Internationalizing Messages in Linux Programs
If the source code changes, the corresponding .po file should be updated without losing any previous translation. Unfortunately, simply calling xgettext again does not work because it overwrites the old .po file. In this case, the program tupdate comes in handy. It merges two .po files, keeping translations already made, as long as the new strings match with the old. Its syntax is simple:
tupdate new.po old.po > latest.po
New strings will obviously still be empty in latest.po, but already translated ones will be there without the need for reprocessing.
It is not always possible to use the gettext function “straight”. Let's look at the source code excerpt in Listing 3 as an example. Two goals must be reached during the internationalization of this code. First, each translatable string must appear in the .po file. Second, before printing each string at runtime, we must pass it through gettext.
The string "You have %d %s" poses a problem. We cannot simply transform each string declared in item_names in a gettext call, because arrays must be initialized with constant values.
One solution is shown in Listing 4. gettext_noop is a marker used to make the string recognizable by xgettext (that is why it is looked for by default). The translation occurs at run time with the normal gettext call.
The .po files have a very simple text structure and can be modified with any text editor. Among others, Emacs can be put in a special po mode when dealing with them.
Each message file consists of a sequence of records. Each record has this structure:
(blank lines) # optional human comments #. optional automatic comments #: optional source code reference msgid original-string msgstr translated-string
Comments introduced by the translator should have a whitespace immediately following the # character. Automatic comments are produced by xgettext and tupdate to enhance the file's readability and to allow the translator to quickly browse the source code and find the line where a string is used. This is sometimes necessary to produce a correct translation.
Strings are formatted just like C. For example, it is legal to write:
msgid "" "Hello " "world!\n" msgstr "" "Ciao " "mondo!\n"
As you can see, strings may span across lines and the backslash is used to introduce special characters such as tabs and newlines.
No POSIX standard for message catalogs exists—the committee could not agree on anything.
GNU gettext is not the only message catalog system that can be used by an internationalized program. Another library, based on the catgets function call, also exists. The catgets interface is supported by the X/Open consortium, while the gettext interface was first used by Sun.
The main disadvantage of catgets is that a unique identifier must be chosen for each message and passed to catgets each time. This makes it quite difficult to manage a large set of messages, where entries are inserted and deleted on a regular basis. However, GNU gettext can use catgets as an underlying interface on systems that support it.
Linux supports both gettext and catgets interfaces. My personal opinion is the gettext system is much easier to use for both programmers and translators.
All listings referred to in this article are available by anonymous download in the file ftp.linuxjournal.com/pub/lj/listings/issue59/3023.tgz
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