gnulib/m4

Many of the files in this directory are shared between the coreutils,
diffutils, tar and gettext packages -- and others, so if you
change them, try to ensure that you don't break those packages.
That's hard without a systematic approach, but here is a set of conventions
that makes it easy.

- The lib/ sources are split into modules.  Usually the module of a
  lib/foo.h and lib/foo.c is called "foo" - not unexpected, hey! -, but
  in more ambiguous cases you can look up the module a file belongs to
  by doing "grep lib/foo.c modules/*".

- For every module there is an autoconf macro file, usually called
  m4/foo.m4 according to the module name.  When you modify lib/foo.h or
  lib/foo.c, remember to modify m4/foo.m4 as well!
  What if you don't find m4/foo.m4? This probably means that the module
  doesn't need autoconf support up to now (again, take a look in modules/*).
  So you might need to create one.

- A module which defines a replacement function (i.e. a function which is
  compiled only on systems which lack it or where it exists but doesn't
  work satisfactorily) has a .m4 file with typically the following structure:

  AC_DEFUN([gl_FUNC_FOO],
  [
    AC_REPLACE_FUNCS(foo)
    if test $ac_cv_func_foo = no; then
      gl_PREREQ_FOO
    fi
  ])

  # Prerequisites of lib/foo.c.
  AC_DEFUN([gl_PREREQ_FOO], [
    dnl Many AC_CHECK_* invocations.
  ])

- A module which is compiled on all platforms can define multiple functions
  and be spread across multiple source files (although each time you do
  this you should consider splitting the module, if the source files could
  be independent). The .m4 file has typically the following structure:

  AC_DEFUN([gl_FOO],
  [
    dnl Prerequisites of lib/foo.c.
    dnl Many AC_CHECK_* invocations.

    dnl Prerequisites of lib/foobar.c.
    dnl Many AC_CHECK_* invocations.
  ])

- When a module FOO depends on a module BAR, you do *not* generally need
  to write

  AC_DEFUN([gl_FOO],
  [
    AC_REQUIRE([gl_BAR])
    ...
  ])

  because the maintainers might want to use locally modified / renamed copies
  of the module BAR.

- If the autoconf tests for the modules FOO and BAR have some checks in
  common, still list them separately. Autoconf has two mechanisms for
  avoiding that a configure file runs the same test twice: AC_REQUIRE
  and AC_CACHE_CHECK. Trying to omit the checks leads to maintenance
  problems: If FOO depends on BAR, and you omit a check from FOO's .m4 file,
  later on, when someone modifies bar.c and removes the check from bar.m4,
  he will not remember that foo.c needs the check as well.

- Now, how can you find the prerequisites of lib/foo.c? Try this:
    "grep '#.*if' lib/foo.c | grep -v endif"
  and for each HAVE_* macro search in the autoconf documentation what could
  be the autoconf macro that provides it. This is only an approximation; in
  general you should look at all preprocessor directives in lib/foo.c.

- In AC_RUN_IFELSE invocations, try to put as much information about failed
  tests as possible in the exit code. The exit code is 0 for success and any
  value between 1 and 127 for failure. The exit code is printed in config.log;
  therefore when an AC_RUN_IFELSE invocation failed, it is possible to analyze
  the failure immediately if sufficient information is contained in the exit
  code.

  For a program that performs a single test, the typical idiom is:

      if (do_test1 ())
        return 1;
      return 0;

  For a test that performs a test with some preparation, the typical idiom is
  to return an enumerated value:

      if (prep1 ())
        return 1;
      else if (prep2 ())
        return 2;
      else if (prep3 ())
        return 3;
      else if (do_test1 ())
        return 4;
      return 0;

  For multiple independent tests in a single program, you can return a bit
  mask with up to 7 bits:

      int result = 0;
      if (do_test1 ())
        result |= 1;
      if (do_test2 ())
        result |= 2;
      if (do_test3 ())
        result |= 4;
      return result;

  For more than 7 independent tests, you have to map some possible test
  failures to same bit.

- After ANY modifications of an m4 file, you should increment its serial
  number (in the first line). Also, if this first line features a particular
  release, _remove_ this release stamp. Example: Change

    # setenv.m4 serial 2 (gettext-0.11.1)

  into

    # setenv.m4 serial 3