GNU gettext utilities - The Programmer's View

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# The Programmer's View

One aim of the current message catalog implementation provided by GNU `gettext` was to use the systems message catalog handling, if the installer wishes to do so. So we perhaps should first take a look at the solutions we know about. The people in the POSIX committee does not manage to agree on one of the semi-official standards which we'll describe below. In fact they couldn't agree on anything, so nothing decide only to include an example of an interface. The major Unix vendors are split in the usage of the two most important specifications: X/Opens catgets vs. Uniforums gettext interface. We'll describe them both and later explain our solution of this dilemma.

## About `catgets`

The `catgets` implementation is defined in the X/Open Portability Guide, Volume 3, XSI Supplementary Definitions, Chapter 5. But the process of creating this standard seemed to be too slow for some of the Unix vendors so they created their implementations on preliminary versions of the standard. Of course this leads again to problems while writing platform independent programs: even the usage of `catgets` does not guarantee a unique interface.

Another, personal comment on this that only a bunch of committee members could have made this interface. They never really tried to program using this interface. It is a fast, memory-saving implementation, an user can happily live with it. But programmers hate it (at least me and some others do...)

But we must not forget one point: after all the trouble with transfering the rights on Unix(tm) they at last came to X/Open, the very same who published this specifications. This leads me to making the prediction that this interface will be in future Unix standards (e.g. Spec1170) and therefore part of all Unix implementation (implementations, which are allowed to wear this name).

### The Interface

The interface to the `catgets` implementation consists of three functions which correspond to those used in file access: `catopen` to open the catalog for using, `catgets` for accessing the message tables, and `catclose` for closing after work is done. Prototypes for the functions and the needed definitions are in the `<nl_types.h>` header file.

`catopen` is used like in this:

    nl_catd catd = catopen ("catalog_name", 0);

The function takes as the argument the name of the catalog. This usual refers to the name of the program or the package. The second parameter is not further specified in the standard. I don't even know whether it is implemented consistently among various systems. So the common advice is to use `0` as the value. The return value is a handle to the message catalog, equivalent to handles to file returned by `open`.

This handle is of course used in the `catgets` function which can be used like this:

    char *translation = catgets (catd, set_no, msg_id, "original string");

The first parameter is this catalog descriptor. The second parameter specifies the set of messages in this catalog, in which the message described by `msg_id` is obtained. `catgets` therefore uses a three-stage addressing:

    catalog name => set number => message ID => translation

The fourth argument is not used to address the translation. It is given as a default value in case when one of the addressing stages fail. One important thing to remember is that although the return type of catgets is `char *` the resulting string must not be changed. It should better `const char *`, but the standard is published in 1988, one year before ANSI C.

The last of these function functions is used and behaves as expected:

    catclose (catd);

After this no `catgets` call using the descriptor is legal anymore.

### Problems with the `catgets` Interface?!

Now that this descriptions seemed to be really easy where are the problem we speak of. In fact the interface could be used in a reasonable way, but constructing the message catalogs is a pain. The reason for this lies in the third argument of `catgets`: the unique message ID. This has to be a numeric value for all messages in a single set. Perhaps you could imagine the problems keeping such list while changing the source code. Add a new message here, remove one there. Of course there have been developed a lot of tools helping to organize this chaos but one as the other fails in one aspect or the other. We don't want to say that the other approach has no problems but they are far more easily to manage.

## About `gettext`

The definition of the `gettext` interface comes from a Uniforum proposal and it is followed by at least one major Unix vendor (Sun) in its last developments. It is not specified in any official standard, though.

The main points about this solution is that it does not follow the method of normal file handling (open-use-close) and that it does not burden the programmer so many task, especially the unique key handling. Of course here is also a unique key needed, but this key is the message itself (how long or short it is). See section Comparing the Two Interfaces for a more detailed comparison of the two methods.

The following section contains a rather detailed description of the interface. We make it that detailed because this is the interface we chose for the GNU `gettext` Library. Programmers interested in using this library will be interested in this description.

### The Interface

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 an 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 return 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 not available the argument itself is returned. If the argument is `NULL` the result is undefined.

One things which should come into mind is that no explicit dependency to the used domain is given. The current value of the domain for the `LC_MESSAGES` locale 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.

### Solving Ambiguities

While this single name domain work good for most applications there might be the need to get translations from more than one domain. Of course one could switch between different domains with calls to `textdomain`, but this is really not convenient nor is it fast. A possible situation could be one case discussing while this writing: all error messages of functions in the set of common used functions should go into a separate domain `error`. By this mean we would only need to translate them once.

For this reasons there are two more functions to retrieve strings:

    char *dgettext (const char *domain_name, const char *msgid);
    char *dcgettext (const char *domain_name, const char *msgid,
                     int category);

Both take an additional argument at the first place, which corresponds to the argument of `textdomain`. The third argument of `dcgettext` allows to use another locale but `LC_MESSAGES`. But I really don't know where this can be useful. If the domain_name is `NULL` or category has an value beside the known ones, the result is undefined. It should also be noted that this function is not part of the second known implementation of this function family, the one found in Solaris.

A second ambiguity can arise by the fact, that perhaps more than one domain has the same name. This can be solved by specifying where the needed message catalog files can be found.

    char *bindtextdomain (const char *domain_name,
                          const char *dir_name);

Calling this function binds the given domain to a file in the specified directory (how this file is determined follows below). Especially a file in the systems default place is not favored against the specified file anymore (as it would be by solely using `textdomain`). A `NULL` pointer for the dir_name parameter returns the binding associated with domain_name. If domain_name itself is `NULL` nothing happens and a `NULL` pointer is returned. Here again as for all the other functions is true that none of the return value must be changed!

It is important to remember that relative path names for the dir_name parameter can be trouble. Since the path is always computed relative to the current directory different results will be achieved when the program executes a `chdir` command. Relative paths should always be avoided to avoid dependencies and unreliabilities.

### Locating Message Catalog Files

Because many different languages for many different packages have to be stored we need some way to add these information to file message catalog files. The way usually used in Unix environments is have this encoding in the file name. This is also done here. The directory name given in `bindtextdomain`s second argument (or the default directory), followed by the value and name of the locale and the domain name are concatenated:

    dir_name/locale/LC_category/domain_name.mo

The default value for dir_name is system specific. For the GNU library, and for packages adhering to its conventions, it's:

    /usr/local/share/locale

locale is the value of the locale whose name is this `LC_category`. For `gettext` and `dgettext` this locale is always `LC_MESSAGES`. `dcgettext` specifies the locale by the third argument.(2) (3)

### Optimization of the *gettext functions

At this point of the discussion we should talk about an advantage of the GNU `gettext` implementation. Some readers might have pointed out that an internationalized program might have a poor performance if some string has to be translated in an inner loop. While this is unavoidable when the string varies from one run of the loop to the other it is simply a waste of time when the string is always the same. Take the following example:

    {
      while (...)
        {
          puts (gettext ("Hello world"));
        }
    }

When the locale selection does not change between two runs the resulting string is always the same. One way to use this is:

    {
      str = gettext ("Hello world");
      while (...)
        {
          puts (str);
        }
    }

But this solution is not usable in all situation (e.g. when the locale selection changes) nor is it good readable.

The GNU C compiler, version 2.7 and above, provide another solution for this. To describe this we show here some lines of the ``intl/libgettext.h'` file. For an explanation of the expression command block see section `Statements and Declarations in Expressions' in The GNU CC Manual.

    #  if defined __GNUC__ && __GNUC__ == 2 && __GNUC_MINOR__ >= 7
    extern int _nl_msg_cat_cntr;
    #   define	dcgettext(domainname, msgid, category)           \
      (__extension__                                                 \
       ({                                                            \
         char *result;                                               \
         if (__builtin_constant_p (msgid))                           \
           {                                                         \
             static char *__translation__;                           \
             static int __catalog_counter__;                         \
             if (! __translation__                                   \
                 || __catalog_counter__ != _nl_msg_cat_cntr)         \
               {                                                     \
                 __translation__ =                                   \
                   dcgettext__ ((domainname), (msgid), (category));  \
                 __catalog_counter__ = _nl_msg_cat_cntr;             \
               }                                                     \
             result = __translation__;                               \
           }                                                         \
         else                                                        \
           result = dcgettext__ ((domainname), (msgid), (category)); \
         result;                                                     \
        }))
    #  endif

The interesting thing here is the `__builtin_constant_p` predicate. This is evaluated at compile time and so optimization can take place immediately. Here two cases are distinguished: the argument to `gettext` is not a constant value in which case simply the function `dcgettext__` is called, the real implementation of the `dcgettext` function.

If the string argument is constant we can reuse the once gained translation when the locale selection has not changed. This is exactly what is done here. The `_nl_msg_cat_cntr` variable is defined in the ``loadmsgcat.c'` which is available in ``libintl.a'` and is changed whenever a new message catalog is loaded.

## Comparing the Two Interfaces

The following discussion is perhaps a little bit colored. As said above we implemented GNU `gettext` following the Uniforum proposal and this surely has its reasons. But it should show how we came to this decision.

First we take a look at the developing process. When we write an application using NLS provided by `gettext` we proceed as always. Only when we come to a string which might be seen by the users and thus has to be translated we use `gettext("...")` instead of `"..."`. At the beginning of each source file (or in a central header file) we define

    #define gettext(String) (String)

Even this definition can be avoided when the system supports the `gettext` function in its C library. When we compile this code the result is the same as if no NLS code is used. When you take a look at the GNU `gettext` code you will see that we use `_("...")` instead of `gettext("...")`. This reduces the number of additional characters per translatable string to 3 (in words: three).

When now a production version of the program is needed we simply replace the definition

    #define _(String) (String)

by

    #include <libintl.h>
    #define _(String) gettext (String)

Additionally we run the program ``xgettext'` on all source code file which contain translatable strings and that's it: we have a running program which does not depend on translations to be available, but which can use any that becomes available.

The same procedure can be done for the `gettext_noop` invocations (see section Special Cases of Translatable Strings). First you can define `gettext_noop` to a no-op macro and later use the definition from ``libintl.h'`. Because this name is not used in Suns implementation of ``libintl.h'`, you should consider the following code for your project:

    #ifdef gettext_noop
    # define N_(String) gettext_noop (String)
    #else
    # define N_(String) (String)
    #endif

`N_` is a short form similar to `_`. The ``Makefile'` in the ``po/'` directory of GNU gettext knows by default both of the mentioned short forms so you are invited to follow this proposal for your own ease.

Now to `catgets`. The main problem is the work for the programmer. Every time he comes to a translatable string he has to define a number (or a symbolic constant) which has also be defined in the message catalog file. He also has to take care for duplicate entries, duplicate message IDs etc. If he wants to have the same quality in the message catalog as the GNU `gettext` program provides he also has to put the descriptive comments for the strings and the location in all source code files in the message catalog. This is nearly a Mission: Impossible.

But there are also some points people might call advantages speaking for `catgets`. If you have a single word in a string and this string is used in different contexts it is likely that in one or the other language the word has different translations. Example:

    printf ("%s: %d", gettext ("number"), number_of_errors)

    printf ("you should see %d %s", number_count,
            number_count == 1 ? gettext ("number") : gettext ("numbers"))

Here we have to translate two times the string `"number"`. Even if you do not speak a language beside English it might be possible to recognize that the two words have a different meaning. In German the first appearance has to be translated to `"Anzahl"` and the second to `"Zahl"`.

Now you can say that this example is really esoteric. And you are right! This is exactly how we felt about this problem and decide that it does not weight that much. The solution for the above problem could be very easy:

    printf ("%s %d", gettext ("number:"), number_of_errors)

    printf (number_count == 1 ? gettext ("you should see %d number")
                              : gettext ("you should see %d numbers"),
            number_count)

We believe that we can solve all conflicts with this method. If it is difficult one can also consider changing one of the conflicting string a little bit. But it is not impossible to overcome.

Translator note: It is perhaps appropriate here to tell those English speaking programmers that the plural form of a noun cannot be formed by appending a single `s'. Most other languages use different methods. Even the above form is not general enough to cope with all languages. Rafal Maszkowski <rzm@mat.uni.torun.pl> reports:

> In Polish we use e.g. plik (file) this way:

>     1 plik
>     2,3,4 pliki
>     5-21 pliko'w
>     22-24 pliki
>     25-31 pliko'w

> and so on (o' means 8859-2 oacute which should be rather okreska, similar to aogonek).

A workable approach might be to consider methods like the one used for `LC_TIME` in the POSIX.2 standard. The value of the `alt_digits` field can be up to 100 strings which represent the numbers 1 to 100. Using this in a situation of an internationalized program means that an array of translatable strings should be indexed by the number which should represent. A small example:

    void
    print_month_info (int month)
    {
      const char *month_pos[12] =
      { N_("first"), N_("second"), N_("third"),    N_("fourth"),
        N_("fifth"), N_("sixth"),  N_("seventh"),  N_("eighth"),
        N_("ninth"), N_("tenth"),  N_("eleventh"), N_("twelfth") };
      printf (_("%s is the %s month\n"), nl_langinfo (MON_1 + month),
              _(month_pos[month]));
    }

It should be obvious that this method is only reasonable for small ranges of numbers.

## Using libintl.a in own programs

Starting with version 0.9.4 the library `libintl.h` should be self-contained. I.e., you can use it in your own programs without providing additional functions. The ``Makefile'` will put the header and the library in directories selected using the `$(prefix)`.

One exception of the above is found on HP-UX systems. Here the C library does not contain the `alloca` function (and the HP compiler does not generate it inlined). But it is not intended to rewrite the whole library just because of this dumb system. Instead include the `alloca` function in all package you use the `libintl.a` in.

## Being a `gettext` grok

To fully exploit the functionality of the GNU `gettext` library it is surely helpful to read the source code. But for those who don't want to spend that much time in reading the (sometimes complicated) code here is a list comments:

- Changing the language at runtime For interactive programs it might be useful to offer a selection of the used language at runtime. To understand how to do this one need to know how the used language is determined while executing the `gettext` function. The method which is presented here only works correctly with the GNU implementation of the `gettext` functions. It is not possible with underlying `catgets` functions or `gettext` functions from the systems C library. The exception is of course the GNU C Library which uses the GNU `gettext` Library for message handling. In the function `dcgettext` at every call the current setting of the highest priority environment variable is determined and used. Highest priority means here the following list with decreasing priority:
  1. `LANGUAGE`
  2. `LC_ALL`
  3. `LC_xxx`, according to selected locale
  4. `LANG`
  Afterwards the path is constructed using the found value and the translation file is loaded if available. What is now when the value for, say, `LANGUAGE` changes. According to the process explained above the new value of this variable is found as soon as the `dcgettext` function is called. But this also means the (perhaps) different message catalog file is loaded. In other words: the used language is changed. But there is one little hook. The code for gcc-2.7.0 and up provides some optimization. This optimization normally prevents the calling of the `dcgettext` function as long as no new catalog is loaded. But if `dcgettext` is not called the program also cannot find the `LANGUAGE` variable be changed (see section Optimization of the *gettext functions). A solution for this is very easy. Include the following code in the language switching function.

        /* Change language.  */
        setenv ("LANGUAGE", "fr", 1);

        /* Make change known.  */
        {
          extern int  _nl_msg_cat_cntr;
          ++_nl_msg_cat_cntr;
        }

  The variable `_nl_msg_cat_cntr` is defined in ``loadmsgcat.c'`. The programmer will find himself in need for a construct like this only when developing programs which do run longer and provide the user to select the language at runtime. Non-interactive programs (like all these little Unix tools) should never need this.

## Temporary Notes for the Programmers Chapter

### Temporary - Two Possible Implementations

There are two competing methods for language independent messages: the X/Open `catgets` method, and the Uniforum `gettext` method. The `catgets` method indexes messages by integers; the `gettext` method indexes them by their English translations. The `catgets` method has been around longer and is supported by more vendors. The `gettext` method is supported by Sun, and it has been heard that the COSE multi-vendor initiative is supporting it. Neither method is a POSIX standard; the POSIX.1 committee had a lot of disagreement in this area.

Neither one is in the POSIX standard. There was much disagreement in the POSIX.1 committee about using the `gettext` routines vs. `catgets` (XPG). In the end the committee couldn't agree on anything, so no messaging system was included as part of the standard. I believe the informative annex of the standard includes the XPG3 messaging interfaces, "...as an example of a messaging system that has been implemented..."

They were very careful not to say anywhere that you should use one set of interfaces over the other. For more on this topic please see the Programming for Internationalization FAQ.

### Temporary - About `catgets`

There have been a few discussions of late on the use of `catgets` as a base. I think it important to present both sides of the argument and hence am opting to play devil's advocate for a little bit.

I'll not deny the fact that `catgets` could have been designed a lot better. It currently has quite a number of limitations and these have already been pointed out.

However there is a great deal to be said for consistency and standardization. A common recurring problem when writing Unix software is the myriad portability problems across Unix platforms. It seems as if every Unix vendor had a look at the operating system and found parts they could improve upon. Undoubtedly, these modifications are probably innovative and solve real problems. However, software developers have a hard time keeping up with all these changes across so many platforms.

And this has prompted the Unix vendors to begin to standardize their systems. Hence the impetus for Spec1170. Every major Unix vendor has committed to supporting this standard and every Unix software developer waits with glee the day they can write software to this standard and simply recompile (without having to use autoconf) across different platforms.

As I understand it, Spec1170 is roughly based upon version 4 of the X/Open Portability Guidelines (XPG4). Because `catgets` and friends are defined in XPG4, I'm led to believe that `catgets` is a part of Spec1170 and hence will become a standardized component of all Unix systems.

### Temporary - Why a single implementation

Now it seems kind of wasteful to me to have two different systems installed for accessing message catalogs. If we do want to remedy `catgets` deficiencies why don't we try to expand `catgets` (in a compatible manner) rather than implement an entirely new system. Otherwise, we'll end up with two message catalog access systems installed with an operating system - one set of routines for packages using GNU `gettext` for their internationalization, and another set of routines (catgets) for all other software. Bloated?

Supposing another catalog access system is implemented. Which do we recommend? At least for Linux, we need to attract as many software developers as possible. Hence we need to make it as easy for them to port their software as possible. Which means supporting `catgets`. We will be implementing the `glocale` code within our `libc`, but does this mean we also have to incorporate another message catalog access scheme within our `libc` as well? And what about people who are going to be using the `glocale` + non-`catgets` routines. When they port their software to other platforms, they're now going to have to include the front-end (`glocale`) code plus the back-end code (the non-`catgets` access routines) with their software instead of just including the `glocale` code with their software.

Message catalog support is however only the tip of the iceberg. What about the data for the other locale categories. They also have a number of deficiencies. Are we going to abandon them as well and develop another duplicate set of routines (should `glocale` expand beyond message catalog support)?

Like many parts of Unix that can be improved upon, we're stuck with balancing compatibility with the past with useful improvements and innovations for the future.

### Temporary - Notes

X/Open agreed very late on the standard form so that many implementations differ from the final form. Both of my system (old Linux catgets and Ultrix-4) have a strange variation.

OK. After incorporating the last changes I have to spend some time on making the GNU/Linux `libc` `gettext` functions. So in future Solaris is not the only system having `gettext`.

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