2 @section Benefits of using Gnulib
4 Gnulib is useful to enhance various aspects of a package:
8 Portability: With Gnulib, a package maintainer can program against the
9 POSIX and GNU libc APIs and nevertheless expect good portability to
10 platforms that don't implement POSIX.
13 Maintainability: When a package uses modules from Gnulib instead of code
14 written specifically for that package, the maintainer has less code to
18 Security: Gnulib provides functions that are immune against vulnerabilities
19 that plague the uses of the corresponding commonplace functions. For
20 example, @code{asprintf}, @code{canonicalize_file_name} are not affected
21 by buffer sizing problems that affect @code{sprintf}, @code{realpath}.
22 @code{openat} does not have the race conditions that @code{open} has. Etc.
25 Reliability: Gnulib provides functions that combine a call to a system
26 function with a check of the result. Examples are @code{xalloc},
27 @code{xprintf}, @code{xstrtod}, @code{xgetcwd}.
30 Structure: Gnulib offers a way to structure code into modules, typically
31 one include file, one source code file, and one autoconf macro for each
32 functionality. Modularity helps maintainability.
35 @node Library vs Reusable Code
36 @section Library vs. Reusable Code
38 Classical libraries are installed as binary object code. Gnulib is
39 different: It is used as a source code library. Each package that uses
40 Gnulib thus ships with part of the Gnulib source code. The used portion
41 of Gnulib is tailored to the package: A build tool, called
42 @code{gnulib-tool}, is provided that copies a tailored subset of Gnulib
45 @node Portability and Application Code
46 @section Portability and Application Code
48 One of the goals of Gnulib is to make portable programming easy, on
49 the basis of the standards relevant for GNU (and Unix). The objective
50 behind that is to avoid a fragmentation of the user community into
51 disjoint user communities according to the operating system, and
52 instead allow synergies between users on different operating systems.
54 Another goal of Gnulib is to provide application code that can be shared
55 between several applications. Some people wonder: "What? glibc doesn't
56 have a function to copy a file?" Indeed, the scope of a system's libc is
57 to implement the relevant standards (ISO C, POSIX) and to provide
58 access functions to the kernel's system calls, and little more.
60 There is no clear borderline between both areas.
62 For example, Gnulib has a facility for generating the name of backup
63 files. While this task is entirely at the application level---no
64 standard specifies an API for it---the na@"{@dotless{i}}ve code has
65 some portability problems because on some platforms the length of file
66 name components is limited to 30 characters or so. Gnulib handles
69 Similarly, Gnulib has a facility for executing a command in a
70 subprocess. It is at the same time a portability enhancement (it
71 works on GNU, Unix, and Windows, compared to the classical
72 @code{fork}/@code{exec} idiom which is not portable to Windows), as well
73 as an application aid: it takes care of redirecting stdin and/or
74 stdout if desired, and emits an error message if the subprocess
77 @node Target Platforms
78 @section Target Platforms
80 Gnulib supports a number of platforms that we call the ``reasonable
81 portability targets''. This class consists of widespread operating systems,
82 for three years after their last availability, or---for proprietary
83 operating systems---as long as the vendor provides commercial support for
84 it. Already existing Gnulib code for older operating systems is usually
85 left in place for longer than these three years. So it comes that programs
86 that use Gnulib run pretty well also on these older operating systems.
88 Some operating systems are not very widespread, but are Free Software and
89 are actively developed. Such platforms are also supported by Gnulib, if
90 that OS's developers community keeps in touch with the Gnulib developers,
91 by providing bug reports, analyses, or patches. For such platforms, Gnulib
92 supports only the versions of the last year or the last few months,
93 depending on the maturity of said OS project, the number of its users, and
94 how often these users upgrade.
96 Niche operating systems are generally unsupported by Gnulib, unless some
97 of their developers or users contribute support to Gnulib.
99 The degree of support Gnulib guarantees for a platform depends on the
100 amount of testing it gets from volunteers. Platforms on which Gnulib
101 is frequently tested are the best supported. Then come platforms with
102 occasional testing, then platforms which are rarely tested. Usually,
103 we fix bugs when they are reported. Except that some rarely tested
104 platforms are also low priority; bug fixes for these platforms can
107 As of 2011, the list of supported platforms is the following:
111 glibc systems. With glibc 2.8 or newer, they are frequently tested. With
112 glibc 2.3 or newer, they are occasionally tested.
114 Mac OS X. In versions 10.5 and 10.6, it's frequently tested. In version
115 10.4, it's rarely tested.
117 FreeBSD 6.0 or newer is occasionally tested. FreeBSD 5.x is rarely tested.
119 NetBSD 5.0 or newer is occasionally tested. NetBSD 3.0 or newer is rarely
122 OpenBSD 4.0 or newer is occasionally tested. OpenBSD 3.8 or newer is rarely
125 AIX 6.1 or newer is occasionally tested. AIX 5.1 or newer is rarely tested.
127 HP-UX 11.11 or newer is occasionally tested. HP-UX 11.00 is rarely tested.
128 HP-UX 10.20 is rarely tested and low priority.
130 IRIX 6.5 is occasionally tested. IRIX 5.3 is rarely tested and low priority.
132 OSF/1 5.1 is occasionally tested. OSF/1 4.0 is rarely tested and low
135 Solaris 8 and newer are occasionally tested. Solaris 7 is rarely tested.
136 Solaris 2.6 and older are rarely tested and low priority.
138 Cygwin 1.7.x is frequently tested. Cygwin 1.5.x is occasionally tested.
140 mingw is frequently tested. But note that some modules are currently
141 unsupported on mingw: @code{mgetgroups}, @code{getugroups}, @code{idcache},
142 @code{userspec}, @code{openpty}, @code{login_tty}, @code{forkpty},
143 @code{pt_chown}, @code{grantpt}, @code{pty}, @code{savewd},
144 @code{mkancesdirs}, @code{mkdir-p}, @code{euidaccess}, @code{faccessat}.
145 The versions of Windows that are supported are Windows XP and newer.
146 Only the latest version of mingw is tested; older versions are not supported.
148 Native Windows, with MSVC as compiler, is rarely tested and low priority.
150 mingw in 64-bit mode is not tested and low priority so far.
152 Interix 6.1 is rarely tested, and requires the @code{suacomp} library
153 (@url{http://sourceforge.net/projects/suacomp/}) in version 0.6.8 or newer.
154 Interix 3.5 is not tested.
156 Haiku is rarely tested, BeOS is not tested and low priority.
158 uClibc on Linux is rarely tested.
160 QNX is not tested and low priority.
163 Gnulib supports these operating systems only in an unvirtualized environment.
164 When you run an OS inside a virtual machine, you have to be aware that the
165 virtual machine can bring in bugs of its own. For example, floating-point
166 operations on Solaris can behave slightly differently in QEMU than on real
167 hardware. And Haiku's @command{bash} program misbehaves in VirtualBox 3,
168 whereas it behaves fine in VirtualBox 4.
170 Similarly, running native Windows binaries on GNU/Linux under WINE is
171 rarely tested and low priority: WINE has a set of behaviours and bugs that
172 is slightly different from native Windows.
174 The following platforms are not supported by Gnulib. The cost of
175 supporting them would exceed the benefit because they are rarely used, or
176 poorly documented, or have been supplanted by other platforms, or diverge
177 too much from POSIX, or some combination of these and other factors.
178 Please don't bother sending us patches for them.
184 DJGPP and EMX (the 32-bit operating systems running in DOS).
186 MSDOS (the 16-bit operating system).
188 Windows Mobile, Symbian OS, iOS.
194 Gnulib is divided into modules. Every module implements a single
195 facility. Modules can depend on other modules.
197 A module consists of a number of files and a module description. The
198 files are copied by @code{gnulib-tool} into the package that will use it,
199 usually verbatim, without changes. Source code files (.h, .c files)
200 reside in the @file{lib/} subdirectory. Autoconf macro files reside in
201 the @file{m4/} subdirectory. Build scripts reside in the
202 @file{build-aux/} subdirectory.
204 The module description contains the list of files; @code{gnulib-tool}
205 copies these files. It contains the module's
206 dependencies; @code{gnulib-tool} installs them as well. It also
207 contains the autoconf macro invocation (usually a single line or
208 nothing at all); @code{gnulib-tool} ensures this is invoked from the
209 package's @file{configure.ac} file. And also a @file{Makefile.am}
210 snippet; @code{gnulib-tool} collects these into a @file{Makefile.am}
211 for the tailored Gnulib part. The module description and include file
212 specification are for documentation purposes; they are combined into
215 The module system serves two purposes:
219 It ensures consistency of the used autoconf macros and @file{Makefile.am}
220 rules with the source code. For example, source code which uses the
221 @code{getopt_long} function---this is a common way to implement parsing
222 of command line options in a way that complies with the GNU standards---needs
223 the source code (@file{lib/getopt.c} and others), the autoconf macro
224 which detects whether the system's libc already has this function (in
225 @file{m4/getopt.m4}), and a few @file{Makefile.am} lines that create the
226 substitute @file{getopt.h} if not. These three pieces belong together.
227 They cannot be used without each other. The module description and
228 @code{gnulib-tool} ensure that they are copied altogether into the
232 It allows for scalability. It is well-known since the inception of the
233 MODULA-2 language around 1978 that dissection into modules with
234 dependencies allows for building large sets of code in a maintainable way.
235 The maintainability comes from the facts that:
239 Every module has a single purpose; you don't worry about other parts of
240 the program while creating, reading or modifying the code of a module.
243 The code you have to read in order to understand a module is limited to
244 the source of the module and the .h files of the modules listed as
245 dependencies. It is for this reason also that we recommend to put the
246 comments describing the functions exported by a module into its .h file.
249 In other words, the module is the elementary unit of code in Gnulib,
250 comparable to a class in object-oriented languages like Java or C#.
253 The module system is the basis of @code{gnulib-tool}. When
254 @code{gnulib-tool} copies a part of Gnulib into a package, it first
255 compiles a module list, starting with the requested modules and adding all
256 the dependencies, and then collects the files, @file{configure.ac}
257 snippets and @file{Makefile.am} snippets.
259 @node Various Kinds of Modules
260 @section Various Kinds of Modules
262 There are modules of various kinds in Gnulib. For a complete list of the
263 modules, see in @file{MODULES.html}.
265 @subsection Support for ISO C or POSIX functions.
267 When a function is not implemented by a system, the Gnulib module provides
268 an implementation under the same name. Examples are the @samp{snprintf}
269 and @samp{readlink} modules.
271 Similarly, when a function is not correctly implemented by a system,
272 Gnulib provides a replacement. For functions, we use the pattern
275 #if !HAVE_WORKING_FOO
281 and implement the @code{foo} function under the name @code{rpl_foo}. This
282 renaming is needed to avoid conflicts at compile time (in case the system
283 header files declare @code{foo}) and at link/run time (because the code
284 making use of @code{foo} could end up residing in a shared library, and
285 the executable program using this library could be defining @code{foo}
288 For header files, such as @code{stdbool.h} or @code{stdint.h}, we provide
289 the substitute only if the system doesn't provide a correct one. The
290 template of this replacement is distributed in a slightly different name,
291 with @samp{.in} inserted before the @samp{.h} extension, so that on
292 systems which do provide a correct
293 header file the system's one is used.
295 @subsection Enhancements of ISO C or POSIX functions
297 These are sometimes POSIX functions with GNU extensions also found in
298 glibc---examples: @samp{getopt}, @samp{fnmatch}---and often new
299 APIs---for example, for all functions that allocate memory in one way
300 or the other, we have variants which also include the error checking
301 against the out-of-memory condition.
303 @subsection Portable general use facilities
305 Examples are a module for copying a file---the portability problems
306 relate to the copying of the file's modification time, access rights,
307 and extended attributes---or a module for extracting the tail
308 component of a file name---here the portability to native Windows
309 requires a different API than the classical POSIX @code{basename} function.
311 @subsection Reusable application code
313 Examples are an error reporting function, a module that allows output of
314 numbers with K/M/G suffixes, or cryptographic facilities.
316 @subsection Object oriented classes
318 Examples are data structures like @samp{list}, or abstract output stream
319 classes that work around the fact that an application cannot implement an
320 stdio @code{FILE} with its logic. Here, while staying in C, we use
321 implementation techniques like tables of function pointers, known from the
322 C++ language or from the Linux kernel.
324 @subsection Interfaces to external libraries
326 Examples are the @samp{iconv} module, which interfaces to the
327 @code{iconv} facility, regardless whether it is contained in libc or in
328 an external @code{libiconv}. Or the @samp{readline} module, which
329 interfaces to the GNU readline library.
331 @subsection Build / maintenance infrastructure
333 An example is the @samp{maintainer-makefile} module, which provides extra
334 Makefile tags for maintaining a package.
336 @node Collaborative Development
337 @section Collaborative Development
339 Gnulib is maintained collaboratively. The mailing list is
340 @code{<bug-gnulib at gnu dot org>}. Be warned that some people on the
341 list may be very active at some times and unresponsive at other times.
343 Every module has one or more maintainers. While issues are discussed
344 collaboratively on the list, the maintainer of a module nevertheless has
345 a veto right regarding changes in his module.
347 All patches should be posted the list, regardless whether they are
348 proposed patches or whether they are committed immediately by the
349 maintainer of the particular module. The purpose is not only to inform
350 the other users of the module, but mainly to allow peer review. It is not
351 uncommon that several people contribute comments or spot bugs after a
354 Conversely, if you are using Gnulib, and a patch is posted that affects
355 one of the modules that your package uses, you have an interest in
356 proofreading the patch.
361 Most modules are under the GPL. Some, mostly modules which can
362 reasonably be used in libraries, are under LGPL. The source files
363 always say "GPL", but the real license specification is in the module
364 description file. If the module description file says "GPL", it means
365 "GPLv3+" (GPLv3 or newer, at the licensee's choice); if it says "LGPL",
366 it means "LGPLv3+" (LGPLv3 or newer, at the licensee's choice).
368 More precisely, the license specification in the module description
369 file applies to the files in @file{lib/} and @file{build-aux/}. Different
370 licenses apply to files in special directories:
374 Module description files are under this copyright:
377 Copyright @copyright{} 20XX--20YY Free Software Foundation, Inc.@*
378 Copying and distribution of this file, with or without modification,
379 in any medium, are permitted without royalty provided the copyright
380 notice and this notice are preserved.
384 Autoconf macro files are under this copyright:
387 Copyright @copyright{} 20XX--20YY Free Software Foundation, Inc.@*
388 This file is free software; the Free Software Foundation
389 gives unlimited permission to copy and/or distribute it,
390 with or without modifications, as long as this notice is preserved.
394 If a license statement is not present in a test module, the test files are
395 under GPL. Even if the corresponding source module is under LGPL, this is
396 not a problem, since compiled tests are not installed by ``make install''.
399 Documentation files are under this copyright:
402 Copyright @copyright{} 2004--20YY Free Software Foundation, Inc.@*
403 Permission is granted to copy, distribute and/or modify this document
404 under the terms of the GNU Free Documentation License, Version 1.3 or
405 any later version published by the Free Software Foundation; with no
406 Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
407 copy of the license is included in the section entitled ``GNU Free
408 Documentation License''.
412 If you want to use some Gnulib modules under LGPL, you can do so by
413 passing the option @samp{--lgpl} to @code{gnulib-tool}. This will
414 replace the GPL header with an LGPL header while copying the source
415 files to your package. Similarly, if you want some Gnulib modules
416 under LGPLv2+ (Lesser GPL version 2.1 or newer), you can do so by
417 passing the option @samp{--lgpl=2} to @code{gnulib-tool}.
419 Keep in mind that when you submit patches to files in Gnulib, you should
420 license them under a compatible license. This means that sometimes the
421 contribution will have to be LGPL, if the original file is available
422 under LGPL. You can find out about it by looking for a "License: LGPL"
423 information in the corresponding module description.
425 @node Steady Development
426 @section Steady Development
428 Gnulib modules are continually adapted, to match new practices, to be
429 consistent with newly added modules, or simply as a response to build
430 failure reports. Gnulib is available in two qualities:
434 There is the newest version of Gnulib from the Git repository.
437 We also make stable releases every two months, at
438 @url{http://erislabs.net/ianb/projects/gnulib/}.
441 If you are willing to report an occasional regression, we recommend to
442 use the newest version always, except in periods of major changes. Most
443 Gnulib users do this. If you prefer stable releases, please use the
444 newest stable release.
449 Gnulib is open in the sense that we gladly accept contributions if they
450 are generally useful, well engineered, and if the contributors have signed
451 the obligatory papers with the FSF.
453 The module system is open in the sense that a package using Gnulib can
456 locally patch or override files in Gnulib,
458 locally add modules that are treated like Gnulib modules by
462 This is achieved by the @samp{--local-dir} option of @code{gnulib-tool}
463 (@pxref{Extending Gnulib}).