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Preparing Program Sources

For the programmer, changes to the C source code fall into three categories. First, you have to make the localization functions known to all modules needing message translation. Second, you should properly trigger the operation of GNU gettext when the program initializes, usually from the main function. Last, you should identify and especially mark all constant strings in your program needing translation.

Presuming that your set of programs, or package, has been adjusted so all needed GNU gettext files are available, and your `Makefile' files are adjusted (see section The Maintainer's View), each C module having translated C strings should contain the line:

#include <libintl.h>

The remaining changes to your C sources are discussed in the further sections of this chapter.

Triggering `gettext` Operations

The initialization of locale data should be done with more or less the same code in every program, as demonstrated below:

int
main (argc, argv)
     int argc;
     char argv;
{
  ...
  setlocale (LC_ALL, "");
  bindtextdomain (PACKAGE, LOCALEDIR);
  textdomain (PACKAGE);
  ...
}

PACKAGE and LOCALEDIR should be provided either by `config.h' or by the Makefile. For now consult the gettext sources for more information.

The use of LC_ALL might not be appropriate for you. LC_ALL includes all locale categories and especially LC_CTYPE. This later category is responsible for determining character classes with the isalnum etc. functions from `ctype.h' which could especially for programs, which process some kind of input language, be wrong. For example this would mean that a source code using the (cedille character) is runnable in France but not in the U.S.

So it is sometimes necessary to replace the LC_ALL line in the code above by a sequence of setlocale lines

{
  ...
  setlocale (LC_TIME, "");
  setlocale (LC_MESSAGES, "");
  ...
}

or to switch for and back to the character class in question.

How Marks Appears in Sources

The C sources should mark all strings requiring translation. Marking is done in such a way that each translatable string appears to be the sole argument of some function or preprocessor macro. There are only a few such possible functions or macros meant for translation, and their names are said to be marking keywords. The marking is attached to strings themselves, rather than to what we do with them. This approach has more uses. A blatant example is an error message produced by formatting. The format string needs translation, as well as some strings inserted through some `%s' specification in the format, while the result from sprintf may have so many different instances that it is unpractical to list them all in some `error_string_out()' routine, say.

This marking operation has two goals. The first goal of marking is for triggering the retrieval of the translation, at run time. The keyword are possibly resolved into a routine able to dynamically return the proper translation, as far as possible or wanted, for the argument string. Most localizable strings are found into executable positions, that is, affected to variables or given as parameter to functions. But this is not universal usage, and some translatable strings appear in structured initializations. See section Special Cases of Translatable Strings.

The second goal of the marking operation is to help xgettext at properly extracting all translatable strings when it scans a set of program sources and produces PO file templates.

The canonical keyword for marking translatable strings is `gettext', it gave its name to the whole GNU gettext package. For packages making only light use of the `gettext' keyword, macro or function, it is easily used as is. However, for packages using the gettext interface more heavily, it is usually more convenient giving the main keyword a shorter, less obtrusive name. Indeed, the keyword might appear on a lot of strings all over the package, and programmers usually do not want nor need that their program sources remind them loud, all the time, that they are internationalized. Further, a long keyword has the disadvantage of using more horizontal space, forcing more indentation work on sources for those trying to keep them within 79 or 80 columns.

Many GNU packages use `_' (a simple underline) as a keyword, and write `_("Translatable string")' instead of `gettext ("Translatable string")'. Further, the usual GNU coding rule wanting that there is a space between the keyword and the opening parenthesis is relaxed, in practice, for this particular usage. So, the textual overhead per translatable string is reduced to only three characters: the underline and the two parentheses. However, even if GNU gettext uses this convention internally, it does not offer it officially. The real, genuine keyword is truly `gettext' indeed. It is fairly easy for those wanting to use `_' instead of `gettext' to declare:

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

instead of merely using `#include <libintl.h>'.

Later on, the maintenance is relatively easy. If, as a programmer, you add or modify a string, you will have to ask yourself if the new or altered string requires translation, and include it within `_()' if you think it should be translated. `"%s: %d"' is an example of string not requiring translation!

Marking Translatable Strings

In PO mode, one set of features is meant more for the programmer than for the translator, and allows him to interactively mark which strings, in a set of program sources, are translatable, and which are not. Even if it is a fairly easy job for a programmer to find and mark such strings by other means, using any editor of his choice, PO mode makes this work more comfortable. Further, this gives translators who feel a little like programmers, or programmers who feel a little like translators, a tool letting them work at marking translatable strings in the program sources, while simultaneously producing a set of translation in some language, for the package being internationalized.

The set of program sources, aimed by the PO mode commands describe here, should have an Emacs tags table constructed for your project, prior to using these PO file commands. This is easy to do. In any shell window, change the directory to the root of your project, then execute a command resembling:

etags src/*.[hc] lib/*.[hc]

presuming here you want to process all `.h' and `.c' files from the `src/' and `lib/' directories. This command will explore all said files and create a `TAGS' file in your root directory, somewhat summarizing the contents using a special file format Emacs can understand.

For official GNU packages which follow the GNU coding standard there is a make goal tags or TAGS which construct the tag files in all directories and for all files containing source code.

Once your `TAGS' file is ready, the following commands assist the programmer at marking translatable strings in his set of sources. But these commands are necessarily driven from within a PO file window, and it is likely that you do not even have such a PO file yet. This is not a problem at all, as you may safely open a new, empty PO file, mainly for using these commands. This empty PO file will slowly fill in while you mark strings as translatable in your program sources.

,: Search through program sources for a string which looks like a candidate for translation.
M-,: Mark the last string found with `_()'.
M-.: Mark the last string found with a keyword taken from a set of possible keywords. This command with a prefix allows some management of these keywords.

The , (po-tags-search) command search for the next occurrence of a string which looks like a possible candidate for translation, and displays the program source in another Emacs window, positioned in such a way that the string is near the top of this other window. If the string is to big to fit whole in this window, it is rather positioned so only its end is shown. In any case, the cursor is left in the PO file window. If the shown string would be better presented differently in different native languages, you may mark it using M-, or M-.. Otherwise, you might rather ignore it and skip to the next string by merely repeating the , command.

A string is a good candidate for translation if it contains a sequence of three or more letters. A string containing at most two letters in a row will be considered as a candidate if it has more letters than non-letters. The command disregards strings containing no letters, or isolated letters only. It also disregards strings within comments, or strings already marked with some keyword PO mode knows (see below).

If you have never told Emacs about some `TAGS' file to use, the command will request that you specify one from the minibuffer, the first time you use the command. You may later change your `TAGS' file by using the regular Emacs command M-x visit-tags-table, which will ask you to name the precise `TAGS' file you want to use. See section `Tag Tables' in The Emacs Editor.

Each time you use the , command, the search resumes where it was left over by the previous search, and goes through all program sources, obeying the `TAGS' file, until all sources have been processed. However, by giving a prefix argument to the command (C-u ,), you may request that the search be restarted all over again from the first program source; but in this case, strings that you recently marked as translatable will be automatically skipped.

Using this , command does not prevent using of other regular Emacs tags commands. For example, regular tags-search or tags-query-replace commands may be used without disrupting the independent , search sequence. However, as implemented, the initial , command (or the , command is used with a prefix) might also reinitialize the regular Emacs tags searching to the first tags file, this reinitialization might be considered spurious.

The M-, (po-mark-translatable) command will mark the recently found string with the `_' keyword. The M-. (po-select-mark-and-mark) command will request that you type one keyword from the minibuffer and use that keyword for marking the string. Both commands will automatically create a new PO file untranslated entry for the string being marked, and make it the current entry (making it easy for you to immediately proceed to its translation, if you feel like doing it right away). It is possible that the modifications made to the program source by M-, or M-. render some source line longer than 80 columns, forcing you to break and re-indent this line differently. You may use the o command from PO mode, or any other window changing command from GNU Emacs, to break out into the program source window, and do any needed adjustments. You will have to use some regular Emacs command to return the cursor to the PO file window, if you want commanding , for the next string, say.

The M-. command has a few built-in speedups, so you do not have to explicitly type all keywords all the time. The first such speedup is that you are presented with a preferred keyword, which you may accept by merely typing RET at the prompt. The second speedup is that you may type any non-ambiguous prefix of the keyword you really mean, and the command will complete it automatically for you. This also means that PO mode has to know all your possible keywords, and that it will not accept mistyped keywords.

If you reply ? to the keyword request, the command gives a list of all known keywords, from which you may choose. When the command is prefixed by an argument (C-u M-.), it inhibits updating any program source or PO file buffer, and does some simple keyword management instead. In this case, the command asks for a keyword, written in full, which becomes a new allowed keyword for later M-. commands. Moreover, this new keyword automatically becomes the preferred keyword for later commands. By typing an already known keyword in response to C-u M-., one merely changes the preferred keyword and does nothing more.

All keywords known for M-. are recognized by the , command when scanning for strings, and strings already marked by any of those known keywords are automatically skipped. If many PO files are opened simultaneously, each one has its own independent set of known keywords. There is no provision in PO mode, currently, for deleting a known keyword, you have to quit the file (maybe using q) and reopen it afresh. When a PO file is newly brought up in an Emacs window, only `gettext' and `_' are known as keywords, and `gettext' is preferred for the M-. command. In fact, this is not useful to prefer `_', as this one is already built in the M-, command.

Special Cases of Translatable Strings

The attentive reader might now point out that it is not always possible to mark translatable string with gettext or something like this. Consider the following case:

{
  static const char *messages[] = {
    "some very meaningful message",
    "and another one"
  };
  const char *string;
  ...
  string
    = index > 1 ? "a default message" : messages[index];

  fputs (string);
  ...
}

While it is no problem to mark the string "a default message" it is not possible to mark the string initializers for messages. What is to do? We have to fulfill two tasks. First we have to mark the strings so that the xgettext program (see section Invoking the xgettext Program) can find them, and second we have to translate the string at runtime before printing them.

The first task can be fulfilled by creating a new keyword, which names a no-op. For the second we have to mark all access points to a string from the array. So one solution can look like this:

#define gettext_noop(String) (String)

{
  static const char *messages[] = {
    gettext_noop ("some very meaningful message"),
    gettext_noop ("and another one")
  };
  const char *string;
  ...
  string
    = index > 1 ? gettext ("a default message") : gettext (messages[index]);

  fputs (string);
  ...
}

Please convince yourself that the string which is written by fputs is translated in any case. How to get xgettext know the additional keyword gettext_noop is explained in section Invoking the xgettext Program.

The above is of course not the only solution. You could also come along with the following one:

#define gettext_noop(String) (String)

{
  static const char *messages[] = {
    gettext_noop ("some very meaningful message",
    gettext_noop ("and another one")
  };
  const char *string;
  ...
  string
    = index > 1 ? gettext_noop ("a default message") : messages[index];

  fputs (gettext (string));
  ...
}

But this has some drawbacks. First the programmer has to take care that he uses gettext_noop for the string "a default message". A use of gettext could have in rare cases unpredictable results. The second reason is found in the internals of the GNU gettext Library which will make this solution less efficient.