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Library Maintenance
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Adding New Functions
====================

   The process of building the library is driven by the makefiles, which
make heavy use of special features of GNU `make'.  The makefiles are
very complex, and you probably don't want to try to understand them.
But what they do is fairly straightforward, and only requires that you
define a few variables in the right places.

   The library sources are divided into subdirectories, grouped by
topic.

   The `string' subdirectory has all the string-manipulation functions,
`math' has all the mathematical functions, etc.

   Each subdirectory contains a simple makefile, called `Makefile',
which defines a few `make' variables and then includes the global
makefile `Rules' with a line like:

     include ../Rules

The basic variables that a subdirectory makefile defines are:

`subdir'
     The name of the subdirectory, for example `stdio'.  This variable
     *must* be defined.

`headers'
     The names of the header files in this section of the library, such
     as `stdio.h'.

`routines'
`aux'
     The names of the modules (source files) in this section of the
     library.  These should be simple names, such as `strlen' (rather
     than complete file names, such as `strlen.c').  Use `routines' for
     modules that define functions in the library, and `aux' for
     auxiliary modules containing things like data definitions.  But the
     values of `routines' and `aux' are just concatenated, so there
     really is no practical difference.

`tests'
     The names of test programs for this section of the library.  These
     should be simple names, such as `tester' (rather than complete file
     names, such as `tester.c').  `make tests' will build and run all
     the test programs.  If a test program needs input, put the test
     data in a file called `TEST-PROGRAM.input'; it will be given to
     the test program on its standard input.  If a test program wants
     to be run with arguments, put the arguments (all on a single line)
     in a file called `TEST-PROGRAM.args'.  Test programs should exit
     with zero status when the test passes, and nonzero status when the
     test indicates a bug in the library or error in building.

`others'
     The names of "other" programs associated with this section of the
     library.  These are programs which are not tests per se, but are
     other small programs included with the library.  They are built by
     `make others'.

`install-lib'
`install-data'
`install'
     Files to be installed by `make install'.  Files listed in
     `install-lib' are installed in the directory specified by `libdir'
     in `configparms' or `Makeconfig' (*note Installation::.).  Files
     listed in `install-data' are installed in the directory specified
     by `datadir' in `configparms' or `Makeconfig'.  Files listed in
     `install' are installed in the directory specified by `bindir' in
     `configparms' or `Makeconfig'.

`distribute'
     Other files from this subdirectory which should be put into a
     distribution tar file.  You need not list here the makefile itself
     or the source and header files listed in the other standard
     variables.  Only define `distribute' if there are files used in an
     unusual way that should go into the distribution.

`generated'
     Files which are generated by `Makefile' in this subdirectory.
     These files will be removed by `make clean', and they will never
     go into a distribution.

`extra-objs'
     Extra object files which are built by `Makefile' in this
     subdirectory.  This should be a list of file names like `foo.o';
     the files will actually be found in whatever directory object
     files are being built in.  These files will be removed by
     `make clean'.  This variable is used for secondary object files
     needed to build `others' or `tests'.

Porting the GNU C Library
=========================

   The GNU C library is written to be easily portable to a variety of
machines and operating systems.  Machine- and operating system-dependent
functions are well separated to make it easy to add implementations for
new machines or operating systems.  This section describes the layout of
the library source tree and explains the mechanisms used to select
machine-dependent code to use.

   All the machine-dependent and operating system-dependent files in the
library are in the subdirectory `sysdeps' under the top-level library
source directory.  This directory contains a hierarchy of
subdirectories (*note Hierarchy Conventions::.).

   Each subdirectory of `sysdeps' contains source files for a
particular machine or operating system, or for a class of machine or
operating system (for example, systems by a particular vendor, or all
machines that use IEEE 754 floating-point format).  A configuration
specifies an ordered list of these subdirectories.  Each subdirectory
implicitly appends its parent directory to the list.  For example,
specifying the list `unix/bsd/vax' is equivalent to specifying the list
`unix/bsd/vax unix/bsd unix'.  A subdirectory can also specify that it
implies other subdirectories which are not directly above it in the
directory hierarchy.  If the file `Implies' exists in a subdirectory,
it lists other subdirectories of `sysdeps' which are appended to the
list, appearing after the subdirectory containing the `Implies' file.
Lines in an `Implies' file that begin with a `#' character are ignored
as comments.  For example, `unix/bsd/Implies' contains:
     # BSD has Internet-related things.
     unix/inet

and `unix/Implies' contains:
     posix

So the final list is `unix/bsd/vax unix/bsd unix/inet unix posix'.

   `sysdeps' has a "special" subdirectory called `generic'.  It is
always implicitly appended to the list of subdirectories, so you
needn't put it in an `Implies' file, and you should not create any
subdirectories under it intended to be new specific categories.
`generic' serves two purposes.  First, the makefiles do not bother to
look for a system-dependent version of a file that's not in `generic'.
This means that any system-dependent source file must have an analogue
in `generic', even if the routines defined by that file are not
implemented on other platforms.  Second. the `generic' version of a
system-dependent file is used if the makefiles do not find a version
specific to the system you're compiling for.

   If it is possible to implement the routines in a `generic' file in
machine-independent C, using only other machine-independent functions in
the C library, then you should do so.  Otherwise, make them stubs.  A
"stub" function is a function which cannot be implemented on a
particular machine or operating system.  Stub functions always return an
error, and set `errno' to `ENOSYS' (Function not implemented).  *Note
Error Reporting::.  If you define a stub function, you must place the
statement `stub_warning(FUNCTION)', where FUNCTION is the name of your
function, after its definition; also, you must include the file
`<stub-tag.h>' into your file.  This causes the function to be listed
in the installed `<gnu/stubs.h>', and makes GNU ld warn when the
function is used.

   Some rare functions are only useful on specific systems and aren't
defined at all on others; these do not appear anywhere in the
system-independent source code or makefiles (including the `generic'
directory), only in the system-dependent `Makefile' in the specific
system's subdirectory.

   If you come across a file that is in one of the main source
directories (`string', `stdio', etc.), and you want to write a machine-
or operating system-dependent version of it, move the file into
`sysdeps/generic' and write your new implementation in the appropriate
system-specific subdirectory.  Note that if a file is to be
system-dependent, it *must not* appear in one of the main source
directories.

   There are a few special files that may exist in each subdirectory of
`sysdeps':

`Makefile'
     A makefile for this machine or operating system, or class of
     machine or operating system.  This file is included by the library
     makefile `Makerules', which is used by the top-level makefile and
     the subdirectory makefiles.  It can change the variables set in the
     including makefile or add new rules.  It can use GNU `make'
     conditional directives based on the variable `subdir' (see above)
     to select different sets of variables and rules for different
     sections of the library.  It can also set the `make' variable
     `sysdep-routines', to specify extra modules to be included in the
     library.  You should use `sysdep-routines' rather than adding
     modules to `routines' because the latter is used in determining
     what to distribute for each subdirectory of the main source tree.

     Each makefile in a subdirectory in the ordered list of
     subdirectories to be searched is included in order.  Since several
     system-dependent makefiles may be included, each should append to
     `sysdep-routines' rather than simply setting it:

          sysdep-routines := $(sysdep-routines) foo bar

`Subdirs'
     This file contains the names of new whole subdirectories under the
     top-level library source tree that should be included for this
     system.  These subdirectories are treated just like the
     system-independent subdirectories in the library source tree, such
     as `stdio' and `math'.

     Use this when there are completely new sets of functions and header
     files that should go into the library for the system this
     subdirectory of `sysdeps' implements.  For example,
     `sysdeps/unix/inet/Subdirs' contains `inet'; the `inet' directory
     contains various network-oriented operations which only make sense
     to put in the library on systems that support the Internet.

`Dist'
     This file contains the names of files (relative to the
     subdirectory of `sysdeps' in which it appears) which should be
     included in the distribution.  List any new files used by rules in
     the `Makefile' in the same directory, or header files used by the
     source files in that directory.  You don't need to list files that
     are implementations (either C or assembly source) of routines
     whose names are given in the machine-independent makefiles in the
     main source tree.

`configure'
     This file is a shell script fragment to be run at configuration
     time.  The top-level `configure' script uses the shell `.' command
     to read the `configure' file in each system-dependent directory
     chosen, in order.  The `configure' files are often generated from
     `configure.in' files using Autoconf.

     A system-dependent `configure' script will usually add things to
     the shell variables `DEFS' and `config_vars'; see the top-level
     `configure' script for details.  The script can check for
     `--with-PACKAGE' options that were passed to the top-level
     `configure'.  For an option `--with-PACKAGE=VALUE' `configure'
     sets the shell variable `with_PACKAGE' (with any dashes in PACKAGE
     converted to underscores) to VALUE; if the option is just
     `--with-PACKAGE' (no argument), then it sets `with_PACKAGE' to
     `yes'.

`configure.in'
     This file is an Autoconf input fragment to be processed into the
     file `configure' in this subdirectory.  *Note Introduction:
     (autoconf.info)Introduction, for a description of Autoconf.  You
     should write either `configure' or `configure.in', but not both.
     The first line of `configure.in' should invoke the `m4' macro
     `GLIBC_PROVIDES'.  This macro does several `AC_PROVIDE' calls for
     Autoconf macros which are used by the top-level `configure'
     script; without this, those macros might be invoked again
     unnecessarily by Autoconf.

   That is the general system for how system-dependencies are isolated.

Layout of the `sysdeps' Directory Hierarchy
-------------------------------------------

   A GNU configuration name has three parts: the CPU type, the
manufacturer's name, and the operating system.  `configure' uses these
to pick the list of system-dependent directories to look for.  If the
`--nfp' option is *not* passed to `configure', the directory
`MACHINE/fpu' is also used.  The operating system often has a "base
operating system"; for example, if the operating system is `Linux', the
base operating system is `unix/sysv'.  The algorithm used to pick the
list of directories is simple: `configure' makes a list of the base
operating system, manufacturer, CPU type, and operating system, in that
order.  It then concatenates all these together with slashes in
between, to produce a directory name; for example, the configuration
`i686-linux-gnu' results in `unix/sysv/linux/i386/i686'.  `configure'
then tries removing each element of the list in turn, so
`unix/sysv/linux' and `unix/sysv' are also tried, among others.  Since
the precise version number of the operating system is often not
important, and it would be very inconvenient, for example, to have
identical `irix6.2' and `irix6.3' directories, `configure' tries
successively less specific operating system names by removing trailing
suffixes starting with a period.

   As an example, here is the complete list of directories that would be
tried for the configuration `i686-linux-gnu' (with the `crypt' and
`linuxthreads' add-on):

     sysdeps/i386/elf
     crypt/sysdeps/unix
     linuxthreads/sysdeps/unix/sysv/linux
     linuxthreads/sysdeps/pthread
     linuxthreads/sysdeps/unix/sysv
     linuxthreads/sysdeps/unix
     linuxthreads/sysdeps/i386/i686
     linuxthreads/sysdeps/i386
     linuxthreads/sysdeps/pthread/no-cmpxchg
     sysdeps/unix/sysv/linux/i386
     sysdeps/unix/sysv/linux
     sysdeps/gnu
     sysdeps/unix/common
     sysdeps/unix/mman
     sysdeps/unix/inet
     sysdeps/unix/sysv/i386/i686
     sysdeps/unix/sysv/i386
     sysdeps/unix/sysv
     sysdeps/unix/i386
     sysdeps/unix
     sysdeps/posix
     sysdeps/i386/i686
     sysdeps/i386/i486
     sysdeps/libm-i387/i686
     sysdeps/i386/fpu
     sysdeps/libm-i387
     sysdeps/i386
     sysdeps/wordsize-32
     sysdeps/ieee754
     sysdeps/libm-ieee754
     sysdeps/generic

   Different machine architectures are conventionally subdirectories at
the top level of the `sysdeps' directory tree.  For example,
`sysdeps/sparc' and `sysdeps/m68k'.  These contain files specific to
those machine architectures, but not specific to any particular
operating system.  There might be subdirectories for specializations of
those architectures, such as `sysdeps/m68k/68020'. Code which is
specific to the floating-point coprocessor used with a particular
machine should go in `sysdeps/MACHINE/fpu'.

   There are a few directories at the top level of the `sysdeps'
hierarchy that are not for particular machine architectures.

`generic'
     As described above (*note Porting::.), this is the subdirectory
     that every configuration implicitly uses after all others.

`ieee754'
     This directory is for code using the IEEE 754 floating-point
     format, where the C type `float' is IEEE 754 single-precision
     format, and `double' is IEEE 754 double-precision format.  Usually
     this directory is referred to in the `Implies' file in a machine
     architecture-specific directory, such as `m68k/Implies'.

`libm-ieee754'
     This directory contains an implementation of a mathematical library
     usable on platforms which use IEEE 754 conformant floating-point
     arithmetic.

`libm-i387'
     This is a special case.  Ideally the code should be in
     `sysdeps/i386/fpu' but for various reasons it is kept aside.

`posix'
     This directory contains implementations of things in the library in
     terms of POSIX.1 functions.  This includes some of the POSIX.1
     functions themselves.  Of course, POSIX.1 cannot be completely
     implemented in terms of itself, so a configuration using just
     `posix' cannot be complete.

`unix'
     This is the directory for Unix-like things.  *Note Porting to
     Unix::.  `unix' implies `posix'.  There are some special-purpose
     subdirectories of `unix':

    `unix/common'
          This directory is for things common to both BSD and System V
          release 4.  Both `unix/bsd' and `unix/sysv/sysv4' imply
          `unix/common'.

    `unix/inet'
          This directory is for `socket' and related functions on Unix
          systems.  `unix/inet/Subdirs' enables the `inet' top-level
          subdirectory.  `unix/common' implies `unix/inet'.

`mach'
     This is the directory for things based on the Mach microkernel
     from CMU (including the GNU operating system).  Other basic
     operating systems (VMS, for example) would have their own
     directories at the top level of the `sysdeps' hierarchy, parallel
     to `unix' and `mach'.

Porting the GNU C Library to Unix Systems
-----------------------------------------

   Most Unix systems are fundamentally very similar.  There are
variations between different machines, and variations in what
facilities are provided by the kernel.  But the interface to the
operating system facilities is, for the most part, pretty uniform and
simple.

   The code for Unix systems is in the directory `unix', at the top
level of the `sysdeps' hierarchy.  This directory contains
subdirectories (and subdirectory trees) for various Unix variants.

   The functions which are system calls in most Unix systems are
implemented in assembly code, which is generated automatically from
specifications in files named `syscalls.list'.  There are several such
files, one in `sysdeps/unix' and others in its subdirectories.  Some
special system calls are implemented in files that are named with a
suffix of `.S'; for example, `_exit.S'.  Files ending in `.S' are run
through the C preprocessor before being fed to the assembler.

   These files all use a set of macros that should be defined in
`sysdep.h'.  The `sysdep.h' file in `sysdeps/unix' partially defines
them; a `sysdep.h' file in another directory must finish defining them
for the particular machine and operating system variant.  See
`sysdeps/unix/sysdep.h' and the machine-specific `sysdep.h'
implementations to see what these macros are and what they should do.

   The system-specific makefile for the `unix' directory
(`sysdeps/unix/Makefile') gives rules to generate several files from
the Unix system you are building the library on (which is assumed to be
the target system you are building the library *for*).  All the
generated files are put in the directory where the object files are
kept; they should not affect the source tree itself.  The files
generated are `ioctls.h', `errnos.h', `sys/param.h', and `errlist.c'
(for the `stdio' section of the library).