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GNU gettext utilities

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1.5 Overview of GNU gettext

The following diagram summarizes the relation between the files handled by GNU gettext and the tools acting on these files. It is followed by somewhat detailed explanations, which you should read while keeping an eye on the diagram. Having a clear understanding of these interrelations will surely help programmers, translators and maintainers.

Original C Sources ---> PO mode ---> Marked C Sources ---.
              .---------<--- GNU gettext Library         |
.--- make <---+                                          |
|             `---------<--------------------+-----------'
|                                            |
|   .-----<--- PACKAGE.pot <--- xgettext <---'   .---<--- PO Compendium
|   |                                            |             ^
|   |                                            `---.         |
|   `---.                                            +---> PO mode ---.
|       +----> msgmerge ------> LANG.po ---->--------'                |
|   .---'                                                             |
|   |                                                                 |
|   `-------------<---------------.                                   |
|                                 +--- New LANG.po <------------------'
|   .--- LANG.gmo <--- msgfmt <---'
|   |
|   `---> install ---> /.../LANG/PACKAGE.mo ---.
|                                              +---> "Hello world!"
`-------> install ---> /.../bin/PROGRAM -------'

The indication `PO mode' appears in two places in this picture, and you may safely read it as merely meaning "hand editing", using any editor of your choice, really. However, for those of you being the lucky users of Emacs, PO mode has been specifically created for providing a cozy environment for editing or modifying PO files. While editing a PO file, PO mode allows for the easy browsing of auxiliary and compendium PO files, as well as for following references into the set of C program sources from which PO files have been derived. It has a few special features, among which are the interactive marking of program strings as translatable, and the validation of PO files with easy repositioning to PO file lines showing errors.

As a programmer, the first step to bringing GNU gettext into your package is identifying, right in the C sources, those strings which are meant to be translatable, and those which are untranslatable. This tedious job can be done a little more comfortably using emacs PO mode, but you can use any means familiar to you for modifying your C sources. Beside this some other simple, standard changes are needed to properly initialize the translation library. See section 3. Preparing Program Sources, for more information about all this.

For newly written software the strings of course can and should be marked while writing it. The gettext approach makes this very easy. Simply put the following lines at the beginning of each file or in a central header file:

#define _(String) (String)
#define N_(String) String
#define textdomain(Domain)
#define bindtextdomain(Package, Directory)

Doing this allows you to prepare the sources for internationalization. Later when you feel ready for the step to use the gettext library simply replace these definitions by the following:

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

and link against `libintl.a' or `libintl.so'. Note that on GNU systems, you don't need to link with libintl because the gettext library functions are already contained in GNU libc. That is all you have to change.

Once the C sources have been modified, the xgettext program is used to find and extract all translatable strings, and create a PO template file out of all these. This `package.pot' file contains all original program strings. It has sets of pointers to exactly where in C sources each string is used. All translations are set to empty. The letter t in `.pot' marks this as a Template PO file, not yet oriented towards any particular language. See section 4.1 Invoking the xgettext Program, for more details about how one calls the xgettext program. If you are really lazy, you might be interested at working a lot more right away, and preparing the whole distribution setup (see section 12. The Maintainer's View). By doing so, you spare yourself typing the xgettext command, as make should now generate the proper things automatically for you!

The first time through, there is no `lang.po' yet, so the msgmerge step may be skipped and replaced by a mere copy of `package.pot' to `lang.po', where lang represents the target language. See 5. Creating a New PO File for details.

Then comes the initial translation of messages. Translation in itself is a whole matter, still exclusively meant for humans, and whose complexity far overwhelms the level of this manual. Nevertheless, a few hints are given in some other chapter of this manual (see section 11. The Translator's View). You will also find there indications about how to contact translating teams, or becoming part of them, for sharing your translating concerns with others who target the same native language.

While adding the translated messages into the `lang.po' PO file, if you do not have Emacs handy, you are on your own for ensuring that your efforts fully respect the PO file format, and quoting conventions (see section 2.2 The Format of PO Files). This is surely not an impossible task, as this is the way many people have handled PO files already for Uniforum or Solaris. On the other hand, by using PO mode in Emacs, most details of PO file format are taken care of for you, but you have to acquire some familiarity with PO mode itself. Besides main PO mode commands (see section 2.3 Main PO mode Commands), you should know how to move between entries (see section 2.4 Entry Positioning), and how to handle untranslated entries (see section 6.4 Untranslated Entries).

If some common translations have already been saved into a compendium PO file, translators may use PO mode for initializing untranslated entries from the compendium, and also save selected translations into the compendium, updating it (see section 6.11 Using Translation Compendia). Compendium files are meant to be exchanged between members of a given translation team.

Programs, or packages of programs, are dynamic in nature: users write bug reports and suggestion for improvements, maintainers react by modifying programs in various ways. The fact that a package has already been internationalized should not make maintainers shy of adding new strings, or modifying strings already translated. They just do their job the best they can. For the Translation Project to work smoothly, it is important that maintainers do not carry translation concerns on their already loaded shoulders, and that translators be kept as free as possible of programming concerns.

The only concern maintainers should have is carefully marking new strings as translatable, when they should be, and do not otherwise worry about them being translated, as this will come in proper time. Consequently, when programs and their strings are adjusted in various ways by maintainers, and for matters usually unrelated to translation, xgettext would construct `package.pot' files which are evolving over time, so the translations carried by `lang.po' are slowly fading out of date.

It is important for translators (and even maintainers) to understand that package translation is a continuous process in the lifetime of a package, and not something which is done once and for all at the start. After an initial burst of translation activity for a given package, interventions are needed once in a while, because here and there, translated entries become obsolete, and new untranslated entries appear, needing translation.

The msgmerge program has the purpose of refreshing an already existing `lang.po' file, by comparing it with a newer `package.pot' template file, extracted by xgettext out of recent C sources. The refreshing operation adjusts all references to C source locations for strings, since these strings move as programs are modified. Also, msgmerge comments out as obsolete, in `lang.po', those already translated entries which are no longer used in the program sources (see section 6.5 Obsolete Entries). It finally discovers new strings and inserts them in the resulting PO file as untranslated entries (see section 6.4 Untranslated Entries). See section 6.1 Invoking the msgmerge Program, for more information about what msgmerge really does.

Whatever route or means taken, the goal is to obtain an updated `lang.po' file offering translations for all strings.

The temporal mobility, or fluidity of PO files, is an integral part of the translation game, and should be well understood, and accepted. People resisting it will have a hard time participating in the Translation Project, or will give a hard time to other participants! In particular, maintainers should relax and include all available official PO files in their distributions, even if these have not recently been updated, without exerting pressure on the translator teams to get the job done. The pressure should rather come from the community of users speaking a particular language, and maintainers should consider themselves fairly relieved of any concern about the adequacy of translation files. On the other hand, translators should reasonably try updating the PO files they are responsible for, while the package is undergoing pretest, prior to an official distribution.

Once the PO file is complete and dependable, the msgfmt program is used for turning the PO file into a machine-oriented format, which may yield efficient retrieval of translations by the programs of the package, whenever needed at runtime (see section 8.3 The Format of GNU MO Files). See section 8.1 Invoking the msgfmt Program, for more information about all modes of execution for the msgfmt program.

Finally, the modified and marked C sources are compiled and linked with the GNU gettext library, usually through the operation of make, given a suitable `Makefile' exists for the project, and the resulting executable is installed somewhere users will find it. The MO files themselves should also be properly installed. Given the appropriate environment variables are set (see section 9.3 Magic for End Users), the program should localize itself automatically, whenever it executes.

The remainder of this manual has the purpose of explaining in depth the various steps outlined above.

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