The catgets function tries to locate the message data file names
cat_name and loads it when found. The return value is of an
opaque type and can be used in calls to the other functions to refer to
this loaded catalog.
The return value is (nl_catd) -1 in case the function failed and
no catalog was loaded. The global variable errno contains a code
for the error causing the failure. But even if the function call
succeeded this does not mean that all messages can be translated.
Locating the catalog file must happen in a way which lets the user of
the program influence the decision. It is up to the user to decide
about the language to use and sometimes it is useful to use alternate
catalog files. All this can be specified by the user by setting some
environment variables.
The first problem is to find out where all the message catalogs are
stored. Every program could have its own place to keep all the
different files but usually the catalog files are grouped by languages
and the catalogs for all programs are kept in the same place.
To tell the catopen function where the catalog for the program
can be found the user can set the environment variable NLSPATH to
a value which describes her/his choice. Since this value must be usable
for different languages and locales it cannot be a simple string.
Instead it is a format string (similar to printf's). An example
is
/usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N
First one can see that more than one directory can be specified (with
the usual syntax of separating them by colons). The next things to
observe are the format string, %L and %N in this case.
The catopen function knows about several of them and the
replacement for all of them is of course different.
%N- This format element is substituted with the name of the catalog file.
This is the value of the cat_name argument given to
catgets.
%L- This format element is substituted with the name of the currently
selected locale for translating messages. How this is determined is
explained below.
%l- (This is the lowercase ell.) This format element is substituted with the
language element of the locale name. The string describing the selected
locale is expected to have the form
lang
[_terr[.codeset]] and this format uses the
first part lang.
%t- This format element is substituted by the territory part terr of
the name of the currently selected locale. See the explanation of the
format above.
%c- This format element is substituted by the codeset part codeset of
the name of the currently selected locale. See the explanation of the
format above.
%%- Since
% is used in a meta character there must be a way to
express the % character in the result itself. Using %%
does this just like it works for printf.
Using NLSPATH allows arbitrary directories to be searched for
message catalogs while still allowing different languages to be used.
If the NLSPATH environment variable is not set, the default value
is
prefix/share/locale/%L/%N:prefix/share/locale/%L/LC_MESSAGES/%N
where prefix is given to configure while installing the GNU
C Library (this value is in many cases /usr or the empty string).
The remaining problem is to decide which must be used. The value
decides about the substitution of the format elements mentioned above.
First of all the user can specify a path in the message catalog name
(i.e., the name contains a slash character). In this situation the
NLSPATH environment variable is not used. The catalog must exist
as specified in the program, perhaps relative to the current working
directory. This situation in not desirable and catalogs names never
should be written this way. Beside this, this behavior is not portable
to all other platforms providing the catgets interface.
Otherwise the values of environment variables from the standard
environment are examined (see Standard Environment). Which
variables are examined is decided by the flag parameter of
catopen. If the value is NL_CAT_LOCALE (which is defined
in nl_types.h) then the catopen function use the name of
the locale currently selected for the LC_MESSAGES category.
If flag is zero the LANG environment variable is examined.
This is a left-over from the early days where the concept of the locales
had not even reached the level of POSIX locales.
The environment variable and the locale name should have a value of the
form lang[_terr[.codeset]] as explained above.
If no environment variable is set the "C" locale is used which
prevents any translation.
The return value of the function is in any case a valid string. Either
it is a translation from a message catalog or it is the same as the
string parameter. So a piece of code to decide whether a
translation actually happened must look like this:
{
char *trans = catgets (desc, set, msg, input_string);
if (trans == input_string)
{
/* Something went wrong. */
}
}
When an error occurred the global variable errno is set to
- EBADF
- The catalog does not exist.
- ENOMSG
- The set/message tuple does not name an existing element in the
message catalog.
While it sometimes can be useful to test for errors programs normally
will avoid any test. If the translation is not available it is no big
problem if the original, untranslated message is printed. Either the
user understands this as well or s/he will look for the reason why the
messages are not translated.
