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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 (see 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). See 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. See 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.