11.5 Being a gettext grok

NOTE: This documentation section is outdated and needs to be revised.

To fully exploit the functionality of the GNU gettext library it is surely helpful to read the source code. But for those who don’t want to spend that much time in reading the (sometimes complicated) code here is a list comments:

  • Changing the language at runtime

    For interactive programs it might be useful to offer a selection of the used language at runtime. To understand how to do this one need to know how the used language is determined while executing the gettext function. The method which is presented here only works correctly with the GNU implementation of the gettext functions.

    In the function dcgettext at every call the current setting of the highest priority environment variable is determined and used. Highest priority means here the following list with decreasing priority:

    1. LANGUAGE
    2. LC_ALL
    3. LC_xxx, according to selected locale category
    4. LANG

    Afterwards the path is constructed using the found value and the translation file is loaded if available.

    What happens now when the value for, say, LANGUAGE changes? According to the process explained above the new value of this variable is found as soon as the dcgettext function is called. But this also means the (perhaps) different message catalog file is loaded. In other words: the used language is changed.

    But there is one little hook. The code for gcc-2.7.0 and up provides some optimization. This optimization normally prevents the calling of the dcgettext function as long as no new catalog is loaded. But if dcgettext is not called the program also cannot find the LANGUAGE variable be changed (see Optimized gettext). A solution for this is very easy. Include the following code in the language switching function.

      /* Change language.  */
      setenv ("LANGUAGE", "fr", 1);
    
      /* Make change known.  */
      {
        extern int  _nl_msg_cat_cntr;
        ++_nl_msg_cat_cntr;
      }

    The variable _nl_msg_cat_cntr is defined in loadmsgcat.c. You don’t need to know what this is for. But it can be used to detect whether a gettext implementation is GNU gettext and not non-GNU system’s native gettext implementation.