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The GNU C Library

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8.1.4.3 How does to this allow to develop

To illustrate the usual way to work with the symbolic version numbers here is a little example. Assume we want to write the very complex and famous greeting program. We start by writing the code as usual:

 
#include <stdio.h>
int
main (void)
{
  printf ("Hello, world!\n");
  return 0;
}

Now we want to internationalize the message and therefore replace the message with whatever the user wants.

 
#include <nl_types.h>
#include <stdio.h>
#include "msgnrs.h"
int
main (void)
{
  nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE);
  printf (catgets (catdesc, SetMainSet, SetMainHello,
                   "Hello, world!\n"));
  catclose (catdesc);
  return 0;
}

We see how the catalog object is opened and the returned descriptor used in the other function calls. It is not really necessary to check for failure of any of the functions since even in these situations the functions will behave reasonable. They simply will be return a translation.

What remains unspecified here are the constants SetMainSet and SetMainHello. These are the symbolic names describing the message. To get the actual definitions which match the information in the catalog file we have to create the message catalog source file and process it using the gencat program.

 
$ Messages for the famous greeting program.
$quote "

$set Main
Hello "Hallo, Welt!\n"

Now we can start building the program (assume the message catalog source file is named `hello.msg' and the program source file `hello.c'):

 
% gencat -H msgnrs.h -o hello.cat hello.msg
% cat msgnrs.h
#define MainSet 0x1     /* hello.msg:4 */
#define MainHello 0x1   /* hello.msg:5 */
% gcc -o hello hello.c -I.
% cp hello.cat /usr/share/locale/de/LC_MESSAGES
% echo $LC_ALL
de
% ./hello
Hallo, Welt!
%

The call of the gencat program creates the missing header file `msgnrs.h' as well as the message catalog binary. The former is used in the compilation of `hello.c' while the later is placed in a directory in which the catopen function will try to locate it. Please check the LC_ALL environment variable and the default path for catopen presented in the description above.


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