Common Usage - The GNU C Library

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#### 8.1.4 How to use the `catgets` interface

The `catgets` functions can be used in two different ways. By following slavishly the X/Open specs and not relying on the extension and by using the GNU extensions. We will take a look at the former method first to understand the benefits of extensions.

##### 8.1.4.1 Not using symbolic names

Since the X/Open format of the message catalog files does not allow symbol names we have to work with numbers all the time. When we start writing a program we have to replace all appearances of translatable strings with something like

         catgets (catdesc, set, msg, "string")

catgets is retrieved from a call to `catopen` which is normally done once at the program start. The `"string"` is the string we want to translate. The problems start with the set and message numbers.

In a bigger program several programmers usually work at the same time on the program and so coordinating the number allocation is crucial. Though no two different strings must be indexed by the same tuple of numbers it is highly desirable to reuse the numbers for equal strings with equal translations (please note that there might be strings which are equal in one language but have different translations due to difference contexts).

The allocation process can be relaxed a bit by different set numbers for different parts of the program. So the number of developers who have to coordinate the allocation can be reduced. But still lists must be keep track of the allocation and errors can easily happen. These errors cannot be discovered by the compiler or the `catgets` functions. Only the user of the program might see wrong messages printed. In the worst cases the messages are so irritating that they cannot be recognized as wrong. Think about the translations for `"true"` and `"false"` being exchanged. This could result in a disaster.

##### 8.1.4.2 Using symbolic names

The problems mentioned in the last section derive from the fact that:

1. the numbers are allocated once and due to the possibly frequent use of them it is difficult to change a number later.
2. the numbers do not allow to guess anything about the string and therefore collisions can easily happen.

By constantly using symbolic names and by providing a method which maps the string content to a symbolic name (however this will happen) one can prevent both problems above. The cost of this is that the programmer has to write a complete message catalog file while s/he is writing the program itself.

This is necessary since the symbolic names must be mapped to numbers before the program sources can be compiled. In the last section it was described how to generate a header containing the mapping of the names. E.g., for the example message file given in the last section we could call the `gencat` program as follow (assume `ex.msg` contains the sources).

         gencat -H ex.h -o ex.cat ex.msg

This generates a header file with the following content:

         #define SetTwoSet 0x2   /* ex.msg:8 */

         #define SetOneSet 0x1   /* ex.msg:4 */
         #define SetOnetwo 0x2   /* ex.msg:6 */

As can be seen the various symbols given in the source file are mangled to generate unique identifiers and these identifiers get numbers assigned. Reading the source file and knowing about the rules will allow to predict the content of the header file (it is deterministic) but this is not necessary. The `gencat` program can take care for everything. All the programmer has to do is to put the generated header file in the dependency list of the source files of her/his project and to add a rules to regenerate the header of any of the input files change.

One word about the symbol mangling. Every symbol consists of two parts: the name of the message set plus the name of the message or the special string `Set`. So `SetOnetwo` means this macro can be used to access the translation with identifier `two` in the message set `SetOne`.

The other names denote the names of the message sets. The special string `Set` is used in the place of the message identifier.

If in the code the second string of the set `SetOne` is used the C code should look like this:

         catgets (catdesc, SetOneSet, SetOnetwo,
                  "   Message with ID \"two\", which gets the value 2 assigned")

Writing the function this way will allow to change the message number and even the set number without requiring any change in the C source code. (The text of the string is normally not the same; this is only for this example.)

##### 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.