1 @node Maintenance, Platform, Installation, Top
2 @c %MENU% How to enhance and port the GNU C Library
3 @appendix Library Maintenance
6 * Source Layout:: How to add new functions or header files
8 * Source Fortification:: Fortification of function calls.
9 * Symbol handling:: How to handle symbols in the GNU C Library.
10 * Porting:: How to port the GNU C Library to
11 a new machine or operating system.
15 @appendixsec Adding New Functions
17 The process of building the library is driven by the makefiles, which
18 make heavy use of special features of GNU @code{make}. The makefiles
19 are very complex, and you probably don't want to try to understand them.
20 But what they do is fairly straightforward, and only requires that you
21 define a few variables in the right places.
23 The library sources are divided into subdirectories, grouped by topic.
25 The @file{string} subdirectory has all the string-manipulation
26 functions, @file{math} has all the mathematical functions, etc.
28 Each subdirectory contains a simple makefile, called @file{Makefile},
29 which defines a few @code{make} variables and then includes the global
30 makefile @file{Rules} with a line like:
37 The basic variables that a subdirectory makefile defines are:
41 The name of the subdirectory, for example @file{stdio}.
42 This variable @strong{must} be defined.
45 The names of the header files in this section of the library,
46 such as @file{stdio.h}.
50 The names of the modules (source files) in this section of the library.
51 These should be simple names, such as @samp{strlen} (rather than
52 complete file names, such as @file{strlen.c}). Use @code{routines} for
53 modules that define functions in the library, and @code{aux} for
54 auxiliary modules containing things like data definitions. But the
55 values of @code{routines} and @code{aux} are just concatenated, so there
56 really is no practical difference.
59 The names of test programs for this section of the library. These
60 should be simple names, such as @samp{tester} (rather than complete file
61 names, such as @file{tester.c}). @w{@samp{make tests}} will build and
62 run all the test programs. If a test program needs input, put the test
63 data in a file called @file{@var{test-program}.input}; it will be given to
64 the test program on its standard input. If a test program wants to be
65 run with arguments, put the arguments (all on a single line) in a file
66 called @file{@var{test-program}.args}. Test programs should exit with
67 zero status when the test passes, and nonzero status when the test
68 indicates a bug in the library or error in building.
71 The names of ``other'' programs associated with this section of the
72 library. These are programs which are not tests per se, but are other
73 small programs included with the library. They are built by
74 @w{@samp{make others}}.
79 Files to be installed by @w{@samp{make install}}. Files listed in
80 @samp{install-lib} are installed in the directory specified by
81 @samp{libdir} in @file{configparms} or @file{Makeconfig}
82 (@pxref{Installation}). Files listed in @code{install-data} are
83 installed in the directory specified by @samp{datadir} in
84 @file{configparms} or @file{Makeconfig}. Files listed in @code{install}
85 are installed in the directory specified by @samp{bindir} in
86 @file{configparms} or @file{Makeconfig}.
89 Other files from this subdirectory which should be put into a
90 distribution tar file. You need not list here the makefile itself or
91 the source and header files listed in the other standard variables.
92 Only define @code{distribute} if there are files used in an unusual way
93 that should go into the distribution.
96 Files which are generated by @file{Makefile} in this subdirectory.
97 These files will be removed by @w{@samp{make clean}}, and they will
98 never go into a distribution.
101 Extra object files which are built by @file{Makefile} in this
102 subdirectory. This should be a list of file names like @file{foo.o};
103 the files will actually be found in whatever directory object files are
104 being built in. These files will be removed by @w{@samp{make clean}}.
105 This variable is used for secondary object files needed to build
106 @code{others} or @code{tests}.
110 * Platform: Adding Platform-specific. Adding platform-specific
114 @node Adding Platform-specific
115 @appendixsubsec Platform-specific types, macros and functions
117 It's sometimes necessary to provide nonstandard, platform-specific
118 features to developers. The C library is traditionally the
119 lowest library layer, so it makes sense for it to provide these
120 low-level features. However, including these features in the C
121 library may be a disadvantage if another package provides them
122 as well as there will be two conflicting versions of them. Also,
123 the features won't be available to projects that do not use
124 @theglibc{} but use other GNU tools, like GCC.
126 The current guidelines are:
129 If the header file provides features that only make sense on a particular
130 machine architecture and have nothing to do with an operating system, then
131 the features should ultimately be provided as GCC built-in functions. Until
132 then, @theglibc{} may provide them in the header file. When the GCC built-in
133 functions become available, those provided in the header file should be made
134 conditionally available prior to the GCC version in which the built-in
135 function was made available.
138 If the header file provides features that are specific to an operating system,
139 both GCC and @theglibc{} could provide it, but @theglibc{} is preferred
140 as it already has a lot of information about the operating system.
143 If the header file provides features that are specific to an operating system
144 but used by @theglibc{}, then @theglibc{} should provide them.
147 The general solution for providing low-level features is to export them as
152 A nonstandard, low-level header file that defines macros and inline
153 functions should be called @file{sys/platform/@var{name}.h}.
156 Each header file's name should include the platform name, to avoid
157 users thinking there is anything in common between the different
158 header files for different platforms. For example, a
159 @file{sys/platform/@var{arch}.h} name such as
160 @file{sys/platform/ppc.h} is better than @file{sys/platform.h}.
163 A platform-specific header file provided by @theglibc{} should coordinate
164 with GCC such that compiler built-in versions of the functions and macros are
165 preferred if available. This means that user programs will only ever need to
166 include @file{sys/platform/@var{arch}.h}, keeping the same names of types,
167 macros, and functions for convenience and portability.
170 Each included symbol must have the prefix @code{__@var{arch}_}, such as
171 @code{__ppc_get_timebase}.
175 The easiest way to provide a header file is to add it to the
176 @code{sysdep_headers} variable. For example, the combination of
177 Linux-specific header files on PowerPC could be provided like this:
180 sysdep_headers += sys/platform/ppc.h
183 Then ensure that you have added a @file{sys/platform/ppc.h}
184 header file in the machine-specific directory, e.g.,
185 @file{sysdeps/powerpc/sys/platform/ppc.h}.
188 @node Source Fortification
189 @appendixsec Fortification of function calls
191 This section contains implementation details of @theglibc{} and may not
192 remain stable across releases.
194 The @code{_FORTIFY_SOURCE} macro may be defined by users to control
195 hardening of calls into some functions in @theglibc{}. The definition
196 should be at the top of the source file before any headers are included
197 or at the pre-processor commandline using the @code{-D} switch. The
198 hardening primarily focuses on accesses to buffers passed to the
199 functions but may also include checks for validity of other inputs to
202 When the @code{_FORTIFY_SOURCE} macro is defined, it enables code that
203 validates inputs passed to some functions in @theglibc to determine if
204 they are safe. If the compiler is unable to determine that the inputs
205 to the function call are safe, the call may be replaced by a call to its
206 hardened variant that does additional safety checks at runtime. Some
207 hardened variants need the size of the buffer to perform access
208 validation and this is provided by the @code{__builtin_object_size} or
209 the @code{__builtin_dynamic_object_size} builtin functions.
210 @code{_FORTIFY_SOURCE} also enables additional compile time diagnostics,
211 such as unchecked return values from some functions, to encourage
212 developers to add error checking for those functions.
214 At runtime, if any of those safety checks fail, the program will
215 terminate with a @code{SIGABRT} signal. @code{_FORTIFY_SOURCE} may be
216 defined to one of the following values:
219 @item @math{1}: This enables buffer bounds checking using the value
220 returned by the @code{__builtin_object_size} compiler builtin function.
221 If the function returns @code{(size_t) -1}, the function call is left
222 untouched. Additionally, this level also enables validation of flags to
223 the @code{open}, @code{open64}, @code{openat} and @code{openat64}
226 @item @math{2}: This behaves like @math{1}, with the addition of some
227 checks that may trap code that is conforming but unsafe, e.g. accepting
228 @code{%n} only in read-only format strings.
230 @item @math{3}: This enables buffer bounds checking using the value
231 returned by the @code{__builtin_dynamic_object_size} compiler builtin
232 function. If the function returns @code{(size_t) -1}, the function call
233 is left untouched. Fortification at this level may have a impact on
234 program performance if the function call that is fortified is frequently
235 encountered and the size expression returned by
236 @code{__builtin_dynamic_object_size} is complex.
239 In general, the fortified variants of the function calls use the name of
240 the function with a @code{__} prefix and a @code{_chk} suffix. There
241 are some exceptions, e.g. the @code{printf} family of functions where,
242 depending on the architecture, one may also see fortified variants have
243 the @code{_chkieee128} suffix or the @code{__nldbl___} prefix to their
246 Another exception is the @code{open} family of functions, where their
247 fortified replacements have the @code{__} prefix and a @code{_2} suffix.
248 The @code{FD_SET}, @code{FD_CLR} and @code{FD_ISSET} macros use the
249 @code{__fdelt_chk} function on fortification.
251 The following functions and macros are fortified in @theglibc{}:
252 @c Generated using the following command:
253 @c find . -name Versions | xargs grep -e "_chk;" -e "_2;" |
254 @c cut -d ':' -f 2 | sed 's/;/\n/g' | sed 's/ *//g' | grep -v "^$" |
255 @c sort -u | grep ^__ |
256 @c grep -v -e ieee128 -e __nldbl -e align_cpy -e "fdelt_warn" |
257 @c sed 's/__fdelt_chk/@item @code{FD_SET}\n\n@item @code{FD_CLR}\n\n@item @code{FD_ISSET}\n/' |
258 @c sed 's/__\(.*\)_\(chk\|2\)/@item @code{\1}\n/'
262 @item @code{asprintf}
268 @item @code{explicit_bzero}
274 @item @code{FD_ISSET}
278 @item @code{fgets_unlocked}
282 @item @code{fgetws_unlocked}
288 @item @code{fread_unlocked}
290 @item @code{fwprintf}
294 @item @code{getdomainname}
296 @item @code{getgroups}
298 @item @code{gethostname}
300 @item @code{getlogin_r}
308 @item @code{mbsnrtowcs}
310 @item @code{mbsrtowcs}
312 @item @code{mbstowcs}
324 @item @code{obstack_printf}
326 @item @code{obstack_vprintf}
334 @item @code{openat64}
348 @item @code{ptsname_r}
352 @item @code{readlinkat}
354 @item @code{readlink}
356 @item @code{realpath}
360 @item @code{recvfrom}
362 @item @code{snprintf}
382 @item @code{swprintf}
386 @item @code{ttyname_r}
388 @item @code{vasprintf}
390 @item @code{vdprintf}
392 @item @code{vfprintf}
394 @item @code{vfwprintf}
398 @item @code{vsnprintf}
400 @item @code{vsprintf}
402 @item @code{vswprintf}
406 @item @code{vwprintf}
426 @item @code{wcsnrtombs}
428 @item @code{wcsrtombs}
430 @item @code{wcstombs}
436 @item @code{wmemmove}
438 @item @code{wmempcpy}
447 @node Symbol handling
448 @appendixsec Symbol handling in the GNU C Library
451 * 64-bit time symbol handling :: How to handle 64-bit time related
452 symbols in the GNU C Library.
455 @node 64-bit time symbol handling
456 @appendixsubsec 64-bit time symbol handling in the GNU C Library
458 With respect to time handling, @glibcadj{} configurations fall in two
459 classes depending on the value of @code{__TIMESIZE}:
463 @item @code{__TIMESIZE == 32}
465 These @dfn{dual-time} configurations have both 32-bit and 64-bit time
466 support. 32-bit time support provides type @code{time_t} and cannot
467 handle dates beyond @dfn{Y2038}. 64-bit time support provides type
468 @code{__time64_t} and can handle dates beyond @dfn{Y2038}.
470 In these configurations, time-related types have two declarations,
471 a 64-bit one, and a 32-bit one; and time-related functions generally
472 have two definitions: a 64-bit one, and a 32-bit one which is a wrapper
473 around the former. Therefore, for every @code{time_t}-related symbol,
474 there is a corresponding @code{__time64_t}-related symbol, the name of
475 which is usually the 32-bit symbol's name with @code{__} (a double
476 underscore) prepended and @code{64} appended. For instance, the
477 64-bit-time counterpart of @code{clock_gettime} is
478 @code{__clock_gettime64}.
480 @item @code{__TIMESIZE == 64}
482 These @dfn{single-time} configurations only have a 64-bit @code{time_t}
483 and related functions, which can handle dates beyond 2038-01-19
484 03:14:07 (aka @dfn{Y2038}).
486 In these configurations, time-related types only have a 64-bit
487 declaration; and time-related functions only have one 64-bit definition.
488 However, for every @code{time_t}-related symbol, there is a
489 corresponding @code{__time64_t}-related macro, the name of which is
490 derived as in the dual-time configuration case, and which expands to
491 the symbol's name. For instance, the macro @code{__clock_gettime64}
492 expands to @code{clock_gettime}.
494 When @code{__TIMESIZE} is set to 64, @theglibc{} will also define
495 the@code{__USE_TIME_BITS64} macro. It is used by the Linux kernel ABI
496 to set the expected @code{time_t} size used on some syscalls.
498 These macros are purely internal to @theglibc{} and exist only so that
499 a single definition of the 64-bit time functions can be used on both
500 single-time and dual-time configurations, and so that glibc code can
501 freely call the 64-bit functions internally in all configurations.
505 @c The following paragraph should be removed once external interfaces
506 @c get support for both time sizes.
508 Note: at this point, 64-bit time support in dual-time configurations is
509 work-in-progress, so for these configurations, the public API only makes
510 the 32-bit time support available. In a later change, the public API
511 will allow user code to choose the time size for a given compilation
514 64-bit variants of time-related types or functions are defined for all
515 configurations and use 64-bit-time symbol names (for dual-time
516 configurations) or macros (for single-time configurations).
518 32-bit variants of time-related types or functions are defined only for
519 dual-time configurations.
521 Here is an example with @code{localtime}:
523 Function @code{localtime} is declared in @file{time/time.h} as
525 extern struct tm *localtime (const time_t *__timer) __THROW;
526 libc_hidden_proto (localtime)
529 For single-time configurations, @code{__localtime64} is a macro which
530 evaluates to @code{localtime}; for dual-time configurations,
531 @code{__localtime64} is a function similar to @code{localtime} except
532 it uses Y2038-proof types:
535 # define __localtime64 localtime
537 extern struct tm *__localtime64 (const __time64_t *__timer) __THROW;
538 libc_hidden_proto (__localtime64)
542 (note: type @code{time_t} is replaced with @code{__time64_t} because
543 @code{time_t} is not Y2038-proof, but @code{struct tm} is not
544 replaced because it is already Y2038-proof.)
546 The 64-bit-time implementation of @code{localtime} is written as follows
547 and is compiled for both dual-time and single-time configuration classes.
551 __localtime64 (const __time64_t *t)
553 return __tz_convert (*t, 1, &_tmbuf);
555 libc_hidden_def (__localtime64)
558 The 32-bit-time implementation is a wrapper and is only compiled for
559 dual-time configurations:
565 localtime (const time_t *t)
568 return __localtime64 (&t64);
570 libc_hidden_def (localtime)
576 @appendixsec Porting @theglibc{}
578 @Theglibc{} is written to be easily portable to a variety of
579 machines and operating systems. Machine- and operating system-dependent
580 functions are well separated to make it easy to add implementations for
581 new machines or operating systems. This section describes the layout of
582 the library source tree and explains the mechanisms used to select
583 machine-dependent code to use.
585 All the machine-dependent and operating system-dependent files in the
586 library are in the subdirectory @file{sysdeps} under the top-level
587 library source directory. This directory contains a hierarchy of
588 subdirectories (@pxref{Hierarchy Conventions}).
590 Each subdirectory of @file{sysdeps} contains source files for a
591 particular machine or operating system, or for a class of machine or
592 operating system (for example, systems by a particular vendor, or all
593 machines that use IEEE 754 floating-point format). A configuration
594 specifies an ordered list of these subdirectories. Each subdirectory
595 implicitly appends its parent directory to the list. For example,
596 specifying the list @file{unix/bsd/vax} is equivalent to specifying the
597 list @file{unix/bsd/vax unix/bsd unix}. A subdirectory can also specify
598 that it implies other subdirectories which are not directly above it in
599 the directory hierarchy. If the file @file{Implies} exists in a
600 subdirectory, it lists other subdirectories of @file{sysdeps} which are
601 appended to the list, appearing after the subdirectory containing the
602 @file{Implies} file. Lines in an @file{Implies} file that begin with a
603 @samp{#} character are ignored as comments. For example,
604 @file{unix/bsd/Implies} contains:
606 # BSD has Internet-related things.
610 and @file{unix/Implies} contains:
617 So the final list is @file{unix/bsd/vax unix/bsd unix/inet unix posix}.
619 @file{sysdeps} has a ``special'' subdirectory called @file{generic}. It
620 is always implicitly appended to the list of subdirectories, so you
621 needn't put it in an @file{Implies} file, and you should not create any
622 subdirectories under it intended to be new specific categories.
623 @file{generic} serves two purposes. First, the makefiles do not bother
624 to look for a system-dependent version of a file that's not in
625 @file{generic}. This means that any system-dependent source file must
626 have an analogue in @file{generic}, even if the routines defined by that
627 file are not implemented on other platforms. Second, the @file{generic}
628 version of a system-dependent file is used if the makefiles do not find
629 a version specific to the system you're compiling for.
631 If it is possible to implement the routines in a @file{generic} file in
632 machine-independent C, using only other machine-independent functions in
633 the C library, then you should do so. Otherwise, make them stubs. A
634 @dfn{stub} function is a function which cannot be implemented on a
635 particular machine or operating system. Stub functions always return an
636 error, and set @code{errno} to @code{ENOSYS} (Function not implemented).
637 @xref{Error Reporting}. If you define a stub function, you must place
638 the statement @code{stub_warning(@var{function})}, where @var{function}
639 is the name of your function, after its definition. This causes the
640 function to be listed in the installed @code{<gnu/stubs.h>}, and
641 makes GNU ld warn when the function is used.
643 Some rare functions are only useful on specific systems and aren't
644 defined at all on others; these do not appear anywhere in the
645 system-independent source code or makefiles (including the
646 @file{generic} directory), only in the system-dependent @file{Makefile}
647 in the specific system's subdirectory.
649 If you come across a file that is in one of the main source directories
650 (@file{string}, @file{stdio}, etc.), and you want to write a machine- or
651 operating system-dependent version of it, move the file into
652 @file{sysdeps/generic} and write your new implementation in the
653 appropriate system-specific subdirectory. Note that if a file is to be
654 system-dependent, it @strong{must not} appear in one of the main source
657 There are a few special files that may exist in each subdirectory of
660 @comment Blank lines after items make the table look better.
664 A makefile for this machine or operating system, or class of machine or
665 operating system. This file is included by the library makefile
666 @file{Makerules}, which is used by the top-level makefile and the
667 subdirectory makefiles. It can change the variables set in the
668 including makefile or add new rules. It can use GNU @code{make}
669 conditional directives based on the variable @samp{subdir} (see above) to
670 select different sets of variables and rules for different sections of
671 the library. It can also set the @code{make} variable
672 @samp{sysdep-routines}, to specify extra modules to be included in the
673 library. You should use @samp{sysdep-routines} rather than adding
674 modules to @samp{routines} because the latter is used in determining
675 what to distribute for each subdirectory of the main source tree.
677 Each makefile in a subdirectory in the ordered list of subdirectories to
678 be searched is included in order. Since several system-dependent
679 makefiles may be included, each should append to @samp{sysdep-routines}
680 rather than simply setting it:
683 sysdep-routines := $(sysdep-routines) foo bar
689 This file contains the names of new whole subdirectories under the
690 top-level library source tree that should be included for this system.
691 These subdirectories are treated just like the system-independent
692 subdirectories in the library source tree, such as @file{stdio} and
695 Use this when there are completely new sets of functions and header
696 files that should go into the library for the system this subdirectory
697 of @file{sysdeps} implements. For example,
698 @file{sysdeps/unix/inet/Subdirs} contains @file{inet}; the @file{inet}
699 directory contains various network-oriented operations which only make
700 sense to put in the library on systems that support the Internet.
704 This file is a shell script fragment to be run at configuration time.
705 The top-level @file{configure} script uses the shell @code{.} command to
706 read the @file{configure} file in each system-dependent directory
707 chosen, in order. The @file{configure} files are often generated from
708 @file{configure.ac} files using Autoconf.
710 A system-dependent @file{configure} script will usually add things to
711 the shell variables @samp{DEFS} and @samp{config_vars}; see the
712 top-level @file{configure} script for details. The script can check for
713 @w{@samp{--with-@var{package}}} options that were passed to the
714 top-level @file{configure}. For an option
715 @w{@samp{--with-@var{package}=@var{value}}} @file{configure} sets the
716 shell variable @w{@samp{with_@var{package}}} (with any dashes in
717 @var{package} converted to underscores) to @var{value}; if the option is
718 just @w{@samp{--with-@var{package}}} (no argument), then it sets
719 @w{@samp{with_@var{package}}} to @samp{yes}.
723 This file is an Autoconf input fragment to be processed into the file
724 @file{configure} in this subdirectory. @xref{Introduction,,,
725 autoconf.info, Autoconf: Generating Automatic Configuration Scripts},
726 for a description of Autoconf. You should write either @file{configure}
727 or @file{configure.ac}, but not both. The first line of
728 @file{configure.ac} should invoke the @code{m4} macro
729 @samp{GLIBC_PROVIDES}. This macro does several @code{AC_PROVIDE} calls
730 for Autoconf macros which are used by the top-level @file{configure}
731 script; without this, those macros might be invoked again unnecessarily
735 That is the general system for how system-dependencies are isolated.
737 The next section explains how to decide what directories in
738 @file{sysdeps} to use. @ref{Porting to Unix}, has some tips on porting
739 the library to Unix variants.
743 * Hierarchy Conventions:: The layout of the @file{sysdeps} hierarchy.
744 * Porting to Unix:: Porting the library to an average
748 @node Hierarchy Conventions
749 @appendixsubsec Layout of the @file{sysdeps} Directory Hierarchy
751 A GNU configuration name has three parts: the CPU type, the
752 manufacturer's name, and the operating system. @file{configure} uses
753 these to pick the list of system-dependent directories to look for. If
754 the @samp{--nfp} option is @emph{not} passed to @file{configure}, the
755 directory @file{@var{machine}/fpu} is also used. The operating system
756 often has a @dfn{base operating system}; for example, if the operating
757 system is @samp{Linux}, the base operating system is @samp{unix/sysv}.
758 The algorithm used to pick the list of directories is simple:
759 @file{configure} makes a list of the base operating system,
760 manufacturer, CPU type, and operating system, in that order. It then
761 concatenates all these together with slashes in between, to produce a
762 directory name; for example, the configuration @w{@samp{i686-linux-gnu}}
763 results in @file{unix/sysv/linux/i386/i686}. @file{configure} then
764 tries removing each element of the list in turn, so
765 @file{unix/sysv/linux} and @file{unix/sysv} are also tried, among others.
766 Since the precise version number of the operating system is often not
767 important, and it would be very inconvenient, for example, to have
768 identical @file{irix6.2} and @file{irix6.3} directories,
769 @file{configure} tries successively less specific operating system names
770 by removing trailing suffixes starting with a period.
772 As an example, here is the complete list of directories that would be
773 tried for the configuration @w{@samp{i686-linux-gnu}}:
777 sysdeps/unix/sysv/linux/i386
778 sysdeps/unix/sysv/linux
783 sysdeps/unix/sysv/i386/i686
784 sysdeps/unix/sysv/i386
791 sysdeps/libm-i387/i686
801 Different machine architectures are conventionally subdirectories at the
802 top level of the @file{sysdeps} directory tree. For example,
803 @w{@file{sysdeps/sparc}} and @w{@file{sysdeps/m68k}}. These contain
804 files specific to those machine architectures, but not specific to any
805 particular operating system. There might be subdirectories for
806 specializations of those architectures, such as
807 @w{@file{sysdeps/m68k/68020}}. Code which is specific to the
808 floating-point coprocessor used with a particular machine should go in
809 @w{@file{sysdeps/@var{machine}/fpu}}.
811 There are a few directories at the top level of the @file{sysdeps}
812 hierarchy that are not for particular machine architectures.
816 As described above (@pxref{Porting}), this is the subdirectory
817 that every configuration implicitly uses after all others.
820 This directory is for code using the IEEE 754 floating-point format,
821 where the C type @code{float} is IEEE 754 single-precision format, and
822 @code{double} is IEEE 754 double-precision format. Usually this
823 directory is referred to in the @file{Implies} file in a machine
824 architecture-specific directory, such as @file{m68k/Implies}.
827 This directory contains an implementation of a mathematical library
828 usable on platforms which use @w{IEEE 754} conformant floating-point
832 This is a special case. Ideally the code should be in
833 @file{sysdeps/i386/fpu} but for various reasons it is kept aside.
836 This directory contains implementations of things in the library in
837 terms of @sc{POSIX.1} functions. This includes some of the @sc{POSIX.1}
838 functions themselves. Of course, @sc{POSIX.1} cannot be completely
839 implemented in terms of itself, so a configuration using just
840 @file{posix} cannot be complete.
843 This is the directory for Unix-like things. @xref{Porting to Unix}.
844 @file{unix} implies @file{posix}. There are some special-purpose
845 subdirectories of @file{unix}:
849 This directory is for things common to both BSD and System V release 4.
850 Both @file{unix/bsd} and @file{unix/sysv/sysv4} imply @file{unix/common}.
853 This directory is for @code{socket} and related functions on Unix systems.
854 @file{unix/inet/Subdirs} enables the @file{inet} top-level subdirectory.
855 @file{unix/common} implies @file{unix/inet}.
859 This is the directory for things based on the Mach microkernel from CMU
860 (including @gnuhurdsystems{}). Other basic operating systems
861 (VMS, for example) would have their own directories at the top level of
862 the @file{sysdeps} hierarchy, parallel to @file{unix} and @file{mach}.
865 @node Porting to Unix
866 @appendixsubsec Porting @theglibc{} to Unix Systems
868 Most Unix systems are fundamentally very similar. There are variations
869 between different machines, and variations in what facilities are
870 provided by the kernel. But the interface to the operating system
871 facilities is, for the most part, pretty uniform and simple.
873 The code for Unix systems is in the directory @file{unix}, at the top
874 level of the @file{sysdeps} hierarchy. This directory contains
875 subdirectories (and subdirectory trees) for various Unix variants.
877 The functions which are system calls in most Unix systems are
878 implemented in assembly code, which is generated automatically from
879 specifications in files named @file{syscalls.list}. There are several
880 such files, one in @file{sysdeps/unix} and others in its subdirectories.
881 Some special system calls are implemented in files that are named with a
882 suffix of @samp{.S}; for example, @file{_exit.S}. Files ending in
883 @samp{.S} are run through the C preprocessor before being fed to the
886 These files all use a set of macros that should be defined in
887 @file{sysdep.h}. The @file{sysdep.h} file in @file{sysdeps/unix}
888 partially defines them; a @file{sysdep.h} file in another directory must
889 finish defining them for the particular machine and operating system
890 variant. See @file{sysdeps/unix/sysdep.h} and the machine-specific
891 @file{sysdep.h} implementations to see what these macros are and what
894 The system-specific makefile for the @file{unix} directory
895 (@file{sysdeps/unix/Makefile}) gives rules to generate several files
896 from the Unix system you are building the library on (which is assumed
897 to be the target system you are building the library @emph{for}). All
898 the generated files are put in the directory where the object files are
899 kept; they should not affect the source tree itself. The files
900 generated are @file{ioctls.h}, @file{errnos.h}, @file{sys/param.h}, and
901 @file{errlist.c} (for the @file{stdio} section of the library).
904 @c This section might be a good idea if it is finished,
905 @c but there's no point including it as it stands. --rms
906 @c @appendixsec Compatibility with Traditional C
908 @c ??? This section is really short now. Want to keep it? --roland
910 @c It's not anymore true. glibc 2.1 cannot be used with K&R compilers.
913 Although @theglibc{} implements the @w{ISO C} library facilities, you
914 @emph{can} use @theglibc{} with traditional, ``pre-ISO'' C
915 compilers. However, you need to be careful because the content and
916 organization of the @glibcadj{} header files differs from that of
917 traditional C implementations. This means you may need to make changes
918 to your program in order to get it to compile.