(gettext.info)Libraries

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4.10 Preparing Library Sources
==============================

   When you are preparing a library, not a program, for the use of
‘gettext’, only a few details are different.  Here we assume that the
library has a translation domain and a POT file of its own.  (If it uses
the translation domain and POT file of the main program, then the
previous sections apply without changes.)

  1. The library code doesn’t call ‘setlocale (LC_ALL, "")’.  It’s the
     responsibility of the main program to set the locale.  The
     library’s documentation should mention this fact, so that
     developers of programs using the library are aware of it.

  2. The library code doesn’t call ‘textdomain (PACKAGE)’, because it
     would interfere with the text domain set by the main program.

  3. The initialization code for a program was

            setlocale (LC_ALL, "");
            bindtextdomain (PACKAGE, LOCALEDIR);
            textdomain (PACKAGE);

     For a library it is reduced to

            bindtextdomain (PACKAGE, LOCALEDIR);

     If your library’s API doesn’t already have an initialization
     function, you need to create one, containing at least the
     ‘bindtextdomain’ invocation.  However, you usually don’t need to
     export and document this initialization function: It is sufficient
     that all entry points of the library call the initialization
     function if it hasn’t been called before.  The typical idiom used
     to achieve this is a static boolean variable that indicates whether
     the initialization function has been called.  Like this:

          static bool libfoo_initialized;

          static void
          libfoo_initialize (void)
          {
            bindtextdomain (PACKAGE, LOCALEDIR);
            libfoo_initialized = true;
          }

          /* This function is part of the exported API.  */
          struct foo *
          create_foo (...)
          {
            /* Must ensure the initialization is performed.  */
            if (!libfoo_initialized)
              libfoo_initialize ();
            ...
          }

          /* This function is part of the exported API.  The argument must be
             non-NULL and have been created through create_foo().  */
          int
          foo_refcount (struct foo *argument)
          {
            /* No need to invoke the initialization function here, because
               create_foo() must already have been called before.  */
            ...
          }

  4. The usual declaration of the ‘_’ macro in each source file was

          #include <libintl.h>
          #define _(String) gettext (String)

     for a program.  For a library, which has its own translation
     domain, it reads like this:

          #include <libintl.h>
          #define _(String) dgettext (PACKAGE, String)

     In other words, ‘dgettext’ is used instead of ‘gettext’.
     Similarly, the ‘dngettext’ function should be used in place of the
     ‘ngettext’ function.