(gettext) Mark Keywords

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 4.4 How Marks Appear in Sources
 All strings requiring translation should be marked in the C sources.
 Marking is done in such a way that each translatable string appears to
 be the sole argument of some function or preprocessor macro.  There are
 only a few such possible functions or macros meant for translation, and
 their names are said to be marking keywords.  The marking is attached
 to strings themselves, rather than to what we do with them.  This
 approach has more uses.  A blatant example is an error message produced
 by formatting.  The format string needs translation, as well as some
 strings inserted through some `%s' specification in the format, while
 the result from `sprintf' may have so many different instances that it
 is impractical to list them all in some `error_string_out()' routine,
    This marking operation has two goals.  The first goal of marking is
 for triggering the retrieval of the translation, at run time.  The
 keyword is possibly resolved into a routine able to dynamically return
 the proper translation, as far as possible or wanted, for the argument
 string.  Most localizable strings are found in executable positions,
 that is, attached to variables or given as parameters to functions.
 But this is not universal usage, and some translatable strings appear
 in structured initializations.   Special cases.
    The second goal of the marking operation is to help `xgettext' at
 properly extracting all translatable strings when it scans a set of
 program sources and produces PO file templates.
    The canonical keyword for marking translatable strings is `gettext',
 it gave its name to the whole GNU `gettext' package.  For packages
 making only light use of the `gettext' keyword, macro or function, it
 is easily used _as is_.  However, for packages using the `gettext'
 interface more heavily, it is usually more convenient to give the main
 keyword a shorter, less obtrusive name.  Indeed, the keyword might
 appear on a lot of strings all over the package, and programmers
 usually do not want nor need their program sources to remind them
 forcefully, all the time, that they are internationalized.  Further, a
 long keyword has the disadvantage of using more horizontal space,
 forcing more indentation work on sources for those trying to keep them
 within 79 or 80 columns.
    Many packages use `_' (a simple underline) as a keyword, and write
 `_("Translatable string")' instead of `gettext ("Translatable
 string")'.  Further, the coding rule, from GNU standards, wanting that
 there is a space between the keyword and the opening parenthesis is
 relaxed, in practice, for this particular usage.  So, the textual
 overhead per translatable string is reduced to only three characters:
 the underline and the two parentheses.  However, even if GNU `gettext'
 uses this convention internally, it does not offer it officially.  The
 real, genuine keyword is truly `gettext' indeed.  It is fairly easy for
 those wanting to use `_' instead of `gettext' to declare:
      #include <libintl.h>
      #define _(String) gettext (String)
 instead of merely using `#include <libintl.h>'.
    The marking keywords `gettext' and `_' take the translatable string
 as sole argument.  It is also possible to define marking functions that
 take it at another argument position.  It is even possible to make the
 marked argument position depend on the total number of arguments of the
 function call; this is useful in C++.  All this is achieved using
 `xgettext''s `--keyword' option.
    Note also that long strings can be split across lines, into multiple
 adjacent string tokens.  Automatic string concatenation is performed at
 compile time according to ISO C and ISO C++; `xgettext' also supports
 this syntax.
    Later on, the maintenance is relatively easy.  If, as a programmer,
 you add or modify a string, you will have to ask yourself if the new or
 altered string requires translation, and include it within `_()' if you
 think it should be translated.  For example, `"%s"' is an example of
 string _not_ requiring translation.  But `"%s: %d"' _does_ require
 translation, because in French, unlike in English, it's customary to
 put a space before a colon.
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