Gnulib Basics
=============

While portability across operating systems is not one of GNU's primary goals, it has helped introduce many people to the GNU system, and is worthwhile when it can be achieved at a low cost.  This collection helps lower that cost.

Gnulib is intended to be the canonical source for most of the important ``portability'' and/or common files for GNU projects.  These are files intended to be shared at the source level; Gnulib is not a typical library meant to be installed and linked against.  Thus, unlike most projects, Gnulib does not normally generate a source tarball distribution; instead, developers grab modules directly from the source repository.

The easiest, and recommended, way to do this is to use the gnulib-tool script.  Since there is no installation procedure for Gnulib, gnulib-tool needs to be run directly in the directory that contains the Gnulib source code.  You can do this either by specifying the absolute filename of gnulib-tool, or by using a symbolic link from a place inside your PATH to the gnulib-tool file of your preferred Gnulib checkout.  For example:

     $ ln -s $HOME/gnu/src/gnulib.git/gnulib-tool $HOME/bin/gnulib-tool

The home page for Gnulib is http://www.gnu.org/software/gnulib.

git and CVS
===========

Gnulib is available for anonymous checkout.  In any Bourne-shell the following should work:

     $ git clone git://git.sv.gnu.org/gnulib.git

For a read-write checkout you need to have a login on savannah.gnu.org and be a member of the Gnulib project at http://savannah.gnu.org/projects/gnulib.  Then, instead of the URL git://git.sv.gnu.org/gnulib, use the URL ssh://USER@git.sv.gnu.org/srv/git/gnulib where USER is your login name on savannah.gnu.org.

git resources:

Overview:
     http://en.wikipedia.org/wiki/Git_(software)

Homepage:
     http://git.or.cz/

Download:
     http://www.kernel.org/pub/software/scm/git/

Tutorial:
     http://git.or.cz/course/ http://www.kernel.org/pub/software/scm/git/docs/tutorial.html

FAQ:
     http://git.or.cz/gitwiki/GitFaq

When you use git annotate or git blame with Gnulib, it's recommended that you use the -w option, in order to ignore massive whitespace changes that happened in 2009.

CVS checkouts are also supported:

     $ cvs -d :pserver:anonymous@pserver.git.sv.gnu.org:/gnulib.git co -d gnulib HEAD

Gnulib is hosted on savannah.gnu.org.  The project page is http://savannah.gnu.org/projects/gnulib.

Keeping Up-to-date
==================

The best way to work with Gnulib is to check it out of git. Subscribing to the bug-gnulib@gnu.org mailing list will help you to plan when to update your local copy of Gnulib (which you use to maintain your software) from git.  To synchronize, you can use git pull, or cvs update -dP if you are still using CVS.

Sometimes, using an updated version of Gnulib will require you to use newer versions of GNU Automake or Autoconf.  You may find it helpful to join the autotools-announce mailing list to be advised of such changes.

Contributing to Gnulib
======================

All software here is copyrighted by the Free Software Foundation---you need to have filled out an assignment form for a project that uses the module for that contribution to be accepted here.

If you have a piece of code that you would like to contribute, please email bug-gnulib@gnu.org.  You can review the archives, subscribe, etc., via http://lists.gnu.org/mailman/listinfo/bug-gnulib.

Generally we are looking for files that fulfill at least one of the following requirements:

  * If your .c and .h files define functions that are broken or missing on some other system, we should be able to include it.

  * If your functions remove arbitrary limits from existing functions (either under the same name, or as a slightly different name), we should be able to include it.

If your functions define completely new but rarely used functionality, you should probably consider packaging it as a separate library.

Gnulib licensing
----------------

Gnulib contains code both under GPL and LGPL.  Because several packages that use Gnulib are GPL, the files state they are licensed under GPL. However, to support LGPL projects as well, you may use some of the files under LGPL.  The ``License:'' information in the files under modules/ clarifies the real license that applies to the module source.

Keep in mind that if you submit patches to files in Gnulib, you should license them under a compatible license, which means that sometimes the contribution will have to be LGPL, if the original file is available under LGPL via a ``License: LGPL'' information in the projects' modules/ file.

Indent with spaces not TABs
---------------------------

We use space-only indentation in nearly all files. This includes all *.h, *.c, *.y files, except for the regex module. Makefile and ChangeLog files are excluded, since TAB characters are part of their format.

In order to tell your editor to produce space-only indentation, you can use these instructions.

  * For Emacs: Add these lines to your Emacs initialization file ($HOME/.emacs or similar):

          ;; In Gnulib, indent with spaces everywhere (not TABs).
          ;; Exceptions: Makefile and ChangeLog modes.
          (add-hook 'find-file-hook '(lambda ()
            (if (and buffer-file-name
                     (string-match "/gnulib\\>" (buffer-file-name))
                     (not (string-equal mode-name "Change Log"))
                     (not (string-equal mode-name "Makefile")))
                (setq indent-tabs-mode nil))))

  * For vi (vim): Add these lines to your $HOME/.vimrc file:

          " Don't use tabs for indentation. Spaces are nicer to work with.
          set expandtab

     For Makefile and ChangeLog files, compensate this by adding this to your $HOME/.vim/after/indent/make.vim and $HOME/.vim/after/indent/changelog.vim files:

          " Use tabs for indentation, regardless of the global setting.
          set noexpandtab

  * For Eclipse: In the ``Window|Preferences'' dialog (or ``Eclipse|Preferences'' dialog on Mac OS),

      1. Under ``General|Editors|Text Editors'', select the ``Insert spaces for tabs'' checkbox.

      2. Under ``C/C++|Code Style'', select a code style profile that has the ``Indentation|Tab policy'' combobox set to ``Spaces only'', such as the ``GNU [built-in]'' policy.

     If you use the GNU indent program, pass it the option --no-tabs.

How to add a new module
-----------------------

  * Add the header files and source files to lib/.

  * If the module needs configure-time checks, write an Autoconf macro for it in m4/MODULE.m4. See m4/README for details.

  * Write a module description modules/MODULE, based on modules/TEMPLATE.

  * If the module contributes a section to the end-user documentation, put this documentation in doc/MODULE.texi and add it to the ``Files'' section of modules/MODULE.  Most modules don't do this; they have only documentation for the programmer (= Gnulib user).  Such documentation usually goes into the lib/ source files.  It may also go into doc/; but don't add it to the module description in this case.

  * Add the module to the list in MODULES.html.sh.

You can test that a module builds correctly with:

     $ ./gnulib-tool --create-testdir --dir=/tmp/testdir module1 ... moduleN
     $ cd /tmp/testdir
     $ ./configure && make

Other things:

  * Check the license and copyright year of headers.

  * Check that the source code follows the GNU coding standards; see http://www.gnu.org/prep/standards.

  * Add source files to config/srclist* if they are identical to upstream and should be upgraded in Gnulib whenever the upstream source changes.

  * Include header files in source files to verify the function prototypes.

  * Make sure a replacement function doesn't cause warnings or clashes on systems that have the function.

  * Autoconf functions can use gl_* prefix. The AC_* prefix is for autoconf internal functions.

  * Build files only if they are needed on a platform.  Look at the alloca and fnmatch modules for how to achieve this.  If for some reason you cannot do this, and you have a .c file that leads to an empty .o file on some platforms (through some big #if around all the code), then ensure that the compilation unit is not empty after preprocessing.  One way to do this is to #include <stddef.h> or <stdio.h> before the big #if.

Portability guidelines
----------------------

Gnulib code is intended to be portable to a wide variety of platforms, not just GNU platforms.  See the documentation section ``Target Platforms'' for details.

Many Gnulib modules exist so that applications need not worry about undesirable variability in implementations.  For example, an application that uses the malloc module need not worry about malloc (0) returning NULL on some Standard C platforms; and time_r users need not worry about localtime_r returning int (not char *) on some platforms that predate POSIX 1003.1-2001.

Currently we assume at least a freestanding C89 compiler, possibly operating with a C library that predates C89.  The oldest environments currently ported to are probably HP-UX 10.20 and IRIX 5.3, though we are not testing these platforms very often.

Because we assume a freestanding C89 compiler, Gnulib code can include <float.h>, <limits.h>, <stdarg.h>, and <stddef.h> unconditionally.  It can also assume the existence of <ctime.h>, <errno.h>, <fcntl.h>, <locale.h>, <signal.h>, <stdio.h>, <stdlib.h>, <string.h>, and <time.h>.  Similarly, many modules include <sys/types.h> even though it's not even in C99; that's OK since <sys/types.h> has been around nearly forever.

Even if the include files exist, they may not conform to C89. However, GCC has a fixincludes script that attempts to fix most C89-conformance problems.  So Gnulib currently assumes include files largely conform to C89 or better.  People still using ancient hosts should use fixincludes or fix their include files manually.

Even if the include files conform to C89, the library itself may not. For example, strtod and mktime have some bugs on some platforms. You can work around some of these problems by requiring the relevant modules, e.g., the Gnulib mktime module supplies a working and conforming mktime.

The GNU coding standards allow one departure from strict C99: Gnulib code can assume that standard internal types like size_t are no wider than long.  POSIX 1003.1-2001 and the GNU coding standards both require int to be at least 32 bits wide, so Gnulib code assumes this as well.  Gnulib code makes the following additional assumptions:

  * Signed integer arithmetic is two's complement.

     Previously, Gnulib code sometimes assumed that signed integer arithmetic wraps around, but modern compiler optimizations sometimes do not guarantee this, and Gnulib code with this assumption is now considered to be questionable. See Integer Properties.

     Some Gnulib modules contain explicit support for the other signed integer representations allowed by C99 (ones' complement and signed magnitude), but these modules are the exception rather than the rule. All practical Gnulib targets use two's complement.

  * There are no ``holes'' in integer values: all the bits of an integer contribute to its value in the usual way.

  * Addresses and sizes behave as if objects reside in a flat address space. In particular:

       * If two nonoverlapping objects have sizes S and T represented as size_t values, then S + T cannot overflow.

       * A pointer P points within an object O if and only if (char *) &O <= (char *) P && (char *) P < (char *) (&O + 1).

       * If an existing object has size S, and if T is sufficiently small (e.g., 8 KiB), then S + T cannot overflow. Overflow in this case would mean that the rest of your program fits into T bytes, which can't happen in realistic flat-address-space hosts.

       * Objects with all bits zero are treated as 0 or NULL.  For example, memset (A, 0, sizeof A) initializes an array A of pointers to NULL.

       * Adding zero to a null pointer does not change the pointer. For example, 0 + (char *) NULL == (char *) NULL.

The above assumptions are not required by the C or POSIX standards but hold on all practical porting targets that we're familiar with.  If you have a porting target where these assumptions are not true, we'd appreciate hearing of any fixes.  We need fixes that do not increase runtime overhead on standard hosts and that are relatively easy to maintain.

With the above caveats, Gnulib code should port without problem to new hosts, e.g., hosts conforming to C99 or to recent POSIX standards. Hence Gnulib code should avoid using constructs (e.g., undeclared functions return int) that do not conform to C99.

High Quality
============

We develop and maintain a testsuite for Gnulib.  The goal is to have a 100% firm interface so that maintainers can feel free to update to the code in git at any time and know that their application will not break.  This means that before any change can be committed to the repository, a test suite program must be produced that exposes the bug for regression testing.  All experimental work should be done on branches to help promote this.