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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.

Hierarchy Conventions

The layout of the `sysdeps' hierarchy.

Porting to Unix

Porting the library to an average

Unix-like system.