GNU gettext utilities - Preparing Program Sources

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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 @,{c} (c-cedilla character) is runnable in France but not in the U.S.

Some systems also have problems with parsing number using the `scanf` functions if an other but the `LC_ALL` locale is used. The standards say that additional formats but the one known in the `"C"` locale might be recognized. But some systems seem to reject numbers in the `"C"` locale format. In some situation, it might also be a problem with the notation itself which makes it impossible to recognize whether the number is in the `"C"` locale or the local format. This can happen if thousands separator characters are used. Some locales define this character accordfing to the national conventions to `'.'` which is the same character used in the `"C"` locale to denote the decimal point.

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. On all POSIX conformant systems the locale categories `LC_CTYPE`, `LC_COLLATE`, `LC_MONETARY`, `LC_NUMERIC`, and `LC_TIME` are available. On some modern systems there is also a locale `LC_MESSAGES` which is called on some old, XPG2 compliant systems `LC_RESPONSES`.

## How Marks Appears in Sources

All strings requiring translation should be marked in the C sources. 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 impractical 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 in executable positions, that is, attached to variables or given as parameters 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 to give 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 their program sources to remind them forcefully, 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 packages use ``_'` (a simple underline) as a keyword, and write ``_("Translatable string")'` instead of ``gettext ("Translatable string")'`. Further, the coding rule, from GNU standards, 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, targetted 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 packages following the GNU coding standards, 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 too big to fit whole in this window, it is 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 from where it was left 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 command `,` 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 Comments preceding Keywords

In C programs strings are often used within calls of functions from the `printf` family. The special thing about these format strings is that they can contain format specifiers introduced with `%`. Assume we have the code

    printf (gettext ("String `%s' has %d characters\n"), s, strlen (s));

A possible German translation for the above string might be:

    "%d Zeichen lang ist die Zeichenkette `%s'"

A C programmer, even if he cannot speak German, will recognize that there is something wrong here. The order of the two format specifiers is changed but of course the arguments in the `printf` don't have. This will most probably lead to problems because now the length of the string is regarded as the address.

To prevent errors at runtime caused by translations the `msgfmt` tool can check statically whether the arguments in the original and the translation string match in type and number. If this is not the case a warning will be given and the error cannot causes problems at runtime.

If the word order in the above German translation would be correct one would have to write

    "%2$d Zeichen lang ist die Zeichenkette `%1$s'"

The routines in `msgfmt` know about this special notation.

Because not all strings in a program must be format strings it is not useful for `msgfmt` to test all the strings in the ``.po'` file. This might cause problems because the string might contain what looks like a format specifier, but the string is not used in `printf`.

Therefore the `xgettext` adds a special tag to those messages it thinks might be a format string. There is no absolute rule for this, only a heuristic. In the ``.po'` file the entry is marked using the `c-format` flag in the `#,` comment line (see section The Format of PO Files).

The careful reader now might say that this again can cause problems. The heuristic might guess it wrong. This is true and therefore `xgettext` knows about special kind of comment which lets the programmer take over the decision. If in the same line or the immediately preceding line of the `gettext` keyword the `xgettext` program find a comment containing the words `xgettext:c-format` it will mark the string in any case with the `c-format` flag. This kind of comment should be used when `xgettext` does not recognize the string as a format string but is really is one and it should be tested. Please note that when the comment is in the same line of the `gettext` keyword, it must be before the string to be translated.

This situation happens quite often. The `printf` function is often called with strings which do not contain a format specifier. Of course one would normally use `fputs` but it does happen. In this case `xgettext` does not recognize this as a format string but what happens if the translation introduces a valid format specifier? The `printf` function will try to access one of the parameter but none exists because the original code does not refer to any parameter.

`xgettext` of course could make a wrong decision the other way round. A string marked as a format string is not really a format string. In this case the `msgfmt` might give too many warnings and would prevent translating the ``.po'` file. The method to prevent this wrong decision is similar to the one used above, only the comment to use must contain the string `xgettext:no-c-format`.

If a string is marked with `c-format` and this is not correct the user can find out who is responsible for the decision. See section Invoking the `xgettext` Program to see how the `--debug` option can be used for solving this problem.

## 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 be done? 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.

One advantage is that you need not make control flow analysis to make sure the output is really translated in any case. But this analysis is generally not very difficult. If it should be in any situation you can use this second method in this situation.

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