(libc.info.gz) Porting

 
 D.2 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
 ( 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).  
 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.  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.
 
 '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.ac' 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.ac'
 
      This file is an Autoconf input fragment to be processed into the
      file 'configure' in this subdirectory.  
      (autoconf.info)Introduction, for a description of Autoconf.  You
      should write either 'configure' or 'configure.ac', but not both.
      The first line of 'configure.ac' 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.
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