(gettext.info)Interface to gettext

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11.2.1 The Interface
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   The minimal functionality an interface must have is a) to select a
domain the strings are coming from (a single domain for all programs is
not reasonable because its construction and maintenance is difficult,
perhaps impossible) and b) to access a string in a selected domain.

   This is principally the description of the ‘gettext’ interface.  It
has a global domain which unqualified usages reference.  Of course this
domain is selectable by the user.

     char *textdomain (const char *domain_name);

   This provides the possibility to change or query the current status
of the current global domain of the ‘LC_MESSAGE’ category.  The argument
is a null-terminated string, whose characters must be legal in the use
in filenames.  If the DOMAIN_NAME argument is ‘NULL’, the function
returns the current value.  If no value has been set before, the name of
the default domain is returned: _messages_.  Please note that although
the return value of ‘textdomain’ is of type ‘char *’ no changing is
allowed.  It is also important to know that no checks of the
availability are made.  If the name is not available you will see this
by the fact that no translations are provided.

To use a domain set by ‘textdomain’ the function

     char *gettext (const char *msgid);

is to be used.  This is the simplest reasonable form one can imagine.
The translation of the string MSGID is returned if it is available in
the current domain.  If it is not available, the argument itself is
returned.  If the argument is ‘NULL’ the result is undefined.

   One thing which should come into mind is that no explicit dependency
to the used domain is given.  The current value of the domain is used.
If this changes between two executions of the same ‘gettext’ call in the
program, both calls reference a different message catalog.

   For the easiest case, which is normally used in internationalized
packages, once at the beginning of execution a call to ‘textdomain’ is
issued, setting the domain to a unique name, normally the package name.
In the following code all strings which have to be translated are
filtered through the gettext function.  That’s all, the package speaks
your language.