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(libc.info)Locating gettext catalog
How to determine which catalog to be used
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The functions to retrieve the translations for a given message have a
remarkable simple interface. But to provide the user of the program
still the opportunity to select exactly the translation s/he wants and
also to provide the programmer the possibility to influence the way to
locate the search for catalogs files there is a quite complicated
underlying mechanism which controls all this. The code is complicated
the use is easy.
Basically we have two different tasks to perform which can also be
performed by the `catgets' functions:
1. Locate the set of message catalogs. There are a number of files
for different languages and which all belong to the package.
Usually they are all stored in the filesystem below a certain
directory.
There can be arbitrary many packages installed and they can follow
different guidelines for the placement of their files.
2. Relative to the location specified by the package the actual
translation files must be searched, based on the wishes of the
user. I.e., for each language the user selects the program should
be able to locate the appropriate file.
This is the functionality required by the specifications for
`gettext' and this is also what the `catgets' functions are able to do.
But there are some problems unresolved:
* The language to be used can be specified in several different ways.
There is no generally accepted standard for this and the user
always expects the program understand what s/he means. E.g., to
select the German translation one could write `de', `german', or
`deutsch' and the program should always react the same.
* Sometimes the specification of the user is too detailed. If s/he,
e.g., specifies `de_DE.ISO-8859-1' which means German, spoken in
Germany, coded using the ISO 8859-1 character set there is the
possibility that a message catalog matching this exactly is not
available. But there could be a catalog matching `de' and if the
character set used on the machine is always ISO 8859-1 there is no
reason why this later message catalog should not be used. (We
call this "message inheritance".)
* If a catalog for a wanted language is not available it is not
always the second best choice to fall back on the language of the
developer and simply not translate any message. Instead a user
might be better able to read the messages in another language and
so the user of the program should be able to define an precedence
order of languages.
We can divide the configuration actions in two parts: the one is
performed by the programmer, the other by the user. We will start with
the functions the programmer can use since the user configuration will
be based on this.
As the functions described in the last sections already mention
separate sets of messages can be selected by a "domain name". This is a
simple string which should be unique for each program part with uses a
separate domain. It is possible to use in one program arbitrary many
domains at the same time. E.g., the GNU C Library itself uses a domain
named `libc' while the program using the C Library could use a domain
named `foo'. The important point is that at any time exactly one
domain is active. This is controlled with the following function.
- Function: char * textdomain (const char *DOMAINNAME)
The `textdomain' function sets the default domain, which is used in
all future `gettext' calls, to DOMAINNAME. Please note that
`dgettext' and `dcgettext' calls are not influenced if the
DOMAINNAME parameter of these functions is not the null pointer.
Before the first call to `textdomain' the default domain is
`messages'. This is the name specified in the specification of
the `gettext' API. This name is as good as any other name. No
program should ever really use a domain with this name since this
can only lead to problems.
The function returns the value which is from now on taken as the
default domain. If the system went out of memory the returned
value is `NULL' and the global variable ERRNO is set to `ENOMEM'.
Despite the return value type being `char *' the return string must
not be changed. It is allocated internally by the `textdomain'
function.
If the DOMAINNAME parameter is the null pointer no new default
domain is set. Instead the currently selected default domain is
returned.
If the DOMAINNAME parameter is the empty string the default domain
is reset to its initial value, the domain with the name `messages'.
This possibility is questionable to use since the domain `messages'
really never should be used.
- Function: char * bindtextdomain (const char *DOMAINNAME, const char
*DIRNAME)
The `bindtextdomain' function can be used to specify the directory
which contains the message catalogs for domain DOMAINNAME for the
different languages. To be correct, this is the directory where
the hierarchy of directories is expected. Details are explained
below.
For the programmer it is important to note that the translations
which come with the program have be placed in a directory
hierarchy starting at, say, `/foo/bar'. Then the program should
make a `bindtextdomain' call to bind the domain for the current
program to this directory. So it is made sure the catalogs are
found. A correctly running program does not depend on the user
setting an environment variable.
The `bindtextdomain' function can be used several times and if the
DOMAINNAME argument is different the previously bound domains will
not be overwritten.
If the program which wish to use `bindtextdomain' at some point of
time use the `chdir' function to change the current working
directory it is important that the DIRNAME strings ought to be an
absolute pathname. Otherwise the addressed directory might vary
with the time.
If the DIRNAME parameter is the null pointer `bindtextdomain'
returns the currently selected directory for the domain with the
name DOMAINNAME.
The `bindtextdomain' function returns a pointer to a string
containing the name of the selected directory name. The string is
allocated internally in the function and must not be changed by the
user. If the system went out of core during the execution of
`bindtextdomain' the return value is `NULL' and the global
variable ERRNO is set accordingly.
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