##### 8.2.1.1 What has to be done to translate a message?

The `gettext` functions have a very simple interface. The most basic function just takes the string which shall be translated as the argument and it returns the translation. This is fundamentally different from the `catgets` approach where an extra key is necessary and the original string is only used for the error case.

If the string which has to be translated is the only argument this of course means the string itself is the key. I.e., the translation will be selected based on the original string. The message catalogs must therefore contain the original strings plus one translation for any such string. The task of the `gettext` function is it to compare the argument string with the available strings in the catalog and return the appropriate translation. Of course this process is optimized so that this process is not more expensive than an access using an atomic key like in `catgets`.

The `gettext` approach has some advantages but also some disadvantages. Please see the GNU `gettext` manual for a detailed discussion of the pros and cons.

All the definitions and declarations for `gettext` can be found in the `libintl.h` header file. On systems where these functions are not part of the C library they can be found in a separate library named `libintl.a` (or accordingly different for shared libraries).

— Function: char * gettext (const char *msgid)

> The `gettext` function searches the currently selected message catalogs for a string which is equal to msgid. If there is such a string available it is returned. Otherwise the argument string msgid is returned.

> Please note that all though the return value is `char *` the returned string must not be changed. This broken type results from the history of the function and does not reflect the way the function should be used.

> Please note that above we wrote “message catalogs” (plural). This is a specialty of the GNU implementation of these functions and we will say more about this when we talk about the ways message catalogs are selected (see Locating gettext catalog).

> The `gettext` function does not modify the value of the global errno variable. This is necessary to make it possible to write something like

>                 printf (gettext ("Operation failed: %m\n"));

> Here the errno value is used in the `printf` function while processing the `%m` format element and if the `gettext` function would change this value (it is called before `printf` is called) we would get a wrong message.

> So there is no easy way to detect a missing message catalog beside comparing the argument string with the result. But it is normally the task of the user to react on missing catalogs. The program cannot guess when a message catalog is really necessary since for a user who speaks the language the program was developed in does not need any translation.

The remaining two functions to access the message catalog add some functionality to select a message catalog which is not the default one. This is important if parts of the program are developed independently. Every part can have its own message catalog and all of them can be used at the same time. The C library itself is an example: internally it uses the `gettext` functions but since it must not depend on a currently selected default message catalog it must specify all ambiguous information.

— Function: char * dgettext (const char *domainname, const char *msgid)

> The `dgettext` functions acts just like the `gettext` function. It only takes an additional first argument domainname which guides the selection of the message catalogs which are searched for the translation. If the domainname parameter is the null pointer the `dgettext` function is exactly equivalent to `gettext` since the default value for the domain name is used.

> As for `gettext` the return value type is `char *` which is an anachronism. The returned string must never be modified.

— Function: char * dcgettext (const char *domainname, const char *msgid, int category)

> The `dcgettext` adds another argument to those which `dgettext` takes. This argument category specifies the last piece of information needed to localize the message catalog. I.e., the domain name and the locale category exactly specify which message catalog has to be used (relative to a given directory, see below).

> The `dgettext` function can be expressed in terms of `dcgettext` by using

>               dcgettext (domain, string, LC_MESSAGES)

> instead of

>               dgettext (domain, string)

> This also shows which values are expected for the third parameter. One has to use the available selectors for the categories available in `locale.h`. Normally the available values are `LC_CTYPE`, `LC_COLLATE`, `LC_MESSAGES`, `LC_MONETARY`, `LC_NUMERIC`, and `LC_TIME`. Please note that `LC_ALL` must not be used and even though the names might suggest this, there is no relation to the environments variables of this name.

> The `dcgettext` function is only implemented for compatibility with other systems which have `gettext` functions. There is not really any situation where it is necessary (or useful) to use a different value but `LC_MESSAGES` in for the category parameter. We are dealing with messages here and any other choice can only be irritating.

> As for `gettext` the return value type is `char *` which is an anachronism. The returned string must never be modified.

When using the three functions above in a program it is a frequent case that the msgid argument is a constant string. So it is worth to optimize this case. Thinking shortly about this one will realize that as long as no new message catalog is loaded the translation of a message will not change. This optimization is actually implemented by the `gettext`, `dgettext` and `dcgettext` functions.