GNU Info
(gawk.info)Explaining gettext
GNU `gettext'
=============
The facilities in GNU `gettext' focus on messages; strings printed
by a program, either directly or via formatting with `printf' or
`sprintf'.(1)
When using GNU `gettext', each application has its own "text
domain". This is a unique name such as `kpilot' or `gawk', that
identifies the application. A complete application may have multiple
components--programs written in C or C++, as well as scripts written in
`sh' or `awk'. All of the components use the same text domain.
To make the discussion concrete, assume we're writing an application
named `guide'. Internationalization consists of the following steps,
in this order:
1. The programmer goes through the source for all of `guide''s
components and marks each string that is a candidate for
translation. For example, `"`-F': option required"' is a good
candidate for translation. A table with strings of option names
is not (e.g., `gawk''s `--profile' option should remain the same,
no matter what the local language).
2. The programmer indicates the application's text domain (`"guide"')
to the `gettext' library, by calling the `textdomain' function.
3. Messages from the application are extracted from the source code
and collected into a Portable Object file (`guide.po'), which
lists the strings and their translations. The translations are
initially empty. The original (usually English) messages serve as
the key for lookup of the translations.
4. For each language with a translator, `guide.po' is copied and
translations are created and shipped with the application.
5. Each language's `.po' file is converted into a binary message
object (`.mo') file. A message object file contains the original
messages and their translations in a binary format that allows
fast lookup of translations at runtime.
6. When `guide' is built and installed, the binary translation files
are installed in a standard place.
7. For testing and development, it is possible to tell `gettext' to
use `.mo' files in a different directory than the standard one by
using the `bindtextdomain' function.
8. At runtime, `guide' looks up each string via a call to `gettext'.
The returned string is the translated string if available, or the
original string if not.
9. If necessary, it is possible to access messages from a different
text domain than the one belonging to the application, without
having to switch the application's default text domain back and
forth.
In C (or C++), the string marking and dynamic translation lookup are
accomplished by wrapping each string in a call to `gettext':
printf(gettext("Don't Panic!\n"));
The tools that extract messages from source code pull out all
strings enclosed in calls to `gettext'.
The GNU `gettext' developers, recognizing that typing `gettext' over
and over again is both painful and ugly to look at, use the macro `_'
(an underscore) to make things easier:
/* In the standard header file: */
#define _(str) gettext(str)
/* In the program text: */
printf(_("Don't Panic!\n"));
This reduces the typing overhead to just three extra characters per
string and is considerably easier to read as well. There are locale
"categories" for different types of locale-related information. The
defined locale categories that `gettext' knows about are:
`LC_MESSAGES'
Text messages. This is the default category for `gettext'
operations, but it is possible to supply a different one
explicitly, if necessary. (It is almost never necessary to supply
a different category.)
`LC_COLLATE'
Text collation information; i.e., how different characters and/or
groups of characters sort in a given language.
`LC_CTYPE'
Character type information (alphabetic, digit, upper- or
lowercase, and so on). This information is accessed via the POSIX
character classes in regular expressions, such as `/[[:alnum:]]/'
(Note: Regular Expression Operators.).
`LC_MONETARY'
Monetary information, such as the currency symbol, and whether the
symbol goes before or after a number.
`LC_NUMERIC'
Numeric information, such as which characters to use for the
decimal point and the thousands separator.(2)
`LC_RESPONSE'
Response information, such as how "yes" and "no" appear in the
local language, and possibly other information as well.
`LC_TIME'
Time and date related information, such as 12- or 24-hour clock,
month printed before or after day in a date, local month
abbreviations, and so on.
`LC_ALL'
All of the above. (Not too useful in the context of `gettext'.)
---------- Footnotes ----------
(1) For some operating systems, the `gawk' port doesn't support GNU
`gettext'. This applies most notably to the PC operating systems. As
such, these features are not available if you are using one of those
operating systems. Sorry.
(2) Americans use a comma every three decimal places and a period
for the decimal point, while many Europeans do exactly the opposite:
`1,234.56' vs. `1.234,56'.
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