GNU Info
(libc.info)Using gettextized software
User influence on `gettext'
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The last sections described what the programmer can do to
internationalize the messages of the program. But it is finally up to
the user to select the message s/he wants to see. S/He must understand
them.
The POSIX locale model uses the environment variables `LC_COLLATE',
`LC_CTYPE', `LC_MESSAGES', `LC_MONETARY', `NUMERIC', and `LC_TIME' to
select the locale which is to be used. This way the user can influence
lots of functions. As we mentioned above the `gettext' functions also
take advantage of this.
To understand how this happens it is necessary to take a look at the
various components of the filename which gets computed to locate a
message catalog. It is composed as follows:
DIR_NAME/LOCALE/LC_CATEGORY/DOMAIN_NAME.mo
The default value for DIR_NAME is system specific. It is computed
from the value given as the prefix while configuring the C library.
This value normally is `/usr' or `/'. For the former the complete
DIR_NAME is:
/usr/share/locale
We can use `/usr/share' since the `.mo' files containing the message
catalogs are system independent, so all systems can use the same files.
If the program executed the `bindtextdomain' function for the message
domain that is currently handled, the `dir_name' component is exactly
the value which was given to the function as the second parameter.
I.e., `bindtextdomain' allows overwriting the only system dependent and
fixed value to make it possible to address files anywhere in the
filesystem.
The CATEGORY is the name of the locale category which was selected
in the program code. For `gettext' and `dgettext' this is always
`LC_MESSAGES', for `dcgettext' this is selected by the value of the
third parameter. As said above it should be avoided to ever use a
category other than `LC_MESSAGES'.
The LOCALE component is computed based on the category used. Just
like for the `setlocale' function here comes the user selection into
the play. Some environment variables are examined in a fixed order and
the first environment variable set determines the return value of the
lookup process. In detail, for the category `LC_xxx' the following
variables in this order are examined:
`LANGUAGE'
`LC_ALL'
`LC_xxx'
`LANG'
This looks very familiar. With the exception of the `LANGUAGE'
environment variable this is exactly the lookup order the `setlocale'
function uses. But why introducing the `LANGUAGE' variable?
The reason is that the syntax of the values these variables can have
is different to what is expected by the `setlocale' function. If we
would set `LC_ALL' to a value following the extended syntax that would
mean the `setlocale' function will never be able to use the value of
this variable as well. An additional variable removes this problem
plus we can select the language independently of the locale setting
which sometimes is useful.
While for the `LC_xxx' variables the value should consist of exactly
one specification of a locale the `LANGUAGE' variable's value can
consist of a colon separated list of locale names. The attentive
reader will realize that this is the way we manage to implement one of
our additional demands above: we want to be able to specify an ordered
list of language.
Back to the constructed filename we have only one component missing.
The DOMAIN_NAME part is the name which was either registered using the
`textdomain' function or which was given to `dgettext' or `dcgettext'
as the first parameter. Now it becomes obvious that a good choice for
the domain name in the program code is a string which is closely
related to the program/package name. E.g., for the GNU C Library the
domain name is `libc'.
A limit piece of example code should show how the programmer is supposed
to work:
{
setlocale (LC_ALL, "");
textdomain ("test-package");
bindtextdomain ("test-package", "/usr/local/share/locale");
puts (gettext ("Hello, world!"));
}
At the program start the default domain is `messages', and the
default locale is "C". The `setlocale' call sets the locale according
to the user's environment variables; remember that correct functioning
of `gettext' relies on the correct setting of the `LC_MESSAGES' locale
(for looking up the message catalog) and of the `LC_CTYPE' locale (for
the character set conversion). The `textdomain' call changes the
default domain to `test-package'. The `bindtextdomain' call specifies
that the message catalogs for the domain `test-package' can be found
below the directory `/usr/local/share/locale'.
If now the user set in her/his environment the variable `LANGUAGE'
to `de' the `gettext' function will try to use the translations from
the file
/usr/local/share/locale/de/LC_MESSAGES/test-package.mo
From the above descriptions it should be clear which component of
this filename is determined by which source.
In the above example we assumed that the `LANGUAGE' environment
variable to `de'. This might be an appropriate selection but what
happens if the user wants to use `LC_ALL' because of the wider
usability and here the required value is `de_DE.ISO-8859-1'? We
already mentioned above that a situation like this is not infrequent.
E.g., a person might prefer reading a dialect and if this is not
available fall back on the standard language.
The `gettext' functions know about situations like this and can
handle them gracefully. The functions recognize the format of the value
of the environment variable. It can split the value is different pieces
and by leaving out the only or the other part it can construct new
values. This happens of course in a predictable way. To understand
this one must know the format of the environment variable value. There
are two more or less standardized forms:
_X/Open Format_
`language[_territory[.codeset]][@modifier]'
_CEN Format (European Community Standard)_
`language[_territory][+audience][+special][,[sponsor][_revision]]'
The functions will automatically recognize which format is used.
Less specific locale names will be stripped of in the order of the
following list:
1. `revision'
2. `sponsor'
3. `special'
4. `codeset'
5. `normalized codeset'
6. `territory'
7. `audience'/`modifier'
From the last entry one can see that the meaning of the `modifier'
field in the X/Open format and the `audience' format have the same
meaning. Beside one can see that the `language' field for obvious
reasons never will be dropped.
The only new thing is the `normalized codeset' entry. This is
another goodie which is introduced to help reducing the chaos which
derives from the inability of the people to standardize the names of
character sets. Instead of ISO-8859-1 one can often see 8859-1, 88591,
iso8859-1, or iso_8859-1. The `normalized codeset' value is generated
from the user-provided character set name by applying the following
rules:
1. Remove all characters beside numbers and letters.
2. Fold letters to lowercase.
3. If the same only contains digits prepend the string `"iso"'.
So all of the above name will be normalized to `iso88591'. This allows
the program user much more freely choosing the locale name.
Even this extended functionality still does not help to solve the
problem that completely different names can be used to denote the same
locale (e.g., `de' and `german'). To be of help in this situation the
locale implementation and also the `gettext' functions know about
aliases.
The file `/usr/share/locale/locale.alias' (replace `/usr' with
whatever prefix you used for configuring the C library) contains a
mapping of alternative names to more regular names. The system manager
is free to add new entries to fill her/his own needs. The selected
locale from the environment is compared with the entries in the first
column of this file ignoring the case. If they match the value of the
second column is used instead for the further handling.
In the description of the format of the environment variables we
already mentioned the character set as a factor in the selection of the
message catalog. In fact, only catalogs which contain text written
using the character set of the system/program can be used (directly;
there will come a solution for this some day). This means for the user
that s/he will always have to take care for this. If in the collection
of the message catalogs there are files for the same language but coded
using different character sets the user has to be careful.
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