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The GNU C Library

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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.)


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