded at prescan
(because @samp{xstr} does not specify stringification or concatenation
of the argument).  The result of prescan then forms the argument for
@samp{str}.  @samp{str} uses its argument without prescan because it
performs stringification; but it cannot prevent or undo the prescanning
already done by @samp{xstr}.

@node Cascaded Macros, Newlines in Args, Argument Prescan, Macro Pitfalls
@subsubsection Cascaded Use of Macros

@cindex cascaded macros
@cindex macro body uses macro
A @dfn{cascade} of macros is when one macro's body contains a reference
to another macro.  This is very common practice.  For example,

@example
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
@end example

This is not at all the same as defining @samp{TABLESIZE} to be
@samp{1020}.  The @samp{#define} for @samp{TABLESIZE} uses exactly the
body you specify --- in this case, @samp{BUFSIZE} --- and does not check
to see whether it too is the name of a macro.

It's only when you @emph{use} @samp{TABLESIZE} that the result of its
expansion is checked for more macro names.

This makes a difference if you change the definition of @samp{BUFSIZE}
at some point in the source file.  @samp{TABLESIZE}, defined as shown,
will always expand using the definition of @samp{BUFSIZE} that is
currently in effect:

@example
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
#undef BUFSIZE
#define BUFSIZE 37
@end example

@noindent
Now @samp{TABLESIZE} expands (in two stages) to @samp{37}.  (The
@samp{#undef} is to prevent any warning about the nontrivial
redefinition of @code{BUFSIZE}.)

@node Newlines in Args,, Cascaded Macros, Macro Pitfalls
@subsection Newlines in Macro Arguments
@cindex newlines in macro arguments

The invocation of a function-like macro can extend over many logical
lines.  The ISO C standard requires that newlines within a macro
invocation be treated as ordinary whitespace.  This means that when the
expansion of a function-like macro replaces its invocation, it appears
on the same line as the macro name did.  Thus line numbers emitted by
the compiler or debugger refer to the line the invocation started on,
which might be different to the line containing the argument causing the
problem.

Here is an example illustrating this:

@example
#define ignore_second_arg(a,b,c) a; c

ignore_second_arg (foo (),
                   ignored (),
                   syntax error);
@end example

@noindent
The syntax error triggered by the tokens @samp{syntax error} results in
an error message citing line three --- the line of ignore_second_arg ---
even though the problematic code comes from line five.

@node Conditionals, Assertions, Macros, Top
@section Conditionals

@cindex conditionals
In a macro processor, a @dfn{conditional} is a directive that allows a
part of the program to be ignored during compilation, on some
conditions.  In the C preprocessor, a conditional can test either an
arithmetic expression or whether a name is defined as a macro.

A conditional in the C preprocessor resembles in some ways an @samp{if}
statement in C, but it is important to understand the difference between
them.  The condition in an @samp{if} statement is tested during the
execution of your program.  Its purpose is to allow your program to
behave differently from run to run, depending on the data it is
operating on.  The condition in a preprocessing conditional directive is
tested when your program is compiled.  Its purpose is to allow different
code to be included in the program depending on the situation at the
time of compilation.

@menu
* Uses: Conditional Uses.       What conditionals are for.
* Syntax: Conditional Syntax.   How conditionals are written.
* Deletion: Deleted Code.       Making code into a comment.
* Macros: Conditionals-Macros.  Why conditionals are used with macros.
* Errors: #error Directive.     Detecting inconsistent compilation parameters.
@end menu

@node Conditional Uses
@subsection Why Conditionals are Used

Generally there are three kinds of reason to use a conditional.

@itemize @bullet
@item
A program may need to use different code depending on the machine or
operating system it is to run on.  In some cases the code for one
operating system may be erroneous on another operating system; for
example, it might refer to library routines that do not exist on the
other system.  When this happens, it is not enough to avoid executing
the invalid code: merely having it in the program makes it impossible to
link the program and run it.  With a preprocessing conditional, the
offending code can be effectively excised from the program when it is
not valid.

@item
You may want to be able to compile the same source file into two
different programs.  Sometimes the difference between the programs is
that one makes frequent time-consuming consistency checks on its
intermediate data, or prints the values of those data for debugging,
while the other does not.

@item
A conditional whose condition is always false is a good way to exclude
code from the program but keep it as a sort of comment for future
reference.
@end itemize

Most simple programs that are intended to run on only one machine will
not need to use preprocessing conditionals.

@node Conditional Syntax
@subsection Syntax of Conditionals

@findex #if
A conditional in the C preprocessor begins with a @dfn{conditional
directive}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}.
@xref{Conditionals-Macros}, for information on @samp{#ifdef} and
@samp{#ifndef}; only @samp{#if} is explained here.

@menu
* If: #if Directive.     Basic conditionals using @samp{#if} and @samp{#endif}.
* Else: #else Directive. Including some text if the condition fails.
* Elif: #elif Directive. Testing several alternative possibilities.
@end menu

@node #if Directive
@subsubsection The @samp{#if} Directive

The @samp{#if} directive in its simplest form consists of

@example
#if @var{expression}
@var{controlled text}
#endif /* @var{expression} */
@end example

The comment following the @samp{#endif} is not required, but it is a
good practice because it helps people match the @samp{#endif} to the
corresponding @samp{#if}.  Such comments should always be used, except
in short conditionals that are not nested.  In fact, you can put
anything at all after the @samp{#endif} and it will be ignored by the
GNU C preprocessor, but only comments are acceptable in ISO Standard C@.

@var{expression} is a C expression of integer type, subject to stringent
restrictions.  It may contain

@itemize @bullet
@item
Integer constants, which are all regarded as @code{long} or
@code{unsigned long}.

@item
Character constants, which are interpreted according to the character
set and conventions of the machine and operating system on which the
preprocessor is running.  The GNU C preprocessor uses the C data type
@samp{char} for these character constants; therefore, whether some
character codes are negative is determined by the C compiler used to
compile the preprocessor.  If it treats @samp{char} as signed, then
character codes large enough to set the sign bit will be considered
negative; otherwise, no character code is considered negative.

@item
Arithmetic operators for addition, subtraction, multiplication,
division, bitwise operations, shifts, comparisons, and logical
operations (@samp{&&} and @samp{||}).  The latter two obey the usual
short-circuiting rules of standard C.

@item
Identifiers that are not macros, which are all treated as zero(!).

@item
Macro calls.  All macro calls in the expression are expanded before
actual computation of the expression's value begins.
@end itemize

Note that @samp{sizeof} operators and @code{enum}-type values are not
allowed.  @code{enum}-type values, like all other identifiers that are
not taken as macro calls and expanded, are treated as zero.

The @var{controlled text} inside of a conditional can include
preprocessing directives.  Then the directives inside the conditional
are obeyed only if that branch of the conditional succeeds.  The text
can also contain other conditional groups.  However, the @samp{#if} and
@samp{#endif} directives must balance.

@node #else Directive
@subsubsection The @samp{#else} Directive

@findex #else
The @samp{#else} directive can be added to a conditional to provide
alternative text to be used if the condition is false.  This is what
it looks like:

@example
#if @var{expression}
@var{text-if-true}
#else /* Not @var{expression} */
@var{text-if-false}
#endif /* Not @var{expression} */
@end example

If @var{expression} is nonzero, and thus the @var{text-if-true} is 
active, then @samp{#else} acts like a failing conditional and the
@var{text-if-false} is ignored.  Conversely, if the @samp{#if}
conditional fails, the @var{text-if-false} is considered included.

@node #elif Directive
@subsubsection The @samp{#elif} Directive

@findex #elif
One common case of nested conditionals is used to check for more than two
possible alternatives.  For example, you might have

@example
#if X == 1
@dots{}
#else /* X != 1 */
#if X == 2
@dots{}
#else /* X != 2 */
@dots{}
#endif /* X != 2 */
#endif /* X != 1 */
@end example

Another conditional directive, @samp{#elif}, allows this to be
abbreviated as follows:

@example
#if X == 1
@dots{}
#elif X == 2
@dots{}
#else /* X != 2 and X != 1*/
@dots{}
#endif /* X != 2 and X != 1*/
@end example

@samp{#elif} stands for ``else if''.  Like @samp{#else}, it goes in the
middle of a @samp{#if}-@samp{#endif} pair and subdivides it; it does not
require a matching @samp{#endif} of its own.  Like @samp{#if}, the
@samp{#elif} directive includes an expression to be tested.

The text following the @samp{#elif} is processed only if the original
@samp{#if}-condition failed and the @samp{#elif} condition succeeds.
More than one @samp{#elif} can go in the same @samp{#if}-@samp{#endif}
group.  Then the text after each @samp{#elif} is processed only if the
@samp{#elif} condition succeeds after the original @samp{#if} and any
previous @samp{#elif} directives within it have failed.  @samp{#else} is
equivalent to @samp{#elif 1}, and @samp{#else} is allowed after any
number of @samp{#elif} directives, but @samp{#elif} may not follow
@samp{#else}.

@node Deleted Code
@subsection Keeping Deleted Code for Future Reference
@cindex commenting out code

If you replace or delete a part of the program but want to keep the old
code around as a comment for future reference, the easy way to do this
is to put @samp{#if 0} before it and @samp{#endif} after it.  This is
better than using comment delimiters @samp{/*} and @samp{*/} since those
won't work if the code already contains comments (C comments do not
nest).

This works even if the code being turned off contains conditionals, but
they must be entire conditionals (balanced @samp{#if} and @samp{#endif}).

Conversely, do not use @samp{#if 0} for comments which are not C code.
Use the comment delimiters @samp{/*} and @samp{*/} instead.  The
interior of @samp{#if 0} must consist of complete tokens; in particular,
single-quote characters must balance.  Comments often contain unbalanced
single-quote characters (known in English as apostrophes).  These
confuse @samp{#if 0}.  They do not confuse @samp{/*}.

@node Conditionals-Macros
@subsection Conditionals and Macros

Conditionals are useful in connection with macros or assertions, because
those are the only ways that an expression's value can vary from one
compilation to another.  A @samp{#if} directive whose expression uses no
macros or assertions is equivalent to @samp{#if 1} or @samp{#if 0}; you
might as well determine which one, by computing the value of the
expression yourself, and then simplify the program.

For example, here is a conditional that tests the expression
@samp{BUFSIZE == 1020}, where @samp{BUFSIZE} must be a macro.

@example
#if BUFSIZE == 1020
  printf ("Large buffers!\n");
#endif /* BUFSIZE is large */
@end example

(Programmers often wish they could test the size of a variable or data
type in @samp{#if}, but this does not work.  The preprocessor does not
understand @code{sizeof}, or typedef names, or even the type keywords
such as @code{int}.)

@findex defined
The special operator @samp{defined} is used in @samp{#if} and
@samp{#elif} expressions to test whether a certain name is defined as a
macro.  Either @samp{defined @var{name}} or @samp{defined (@var{name})}
is an expression whose value is 1 if @var{name} is defined as macro at
the current point in the program, and 0 otherwise.  To the
@samp{defined} operator it makes no difference what the definition of
the macro is; all that matters is whether there is a definition.  Thus,
for example,@refill

@example
#if defined (vax) || defined (ns16000)
@end example

@noindent
would succeed if either of the names @samp{vax} and @samp{ns16000} is
defined as a macro.  You can test the same condition using assertions
(@pxref{Assertions}), like this:

@example
#if #cpu (vax) || #cpu (ns16000)
@end example

If a macro is defined and later undefined with @samp{#undef}, subsequent
use of the @samp{defined} operator returns 0, because the name is no
longer defined.  If the macro is defined again with another
@samp{#define}, @samp{defined} will recommence returning 1.

If the @samp{defined} operator appears as a result of a macro expansion,
the C standard says the behavior is undefined.  GNU cpp treats it as a
genuine @samp{defined} operator and evaluates it normally.  It will warn
wherever your code uses this feature if you use the command-line option
@samp{-pedantic}, since other compilers may handle it differently.

@findex #ifdef
@findex #ifndef
Conditionals that test whether a single macro is defined are very common,
so there are two special short conditional directives for this case.

@table @code
@item #ifdef @var{name}
is equivalent to @samp{#if defined (@var{name})}.

@item #ifndef @var{name}
is equivalent to @samp{#if ! defined (@var{name})}.
@end table

Macro definitions can vary between compilations for several reasons.

@itemize @bullet
@item
Some macros are predefined on each kind of machine.  For example, on a
Vax, the name @samp{vax} is a predefined macro.  On other machines, it
would not be defined.

@item
Many more macros are defined by system header files.  Different systems
and machines define different macros, or give them different values.  It
is useful to test these macros with conditionals to avoid using a system
feature on a machine where it is not implemented.

@item
Macros are a common way of allowing users to customize a program for
different machines or applications.  For example, the macro
@samp{BUFSIZE} might be defined in a configuration file for your program
that is included as a header file in each source file.  You would use
@samp{BUFSIZE} in a preprocessing conditional in order to generate
different code depending on the chosen configuration.

@item
Macros can be defined or undefined with @samp{-D} and @samp{-U} command
options when you compile the program.  You can arrange to compile the
same source file into two different programs by choosing a macro name to
specify which program you want, writing conditionals to test whether or
how this macro is defined, and then controlling the state of the macro
with compiler command options.  @xref{Invocation}.
@end itemize

@ifinfo
Assertions are usually predefined, but can be defined with preprocessor
directives or command-line options.
@end ifinfo

@node #error Directive
@subsection The @samp{#error} and @samp{#warning} Directives

@findex #error
The directive @samp{#error} causes the preprocessor to report a fatal
error.  The tokens forming the rest of the line following @samp{#error}
are used as the error message, and not macro-expanded.  Internal
whitespace sequences are each replaced with a single space.  The line
must consist of complete tokens.

You would use @samp{#error} inside of a conditional that detects a
combination of parameters which you know the program does not properly
support.  For example, if you know that the program will not run
properly on a Vax, you might write

@smallexample
@group
#ifdef __vax__
#error "Won't work on Vaxen.  See comments at get_last_object."
#endif
@end group
@end smallexample

@noindent
@xref{Nonstandard Predefined}, for why this works.

If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with @samp{#error}.  For example,

@smallexample
#if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
    || HASH_TABLE_SIZE % 5 == 0
#error HASH_TABLE_SIZE should not be divisible by a small prime
#endif
@end smallexample

@findex #warning
The directive @samp{#warning} is like the directive @samp{#error}, but
causes the preprocessor to issue a warning and continue preprocessing.
The tokens following @samp{#warning} are used as the warning message,
and not macro-expanded.

You might use @samp{#warning} in obsolete header files, with a message
directing the user to the header file which should be used instead.

@node Assertions, Line Control, Conditionals, Top
@section Assertions
@cindex assertions
@dfn{Assertions} are a more systematic alternative to macros in writing
conditionals to test what sort of computer or system the compiled
program will run on.  Assertions are usually predefined, but you can
define them with preprocessing directives or command-line options.

@cindex predicates
The macros traditionally used to describe the type of target are not
classified in any way according to which question they answer; they may
indicate a hardware architecture, a particular hardware model, an
operating system, a particular version of an operating system, or
specific configuration options.  These are jumbled together in a single
namespace.  In contrast, each assertion consists of a named question and
an answer.  The question is usually called the @dfn{predicate}.  An
assertion looks like this:

@example
#@var{predicate} (@var{answer})
@end example

@noindent
You must use a properly formed identifier for @var{predicate}.  The
value of @var{answer} can be any sequence of words; all characters are
significant except for leading and trailing whitespace, and differences
in internal whitespace sequences are ignored.  (This is similar to the
rules governing macro redefinition.)  Thus, @samp{x + y} is different
from @samp{x+y} but equivalent to @samp{ x + y }.  @samp{)} is not
allowed in an answer.

@cindex testing predicates
Here is a conditional to test whether the answer @var{answer} is asserted
for the predicate @var{predicate}:

@example
#if #@var{predicate} (@var{answer})
@end example

@noindent
There may be more than one answer asserted for a given predicate.  If
you omit the answer, you can test whether @emph{any} answer is asserted
for @var{predicate}:

@example
#if #@var{predicate}
@end example

@findex #system
@findex #machine
@findex #cpu
Most of the time, the assertions you test will be predefined assertions.
GNU C provides three predefined predicates: @code{system}, @code{cpu},
and @code{machine}.  @code{system} is for assertions about the type of
software, @code{cpu} describes the type of computer architecture, and
@code{machine} gives more information about the computer.  For example,
on a GNU system, the following assertions would be true:

@example
#system (gnu)
#system (mach)
#system (mach 3)
#system (mach 3.@var{subversion})
#system (hurd)
#system (hurd @var{version})
@end example

@noindent
and perhaps others.  The alternatives with
more or less version information let you ask more or less detailed
questions about the type of system software.

On a Unix system, you would find @code{#system (unix)} and perhaps one of:
@code{#system (aix)}, @code{#system (bsd)}, @code{#system (hpux)},
@code{#system (lynx)}, @code{#system (mach)}, @code{#system (posix)},
@code{#system (svr3)}, @code{#system (svr4)}, or @code{#system (xpg4)}
with possible version numbers following.

Other values for @code{system} are @code{#system (mvs)}
and @code{#system (vms)}.

@strong{Portability note:} Many Unix C compilers provide only one answer
for the @code{system} assertion: @code{#system (unix)}, if they support
assertions at all.  This is less than useful.

An assertion with a multi-word answer is completely different from several
assertions with individual single-word answers.  For example, the presence
of @code{system (mach 3.0)} does not mean that @code{system (3.0)} is true.
It also does not directly imply @code{system (mach)}, but in GNU C, that
last will normally be asserted as well.

The current list of possible assertion values for @code{cpu} is:
@code{#cpu (a29k)}, @code{#cpu (alpha)}, @code{#cpu (arm)}, @code{#cpu
(clipper)}, @code{#cpu (convex)}, @code{#cpu (elxsi)}, @code{#cpu
(tron)}, @code{#cpu (h8300)}, @code{#cpu (i370)}, @code{#cpu (i386)},
@code{#cpu (i860)}, @code{#cpu (i960)}, @code{#cpu (m68k)}, @code{#cpu
(m88k)}, @code{#cpu (mips)}, @code{#cpu (ns32k)}, @code{#cpu (hppa)},
@code{#cpu (pyr)}, @code{#cpu (ibm032)}, @code{#cpu (rs6000)},
@code{#cpu (sh)}, @code{#cpu (sparc)}, @code{#cpu (spur)}, @code{#cpu
(tahoe)}, @code{#cpu (vax)}, @code{#cpu (we32000)}.

@findex #assert
You can create assertions within a C program using @samp{#assert}, like
this:

@example
#assert @var{predicate} (@var{answer})
@end example

@noindent
(Note the absence of a @samp{#} before @var{predicate}.)

@cindex unassert
@cindex assertions, undoing
@cindex retracting assertions
@findex #unassert
Each time you do this, you assert a new true answer for @var{predicate}.
Asserting one answer does not invalidate previously asserted answers;
they all remain true.  The only way to remove an answer is with
@samp{#unassert}.  @samp{#unassert} has the same syntax as
@samp{#assert}.  You can also remove all answers to a @var{predicate}
like this:

@example
#unassert @var{predicate}
@end example

You can also add or cancel assertions using command options
when you run @code{gcc} or @code{cpp}.  @xref{Invocation}.

@node Line Control, Other Directives, Assertions, Top
@section Combining Source Files

@cindex line control
One of the jobs of the C preprocessor is to inform the C compiler of where
each line of C code came from: which source file and which line number.

C code can come from multiple source files if you use @samp{#include};
both @samp{#include} and the use of conditionals and macros can cause
the line number of a line in the preprocessor output to be different
from the line's number in the original source file.  You will appreciate
the value of making both the C compiler (in error messages) and symbolic
debuggers such as GDB use the line numbers in your source file.

The C preprocessor builds on this feature by offering a directive by
which you can control the feature explicitly.  This is useful when a
file for input to the C preprocessor is the output from another program
such as the @code{bison} parser generator, which operates on another
file that is the true source file.  Parts of the output from
@code{bison} are generated from scratch, other parts come from a
standard parser file.  The rest are copied nearly verbatim from the
source file, but their line numbers in the @code{bison} output are not
the same as their original line numbers.  Naturally you would like
compiler error messages and symbolic debuggers to know the original
source file and line number of each line in the @code{bison} input.

@findex #line
@code{bison} arranges this by writing @samp{#line} directives into the output
file.  @samp{#line} is a directive that specifies the original line number
and source file name for subsequent input in the current preprocessor input
file.  @samp{#line} has three variants:

@table @code
@item #line @var{linenum}
Here @var{linenum} is a decimal integer constant.  This specifies that
the line number of the following line of input, in its original source file,
was @var{linenum}.

@item #line @var{linenum} @var{filename}
Here @var{linenum} is a decimal integer constant and @var{filename} is a
string constant.  This specifies that the following line of input came
originally from source file @var{filename} and its line number there was
@var{linenum}.  Keep in mind that @var{filename} is not just a file
name; it is surrounded by double-quote characters so that it looks like
a string constant.

@item #line @var{anything else}
@var{anything else} is checked for macro calls, which are expanded.
The result should be a decimal integer constant followed optionally
by a string constant, as described above.
@end table

@samp{#line} directives alter the results of the @samp{__FILE__} and
@samp{__LINE__} predefined macros from that point on.  @xref{Standard
Predefined}.

The output of the preprocessor (which is the input for the rest of the
compiler) contains directives that look much like @samp{#line}
directives.  They start with just @samp{#} instead of @samp{#line}, but
this is followed by a line number and file name as in @samp{#line}.
@xref{Output}.

@node Other Directives, Output, Line Control, Top
@section Miscellaneous Preprocessing Directives

This section describes some additional, rarely used, preprocessing
directives.

@findex #pragma
@findex #pragma GCC

The ISO standard specifies that the effect of the @samp{#pragma}
directive is implementation-defined.  The GNU C preprocessor recognizes
some pragmas, and passes unrecognized ones through to the preprocessor
output, so they are available to the compilation pass.

In line with the C99 standard, which introduces a STDC namespace for C99
pragmas, the preprocessor introduces a GCC namespace for GCC pragmas.
Supported GCC preprocessor pragmas are of the form @samp{#pragma GCC
...}.  For backwards compatibility previously supported pragmas are also
recognized without the @samp{GCC} prefix, however that use is
deprecated.  Pragmas that are already deprecated are not recognized with
a @samp{GCC} prefix.

@findex #pragma GCC dependency
The @samp{#pragma GCC dependency} allows you to check the relative dates
of the current file and another file. If the other file is more recent
than the current file, a warning is issued. This is useful if the
include file is derived from the other file, and should be regenerated.
The other file is searched for using the normal include search path.
Optional trailing text can be used to give more information in the
warning message.

@smallexample
#pragma GCC dependency "parse.y"
#pragma GCC dependency "/usr/include/time.h" rerun /path/to/fixincludes
@end smallexample

@findex _Pragma
The C99 standard also introduces the @samp{_Pragma} operator.  The
syntax is @code{_Pragma (string-literal)}, where @samp{string-literal}
can be either a normal or wide-character string literal.  It is
destringized, by replacing all @samp{\\} with a single @samp{\} and all
@samp{\"} with a @samp{"}.  The result is then processed as if it had
appeared as the right hand side of a @samp{#pragma} directive.  For
example,

@smallexample
_Pragma ("GCC dependency \"parse.y\"")
@end smallexample

@noindent has the same effect as @samp{#pragma GCC dependency
"parse.y"}.  The same effect could be achieved using macros, for example

@smallexample
#define DO_PRAGMA(x) _Pragma (#x)
DO_PRAGMA (GCC dependency "parse.y")
@end smallexample

The standard is unclear on where a @samp{_Pragma} operator can appear.
The preprocessor accepts it even within a preprocessing conditional
directive like @samp{#if}.  To be safe, you are probably best keeping it
out of directives other than @samp{#define}, and putting it on a line of
its own.

@findex #ident
The @samp{#ident} directive is supported for compatibility with certain
other systems.  It is followed by a line of text.  On some systems, the
text is copied into a special place in the object file; on most systems,
the text is ignored and this directive has no effect.  Typically
@samp{#ident} is only used in header files supplied with those systems
where it is meaningful.

@cindex null directive
The @dfn{null directive} consists of a @samp{#} followed by a newline,
with only whitespace (including comments) in between.  A null directive
is understood as a preprocessing directive but has no effect on the
preprocessor output.  The primary significance of the existence of the
null directive is that an input line consisting of just a @samp{#} will
produce no output, rather than a line of output containing just a
@samp{#}.  Supposedly some old C programs contain such lines.

@node Output, Implementation, Other Directives, Top
@section C Preprocessor Output

@cindex output format
The output from the C preprocessor looks much like the input, except
that all preprocessing directive lines have been replaced with blank
lines and all comments with spaces.

The ISO standard specifies that it is implementation defined whether a
preprocessor preserves whitespace between tokens, or replaces it with
e.g. a single space.  In the GNU C preprocessor, whitespace between
tokens is collapsed to become a single space, with the exception that
the first token on a non-directive line is preceded with sufficient
spaces that it appears in the same column in the preprocessed output
that it appeared in in the original source file.  This is so the output
is easy to read.  @xref{Unreliable Features}.

Source file name and line number information is conveyed by lines
of the form

@example
# @var{linenum} @var{filename} @var{flags}
@end example

@noindent
which are inserted as needed into the output (but never within a string
or character constant), and in place of long sequences of empty lines.
Such a line means that the following line originated in file
@var{filename} at line @var{linenum}.

After the file name comes zero or more flags, which are @samp{1},
@samp{2}, @samp{3}, or @samp{4}.  If there are multiple flags, spaces
separate them.  Here is what the flags mean:

@table @samp
@item 1
This indicates the start of a new file.
@item 2
This indicates returning to a file (after having included another file).
@item 3
This indicates that the following text comes from a system header file,
so certain warnings should be suppressed.
@item 4
This indicates that the following text should be treated as C@.
@c maybe cross reference NO_IMPLICIT_EXTERN_C
@end table

@node Implementation, Unreliable Features, Output, Top
@section Implementation-defined Behavior and Implementation Limits
@cindex implementation limits
@cindex implementation-defined behavior

The ISO C standard mandates that implementations document various
aspects of preprocessor behavior.  You should try to avoid undue
reliance on behaviour described here, as it is possible that it will
change subtly in future implementations.

@itemize @bullet

@item The mapping of physical source file multi-byte characters to the
execution character set.

Currently, GNU cpp only supports character sets that are strict supersets
of ASCII, and performs no translation of characters.

@item Non-empty sequences of whitespace characters.

Each whitespace sequence is not preserved, but collapsed to a single
space.  For aesthetic reasons, the first token on each non-directive
line of output is preceded with sufficient spaces that it appears in the
same column as it did in the original source file.

@item The numeric value of character constants in preprocessor expressions.

The preprocessor interprets character constants in preprocessing
directives on the host machine.  Expressions outside preprocessing
directives are compiled to be interpreted on the target machine.  In the
normal case of a native compiler, these two environments are the same
and so character constants will be evaluated identically in both cases.
However, in the case of a cross compiler, the values may be different.

Multi-character character constants are interpreted a character at a
time, shifting the previous result left by the number of bits per
character on the host, and adding the new character.  For example, 'ab'
on an 8-bit host would be interpreted as 'a' * 256 + 'b'.  If there are
more characters in the constant than can fit in the widest native
integer type on the host, usually a @samp{long}, the behavior is
undefined.

Evaluation of wide character constants is not properly implemented yet.

@item Source file inclusion.

For a discussion on how the preprocessor locates header files,
@pxref{Include Operation}.

@item Interpretation of the filename resulting from a macro-expanded
@samp{#include} directive.

If the macro expands to a string literal, the @samp{#include} directive
is processed as if the string had been specified directly.  Otherwise,
the macro must expand to a token stream beginning with a @samp{<} token
and including a @samp{>} token.  In this case, the tokens between the
@samp{<} and the first @samp{>} are combined to form the filename to be
included.  Any whitespace between tokens is reduced to a single space;
then any space after the initial @samp{<} is retained, but a trailing
space before the closing @samp{>} is ignored.

In either case, if any excess tokens remain, an error occurs and the
directive is not processed.

@item Treatment of a @samp{#pragma} directive that after macro-expansion
results in a standard pragma.

The pragma is processed as if it were a normal standard pragma.

@end itemize

The following documents internal limits of GNU cpp.

@itemize @bullet

@item Nesting levels of @samp{#include} files.

We impose an arbitrary limit of 200 levels, to avoid runaway recursion.
The standard requires at least 15 levels.

@item Nesting levels of conditional inclusion.

The C standard mandates this be at least 63.  The GNU C preprocessor
is limited only by available memory.

@item Levels of parenthesised expressions within a full expression.

The C standard requires this to be at least 63.  In preprocessor
conditional expressions it is limited only by available memory.

@item Significant initial characters in an identifier or macro name.

The preprocessor treats all characters as significant.  The C standard
requires only that the first 63 be significant.

@item Number of macros simultaneously defined in a single translation unit.

The standard requires at least 4095 be possible; GNU cpp is limited only
by available memory.

@item Number of parameters in a macro definition and arguments in a macro call.

We allow USHRT_MAX, which is normally 65,535, and above the minimum of
127 required by the standard.

@item Number of characters on a logical source line.

The C standard requires a minimum of 4096 be permitted.  GNU cpp places
no limits on this, but you may get incorrect column numbers reported in
diagnostics for lines longer than 65,535 characters.

@end itemize

@node Unreliable Features, Invocation, Implementation, Top
@section Undefined Behavior and Deprecated Features
@cindex undefined behavior
@cindex deprecated features

This section details GNU C preprocessor behavior that is subject to
change or deprecated.  You are @emph{strongly advised} to write your
software so it does not rely on anything described here; future versions
of the preprocessor may subtly change such behavior or even remove the
feature altogether.

Preservation of the form of whitespace between tokens is unlikely to
change from current behavior (@ref{Output}), but you are advised not
to rely on it.

The following are undocumented and subject to change:-

@itemize @bullet

@item Precedence of ## operators with respect to each other

Whether a sequence of ## operators is evaluated left-to-right,
right-to-left or indeed in a consistent direction at all is not
specified.  An example of where this might matter is pasting the
arguments @samp{1}, @samp{e} and @samp{-2}.  This would be fine for
left-to-right pasting, but right-to-left pasting would produce an
invalid token @samp{e-2}.  It is possible to guarantee precedence by
suitable use of nested macros.

@item Precedence of # operator with respect to the ## operator

Which of these two operators is evaluated first is not specified.

@end itemize

The following features are in flux and should not be used in portable
code:

@itemize @bullet

@item Optional argument when invoking rest argument macros

As an extension, GCC permits you to omit the variable arguments entirely
when you use a variable argument macro.  This works whether or not you
give the variable argument a name.  For example, the two macro
invocations in the example below expand to the same thing:

@smallexample
#define debug(format, ...) printf (format, __VA_ARGS__)
debug("string");       /* Not permitted by C standard.  */
debug("string",);      /* OK.  */
@end smallexample

This extension will be preserved, but the special behavior of @samp{##}
in this context has changed in the past and may change again in the
future.

@item ## swallowing preceding text in rest argument macros

Formerly, in a macro expansion, if @samp{##} appeared before a variable
arguments parameter, and the set of tokens specified for that argument in
the macro invocation was empty, previous versions of the GNU C
preprocessor would back up and remove the preceding sequence of
non-whitespace characters (@strong{not} the preceding token).  This
extension is in direct conflict with the 1999 C standard and has been
drastically pared back.

In the current version of the preprocessor, if @samp{##} appears between
a comma and a variable arguments parameter, and the variable argument is
omitted entirely, the comma will be removed from the expansion.  If the
variable argument is empty, or the token before @samp{##} is not a
comma, then @samp{##} behaves as a normal token paste.  

Portable code should avoid this extension at all costs.

@end itemize

The following features are deprecated and will likely be removed at some
point in the future:-

@itemize @bullet

@item Attempting to paste two tokens which together do not form a valid
preprocessing token

The preprocessor currently warns about this and outputs the two tokens
adjacently, which is probably the behavior the programmer intends.  It
may not work in future, though.

Most of the time, when you get this warning, you will find that @samp{##}
is being used superstitiously, to guard against whitespace appearing
between two tokens.  It is almost always safe to delete the @samp{##}.

@findex #pragma once
@item #pragma once

This pragma was once used to tell the preprocessor that it need not
include a file more than once.  It is now obsolete and should not be
used at all.

@item #pragma poison

This pragma has been superseded by @samp{#pragma GCC poison}.
@xref{Poisoning}.

@item Multi-line string literals

The preprocessor currently allows raw newlines in string literals.  This
extension is deprecated and will be removed in a future version of GCC.
The preprocessor already forbids such string literals in all directives
apart from #define.

Instead, make use of ISO C concatenation of adjacent string literals, or
use @samp{\n} followed by an escaped newline.

@item Preprocessing things which are not C

The C preprocessor is intended to be used only with C, C++, and
Objective C source code.  In the past, it has been abused as a general
text processor.  It will choke on input which is not lexically valid C;
for example, apostrophes will be interpreted as the beginning of
character constants, and cause errors.  Also, you cannot rely on it
preserving characteristics of the input which are not significant to
C-family languages.  For instance, if a Makefile is preprocessed, all
the hard tabs will be lost, and the Makefile will not work.

Having said that, you can often get away with using cpp on things which
are not C.  Other Algol-ish programming languages are often safe
(Pascal, Ada, ...)  and so is assembly, with caution. @samp{-traditional}
mode is much more permissive, and can safely be used with e.g. Fortran.
Many of the problems go away if you write C or C++ style comments
instead of native language comments, and if you avoid elaborate macros.

Wherever possible, you should use a preprocessor geared to the language
you are writing in.  Modern versions of the GNU assembler have macro
facilities.  Most high level programming languages have their own
conditional compilation and inclusion mechanism.  If all else fails,
try a true general text processor, such as @xref{Top, M4, , m4, GNU `m4'}.

@end itemize

@node Invocation, Concept Index, Unreliable Features, Top
@section Invoking the C Preprocessor
@cindex invocation of the preprocessor

Most often when you use the C preprocessor you will not have to invoke it
explicitly: the C compiler will do so automatically.  However, the
preprocessor is sometimes useful on its own.

@ignore
@c man begin SYNOPSIS
cpp [@samp{-P}] [@samp{-C}] [@samp{-gcc}] [@samp{-traditional}]
    [@samp{-undef}] [@samp{-trigraphs}] [@samp{-pedantic}]
    [@samp{-W}@var{warn}...] [@samp{-I}@var{dir}...]
    [@samp{-D}@var{macro}[=@var{defn}]...] [@samp{-U}@var{macro}]
    [@samp{-A}@var{predicate}(@var{answer})]
    [@samp{-M}|@samp{-MM}][@samp{-MG}][@samp{-MF}@var{filename}]
    [@samp{-MP}][@samp{-MQ}@var{target}...][@samp{-MT}@var{target}...]
    [@samp{-x} @var{language}] [@samp{-std=}@var{standard}]
    @var{infile} @var{outfile}

Only the most useful options are listed here; see below for the remainder.
@c man end
@c man begin SEEALSO
gcc(1), as(1), ld(1), and the Info entries for @file{cpp}, @file{gcc}, and
@file{binutils}.
@c man end
@end ignore

@c man begin OPTIONS
The C preprocessor expects two file names as arguments, @var{infile} and
@var{outfile}.  The preprocessor reads @var{infile} together with any
other files it specifies with @samp{#include}.  All the output generated
by the combined input files is written in @var{outfile}.

Either @var{infile} or @var{outfile} may be @samp{-}, which as
@var{infile} means to read from standard input and as @var{outfile}
means to write to standard output.  Also, if either file is omitted, it
means the same as if @samp{-} had been specified for that file.

@cindex options
Here is a table of command options accepted by the C preprocessor.
These options can also be given when compiling a C program; they are
passed along automatically to the preprocessor when it is invoked by the
compiler.

@table @samp
@item -P
@findex -P
Inhibit generation of @samp{#}-lines with line-number information in the
output from the preprocessor.  This might be useful when running the
preprocessor on something that is not C code and will be sent to a
program which might be confused by the @samp{#}-lines.  @xref{Output}.

@item -C
@findex -C
Do not discard comments.  All comments are passed through to the output
file, except for comments in processed directives, which are deleted
along with the directive.  Comments appearing in the expansion list of a
macro will be preserved, and appear in place wherever the macro is
invoked.

You should be prepared for side effects when using @samp{-C}; it causes
the preprocessor to treat comments as tokens in their own right.  For
example, macro redefinitions that were trivial when comments were
replaced by a single space might become significant when comments are
retained.  Also, comments appearing at the start of what would be a
directive line have the effect of turning that line into an ordinary
source line, since the first token on the line is no longer a @samp{#}.

@item -traditional
@findex -traditional
Try to imitate the behavior of old-fashioned C, as opposed to ISO C@.

@itemize @bullet
@item
Traditional macro expansion pays no attention to single-quote or
double-quote characters; macro argument symbols are replaced by the
argument values even when they appear within apparent string or
character constants.

@item
Traditionally, it is permissible for a macro expansion to end in the
middle of a string or character constant.  The constant continues into
the text surrounding the macro call.

@item
However, traditionally the end of the line terminates a string or
character constant, with no error.

@item
In traditional C, a comment is equivalent to no text at all.  (In ISO
C, a comment counts as whitespace.)

@item
Traditional C does not have the concept of a ``preprocessing number''.
It considers @samp{1.0e+4} to be three tokens: @samp{1.0e}, @samp{+},
and @samp{4}.

@item
A macro is not suppressed within its own definition, in traditional C@.
Thus, any macro that is used recursively inevitably causes an error.

@item
The character @samp{#} has no special meaning within a macro definition
in traditional C@.

@item
In traditional C, the text at the end of a macro expansion can run
together with the text after the macro call, to produce a single token.
(This is impossible in ISO C@.)

@item
None of the GNU extensions to the preprocessor are available in
@samp{-traditional} mode.

@end itemize

@cindex Fortran
@cindex unterminated
Use the @samp{-traditional} option when preprocessing Fortran code, so
that single-quotes and double-quotes within Fortran comment lines (which
are generally not recognized as such by the preprocessor) do not cause
diagnostics about unterminated character or string constants.

However, this option does not prevent diagnostics about unterminated
comments when a C-style comment appears to start, but not end, within
Fortran-style commentary.

So, the following Fortran comment lines are accepted with
@samp{-traditional}:

@smallexample
C This isn't an unterminated character constant
C Neither is "20000000000, an octal constant
C in some dialects of Fortran
@end smallexample

However, this type of comment line will likely produce a diagnostic, or
at least unexpected output from the preprocessor, due to the
unterminated comment:

@smallexample
C Some Fortran compilers accept /* as starting
C an inline comment.
@end smallexample

@cindex g77
Note that @code{g77} automatically supplies the @samp{-traditional}
option when it invokes the preprocessor.  However, a future version of
@code{g77} might use a different, more-Fortran-aware preprocessor in
place of @code{cpp}.

@item -trigraphs
@findex -trigraphs
Process ISO standard trigraph sequences.  These are three-character
sequences, all starting with @samp{??}, that are defined by ISO C to
stand for single characters.  For example, @samp{??/} stands for
@samp{\}, so @samp{'??/n'} is a character constant for a newline.  By
default, GCC ignores trigraphs, but in standard-conforming modes it
converts them.  See the @samp{-std} option.

The nine trigraph sequences are
@table @samp
@item ??(
-> @samp{[}

@item ??)
-> @samp{]}

@item ??<
-> @samp{@{}

@item ??>
-> @samp{@}}

@item ??=
-> @samp{#}

@item ??/
-> @samp{\}

@item ??'
-> @samp{^}

@item ??!
-> @samp{|}

@item ??-
-> @samp{~}

@end table

Trigraph support is not popular, so many compilers do not implement it
properly.  Portable code should not rely on trigraphs being either
converted or ignored.

@item -pedantic
@findex -pedantic
Issue warnings required by the ISO C standard in certain cases such
as when text other than a comment follows @samp{#else} or @samp{#endif}.

@item -pedantic-errors
@findex -pedantic-errors
Like @samp{-pedantic}, except that errors are produced rather than
warnings.

@item -Wcomment
@findex -Wcomment
@itemx -Wcomments
(Both forms have the same effect).
Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
comment, or whenever a backslash-newline appears in a @samp{//} comment.

@item -Wtrigraphs
@findex -Wtrigraphs
Warn if any trigraphs are encountered.  This option used to take effect
only if @samp{-trigraphs} was also specified, but now works
independently.  Warnings are not given for trigraphs within comments, as
we feel this is obnoxious.

@item -Wall
@findex -Wall
Requests @samp{-Wcomment} and @samp{-Wtrigraphs} (but not
@samp{-Wtraditional} or @samp{-Wundef}).

@item -Wtraditional
@findex -Wtraditional
Warn about certain constructs that behave differently in traditional and
ISO C@.  Also warn about ISO C constructs that have no traditional C
equivalent, and/or problematic constructs which should be avoided.

@itemize @bullet
@item
Macro parameters that appear within string literals in the macro body.
In traditional C macro replacement takes place within string literals,
but does not in ISO C.

@item
In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a directive
if the @samp{#} appeared in column 1 on the line.  Therefore
@samp{-Wtraditional} warns about directives that traditional C
understands but would ignore because the @samp{#} does not appear as the
first character on the line.  It also suggests you hide directives like
@samp{#pragma} not understood by traditional C by indenting them.  Some
traditional implementations would not recognise @samp{#elif}, so it
suggests avoiding it altogether.

@item
A function-like macro that appears without arguments.

@item
The unary plus operator.

@item
The `U' integer constant suffix.  (Traditonal C does support the `L'
suffix on integer constants.)  Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the _MIN/_MAX
macros in limits.h.  Use of these macros in user code might normally
lead to spurious warnings, however gcc's integrated preprocessor has
enough context to avoid warning in these cases.
@end itemize

@item -Wundef
@findex -Wundef
Warn if an undefined identifier is evaluated in an @samp{#if} directive.

@item -I @var{directory}
@findex -I
Add the directory @var{directory} to the head of the list of
directories to be searched for header files (@pxref{Include Syntax}).
This can be used to override a system header file, substituting your
own version, since these directories are searched before the system
header file directories.  If you use more than one @samp{-I} option,
the directories are scanned in left-to-right order; the standard
system directories come after.

@item -I-
Any directories specified with @samp{-I} options before the @samp{-I-}
option are searched only for the case of @samp{#include "@var{file}"};
they are not searched for @samp{#include <@var{file}>}.

If additional directories are specified with @samp{-I} options after
the @samp{-I-}, these directories are searched for all @samp{#include}
directives.

In addition, the @samp{-I-} option inhibits the use of the current
directory as the first search directory for @samp{#include "@var{file}"}.
Therefore, the current directory is searched only if it is requested
explicitly with @samp{-I.}.  Specifying both @samp{-I-} and @samp{-I.}
allows you to control precisely which directories are searched before
the current one and which are searched after.

@item -nostdinc
@findex -nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with @samp{-I} options
(and the current directory, if appropriate) are searched.

By using both @samp{-nostdinc} and @samp{-I-}, you can limit the include-file
search path to only those directories you specify explicitly.

@item -nostdinc++
@findex -nostdinc++
Do not search for header files in the C++-specific standard directories,
but do still search the other standard directories.  (This option is
used when building the C++ library.)

@item -remap
@findex -remap
When searching for a header file in a directory, remap file names if a
file named @file{header.gcc} exists in that directory.  This can be used
to work around limitations of file systems with file name restrictions.
The @file{header.gcc} file should contain a series of lines with two
tokens on each line: the first token is the name to map, and the second
token is the actual name to use.

@item -D @var{name}
@findex -D
Predefine @var{name} as a macro, with definition @samp{1}.

@item -D @var{name}=@var{definition}
Predefine @var{name} as a macro, with definition @var{definition}.
There are no restrictions on the contents of @var{definition}, but if
you are invoking the preprocessor from a shell or shell-like program you
may need to use the shell's quoting syntax to protect characters such as
spaces that have a meaning in the shell syntax.  If you use more than
one @samp{-D} for the same @var{name}, the rightmost definition takes
effect.

Any @samp{-D} and @samp{-U} options on the command line are processed in
order, and always before @samp{-imacros @var{file}}, regardless of the
order in which they are written.

@item -U @var{name}
@findex -U
Do not predefine @var{name}.

Any @samp{-D} and @samp{-U} options on the command line are processed in
order, and always before @samp{-imacros @var{file}}, regardless of the
order in which they are written.

@item -undef
@findex -undef
Do not predefine any nonstandard macros.

@item -gcc
@findex -gcc
Define the macros @var{__GNUC__}, @var{__GNUC_MINOR__} and
@var{__GNUC_PATCHLEVEL__}. These are defined automatically when you use
@samp{gcc -E}; you can turn them off in that case with @samp{-no-gcc}.

@item -A @var{predicate}=@var{answer}
@findex -A
Make an assertion with the predicate @var{predicate} and answer
@var{answer}.  This form is preferred to the older form @samp{-A
@var{predicate}(@var{answer})}, which is still supported, because
it does not use shell special characters.  @xref{Assertions}.

@item -A -@var{predicate}=@var{answer}
Disable an assertion with the predicate @var{predicate} and answer
@var{answer}.  Specifying no predicate, by @samp{-A-} or @samp{-A -},
disables all predefined assertions and all assertions preceding it on
the command line; and also undefines all predefined macros and all
macros preceding it on the command line.

@item -dM
@findex -dM
Instead of outputting the result of preprocessing, output a list of
@samp{#define} directives for all the macros defined during the
execution of the preprocessor, including predefined macros.  This gives
you a way of finding out what is predefined in your version of the
preprocessor; assuming you have no file @samp{foo.h}, the command

@example
touch foo.h; cpp -dM foo.h
@end example

@noindent 
will show the values of any predefined macros.

@item -dD
@findex -dD
Like @samp{-dM} except in two respects: it does @emph{not} include the
predefined macros, and it outputs @emph{both} the @samp{#define}
directives and the result of preprocessing.  Both kinds of output go to
the standard output file.

@item -dN
@findex -dN
Like @samp{-dD}, but emit only the macro names, not their expansions.

@item -dI
@findex -dI
Output @samp{#include} directives in addition to the result of
preprocessing.

@item -M
@findex -M
Instead of outputting the result of preprocessing, output a rule
suitable for @code{make} describing the dependencies of the main source
file.  The preprocessor outputs one @code{make} rule containing the
object file name for that source file, a colon, and the names of all the
included files, including those coming from @samp{-include} or
@samp{-imacros} command line options.  Unless specified explicitly (with
@samp{-MT} or @samp{-MQ}), the object file name consists of the basename
of the source file with any suffix replaced with object file suffix.
If there are many included files
then the rule is split into several lines using @samp{\}-newline.

@item -MM
@findex -MM
Like @samp{-M}, but mention only the files included with @samp{#include
"@var{file}"} or with @samp{-include} or @samp{-imacros} command line
options.  System header files included with @samp{#include <@var{file}>}
are omitted.

@item -MF @var{file}
@findex -MF
When used with @samp{-M} or @samp{-MM}, specifies a file to write the
dependencies to.  This allows the preprocessor to write the preprocessed
file to stdout normally.  If no @samp{-MF} switch is given, CPP sends
the rules to stdout and suppresses normal preprocessed output.

@item -MG
@findex -MG
When used with @samp{-M} or @samp{-MM}, @samp{-MG} says to treat missing
header files as generated files and assume they live in the same
directory as the source file.  It suppresses preprocessed output, as a
missing header file is ordinarily an error.

This feature is used in automatic updating of makefiles.

@item -MP
@findex -MP
This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing.  These
dummy rules work around errors @code{make} gives if you remove header
files without updating the @code{Makefile} to match.

This is typical output:-

@smallexample
/tmp/test.o: /tmp/test.c /tmp/test.h

/tmp/test.h:
@end smallexample

@item -MQ @var{target}
@item -MT @var{target}
@findex -MQ
@findex -MT
By default CPP uses the main file name, including any path, and appends
the object suffix, normally ``.o'', to it to obtain the name of the
target for dependency generation.  With @samp{-MT} you can specify a
target yourself, overriding the default one.

If you want multiple targets, you can specify them as a single argument
to @samp{-MT}, or use multiple @samp{-MT} options.

The targets you specify are output in the order they appear on the
command line.  @samp{-MQ} is identical to @samp{-MT}, except that the
target name is quoted for Make, but with @samp{-MT} it isn't.  For
example, -MT '$(objpfx)foo.o' gives

@smallexample
$(objpfx)foo.o: /tmp/foo.c
@end smallexample

but -MQ '$(objpfx)foo.o' gives

@smallexample
$$(objpfx)foo.o: /tmp/foo.c
@end smallexample

The default target is automatically quoted, as if it were given with
@samp{-MQ}.

@item -H
@findex -H
Print the name of each header file used, in addition to other normal
activities.

@item -imacros @var{file}
@findex -imacros
Process @var{file} as input, discarding the resulting output, before
processing the regular input file.  Because the output generated from
@var{file} is discarded, the only effect of @samp{-imacros @var{file}}
is to make the macros defined in @var{file} available for use in the
main input.

@item -include @var{file}
@findex -include
Process @var{file} as input, and include all the resulting output,
before processing the regular input file.  

@item -idirafter @var{dir}
@findex -idirafter
@cindex second include path
Add the directory @var{dir} to the second include path, marking it as a
system directory.  The directories on the second include path are searched
when a header file is not found in any of the directories in the main
include path (the one that @samp{-I} adds to).

@item -iprefix @var{prefix}
@findex -iprefix
Specify @var{prefix} as the prefix for subsequent @samp{-iwithprefix}
options.  If the prefix represents a directory, you should include the
final @samp{/}.

@item -iwithprefix @var{dir}
@findex -iwithprefix
Add a directory to the second include path, marking it as a system
directory.  The directory's name is made by concatenating @var{prefix}
and @var{dir}, where @var{prefix} was specified previously with
@samp{-iprefix}.

@item -isystem @var{dir}
@findex -isystem
Add a directory to the beginning of the second include path, marking it
as a system directory, so that it gets the same special treatment as
is applied to the standard system directories.  @xref{System Headers}.

@item -x c
@itemx -x c++
@itemx -x objective-c
@itemx -x assembler-with-cpp
@findex -x c
@findex -x objective-c
@findex -x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly.  This has
nothing to do with standards conformance or extensions; it merely
selects which base syntax to expect.  If you give none of these options,
cpp will deduce the language from the extension of the source file:
@samp{.c}, @samp{.cc}, @samp{.m}, or @samp{.S}.  Some other common
extensions for C++ and assembly are also recognized.  If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.

@strong{Note:} Previous versions of cpp accepted a @samp{-lang} option
which selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the @samp{-l}
option.

@item -std=@var{standard}
@itemx -ansi
@findex -std
@findex -ansi
Specify the standard to which the code should conform.  Currently cpp
only knows about the standards for C; other language standards will be
added in the future.

@var{standard}
may be one of:
@table @code
@item iso9899:1990
@itemx c89
The ISO C standard from 1990.  @samp{c89} is the customary shorthand for
this version of the standard.

The @samp{-ansi} option is equivalent to @samp{-std=c89}.

@item iso9899:199409
The 1990 C standard, as amended in 1994.

@item iso9899:1999
@itemx c99
@itemx iso9899:199x
@itemx c9x
The revised ISO C standard, published in December 1999.  Before
publication, this was known as C9X.

@item gnu89
The 1990 C standard plus GNU extensions.  This is the default.

@item gnu99
@itemx gnu9x
The 1999 C standard plus GNU extensions.
@end table

@item -ftabstop=NUMBER
@findex -ftabstop
Set the distance between tab stops.  This helps the preprocessor
report correct column numbers in warnings or errors, even if tabs appear
on the line.  Values less than 1 or greater than 100 are ignored.  The
default is 8.

@item -$
@findex -$
Forbid the use of @samp{$} in identifiers.  The C standard allows
implementations to define extra characters that can appear in
identifiers.  By default the GNU C preprocessor permits @samp{$}, a
common extension.
@end table
@c man end

@node Concept Index, Index, Invocation, Top
@unnumbered Concept Index
@printindex cp

@node Index,, Concept Index, Top
@unnumbered Index of Directives, Macros and Options
@printindex fn

@contents
@bye