Message Translation
*******************

The program's interface with the human should be designed in a way to ease the human the task.  One of the possibilities is to use messages in whatever language the user prefers.

Printing messages in different languages can be implemented in different ways.  One could add all the different languages in the source code and add among the variants every time a message has to be printed.  This is certainly no good solution since extending the set of languages is difficult (the code must be changed) and the code itself can become really big with dozens of message sets.

A better solution is to keep the message sets for each language are kept in separate files which are loaded at runtime depending on the language selection of the user.

The GNU C Library provides two different sets of functions to support message translation.  The problem is that neither of the interfaces is officially defined by the POSIX standard.  The catgets family of functions is defined in the X/Open standard but this is derived from industry decisions and therefore not necessarily based on reasonable decisions.

As mentioned above the message catalog handling provides easy extendibility by using external data files which contain the message translations.  I.e., these files contain for each of the messages used in the program a translation for the appropriate language.  So the tasks of the message handling functions are

  * locate the external data file with the appropriate translations.

  * load the data and make it possible to address the messages

  * map a given key to the translated message

The two approaches mainly differ in the implementation of this last step.  The design decisions made for this influences the whole rest.

X/Open Message Catalog Handling
===============================

The catgets functions are based on the simple scheme:

     Associate every message to translate in the source code with a unique identifier.  To retrieve a message from a catalog file solely the identifier is used.

This means for the author of the program that s/he will have to make sure the meaning of the identifier in the program code and in the message catalogs are always the same.

Before a message can be translated the catalog file must be located. The user of the program must be able to guide the responsible function to find whatever catalog the user wants.  This is separated from what the programmer had in mind.

All the types, constants and functions for the catgets functions are defined/declared in the nl_types.h header file.

The catgets function family
---------------------------

 -- Function: nl_catd catopen (const char *CAT_NAME, int FLAG)
     The catgets function tries to locate the message data file names CAT_NAME and loads it when found.  The return value is of an opaque type and can be used in calls to the other functions to refer to this loaded catalog.

     The return value is (nl_catd) -1 in case the function failed and no catalog was loaded.  The global variable ERRNO contains a code for the error causing the failure.  But even if the function call succeeded this does not mean that all messages can be translated.

     Locating the catalog file must happen in a way which lets the user of the program influence the decision.  It is up to the user to decide about the language to use and sometimes it is useful to use alternate catalog files.  All this can be specified by the user by setting some environment variables.

     The first problem is to find out where all the message catalogs are stored.  Every program could have its own place to keep all the different files but usually the catalog files are grouped by languages and the catalogs for all programs are kept in the same place.

     To tell the catopen function where the catalog for the program can be found the user can set the environment variable NLSPATH to a value which describes her/his choice.  Since this value must be usable for different languages and locales it cannot be a simple string. Instead it is a format string (similar to printf's).  An example is

          /usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N

     First one can see that more than one directory can be specified (with the usual syntax of separating them by colons).  The next things to observe are the format string, %L and %N in this case. The catopen function knows about several of them and the replacement for all of them is of course different.

     %N
          This format element is substituted with the name of the catalog file. This is the value of the CAT_NAME argument given to catgets.

     %L
          This format element is substituted with the name of the currently selected locale for translating messages.  How this is determined is explained below.

     %l
          (This is the lowercase ell.) This format element is substituted with the language element of the locale name.  The string describing the selected locale is expected to have the form LANG[_TERR[.CODESET]] and this format uses the first part LANG.

     %t
          This format element is substituted by the territory part TERR of the name of the currently selected locale.  See the explanation of the format above.

     %c
          This format element is substituted by the codeset part CODESET of the name of the currently selected locale.  See the explanation of the format above.

     %%
          Since % is used in a meta character there must be a way to express the % character in the result itself.  Using %% does this just like it works for printf.

     Using NLSPATH allows arbitrary directories to be searched for message catalogs while still allowing different languages to be used. If the NLSPATH environment variable is not set, the default value is

          PREFIX/share/locale/%L/%N:PREFIX/share/locale/%L/LC_MESSAGES/%N

     where PREFIX is given to configure while installing the GNU C Library (this value is in many cases /usr or the empty string).

     The remaining problem is to decide which must be used.  The value decides about the substitution of the format elements mentioned above. First of all the user can specify a path in the message catalog name (i.e., the name contains a slash character).  In this situation the NLSPATH environment variable is not used.  The catalog must exist as specified in the program, perhaps relative to the current working directory.  This situation in not desirable and catalogs names never should be written this way.  Beside this, this behavior is not portable to all other platforms providing the catgets interface.

     Otherwise the values of environment variables from the standard environment are examined (see Standard Environment).  Which variables are examined is decided by the FLAG parameter of catopen.  If the value is NL_CAT_LOCALE (which is defined in nl_types.h) then the catopen function use the name of the locale currently selected for the LC_MESSAGES category.

     If FLAG is zero the LANG environment variable is examined. This is a left-over from the early days where the concept of the locales had not even reached the level of POSIX locales.

     The environment variable and the locale name should have a value of the form LANG[_TERR[.CODESET]] as explained above. If no environment variable is set the "C" locale is used which prevents any translation.

     The return value of the function is in any case a valid string.  Either it is a translation from a message catalog or it is the same as the STRING parameter.  So a piece of code to decide whether a translation actually happened must look like this:

          {
            char *trans = catgets (desc, set, msg, input_string);
            if (trans == input_string)
              {
                /* Something went wrong.  */
              }
          }

     When an error occurred the global variable ERRNO is set to

     EBADF
          The catalog does not exist.

     ENOMSG
          The set/message tuple does not name an existing element in the message catalog.

     While it sometimes can be useful to test for errors programs normally will avoid any test.  If the translation is not available it is no big problem if the original, untranslated message is printed.  Either the user understands this as well or s/he will look for the reason why the messages are not translated.

Please note that the currently selected locale does not depend on a call to the setlocale function.  It is not necessary that the locale data files for this locale exist and calling setlocale succeeds. The catopen function directly reads the values of the environment variables.

 -- Function: char * catgets (nl_catd CATALOG_DESC, int SET, int MESSAGE, const char *STRING)
     The function catgets has to be used to access the massage catalog previously opened using the catopen function.  The CATALOG_DESC parameter must be a value previously returned by catopen.

     The next two parameters, SET and MESSAGE, reflect the internal organization of the message catalog files.  This will be explained in detail below.  For now it is interesting to know that a catalog can consists of several set and the messages in each thread are individually numbered using numbers.  Neither the set number nor the message number must be consecutive.  They can be arbitrarily chosen. But each message (unless equal to another one) must have its own unique pair of set and message number.

     Since it is not guaranteed that the message catalog for the language selected by the user exists the last parameter STRING helps to handle this case gracefully.  If no matching string can be found STRING is returned.  This means for the programmer that

       * the STRING parameters should contain reasonable text (this also helps to understand the program seems otherwise there would be no hint on the string which is expected to be returned.

       * all STRING arguments should be written in the same language.

It is somewhat uncomfortable to write a program using the catgets functions if no supporting functionality is available.  Since each set/message number tuple must be unique the programmer must keep lists of the messages at the same time the code is written.  And the work between several people working on the same project must be coordinated. We will see some how these problems can be relaxed a bit (see Common Usage).

 -- Function: int catclose (nl_catd CATALOG_DESC)
     The catclose function can be used to free the resources associated with a message catalog which previously was opened by a call to catopen.  If the resources can be successfully freed the function returns 0.  Otherwise it return -1 and the global variable ERRNO is set.  Errors can occur if the catalog descriptor CATALOG_DESC is not valid in which case ERRNO is set to EBADF.

Format of the message catalog files
-----------------------------------

The only reasonable way the translate all the messages of a function and store the result in a message catalog file which can be read by the catopen function is to write all the message text to the translator and let her/him translate them all.  I.e., we must have a file with entries which associate the set/message tuple with a specific translation.  This file format is specified in the X/Open standard and is as follows:

  * Lines containing only whitespace characters or empty lines are ignored.

  * Lines which contain as the first non-whitespace character a $ followed by a whitespace character are comment and are also ignored.

  * If a line contains as the first non-whitespace characters the sequence $set followed by a whitespace character an additional argument is required to follow.  This argument can either be:

       - a number.  In this case the value of this number determines the set to which the following messages are added.

       - an identifier consisting of alphanumeric characters plus the underscore character.  In this case the set get automatically a number assigned. This value is one added to the largest set number which so far appeared.

          How to use the symbolic names is explained in section Common Usage.

          It is an error if a symbol name appears more than once.  All following messages are placed in a set with this number.

  * If a line contains as the first non-whitespace characters the sequence $delset followed by a whitespace character an additional argument is required to follow.  This argument can either be:

       - a number.  In this case the value of this number determines the set which will be deleted.

       - an identifier consisting of alphanumeric characters plus the underscore character.  This symbolic identifier must match a name for a set which previously was defined.  It is an error if the name is unknown.

     In both cases all messages in the specified set will be removed.  They will not appear in the output.  But if this set is later again selected with a $set command again messages could be added and these messages will appear in the output.

  * If a line contains after leading whitespaces the sequence $quote, the quoting character used for this input file is changed to the first non-whitespace character following the $quote.  If no non-whitespace character is present before the line ends quoting is disable.

     By default no quoting character is used.  In this mode strings are terminated with the first unescaped line break.  If there is a $quote sequence present newline need not be escaped.  Instead a string is terminated with the first unescaped appearance of the quote character.

     A common usage of this feature would be to set the quote character to ".  Then any appearance of the " in the strings must be escaped using the backslash (i.e., \" must be written).

  * Any other line must start with a number or an alphanumeric identifier (with the underscore character included).  The following characters (starting after the first whitespace character) will form the string which gets associated with the currently selected set and the message number represented by the number and identifier respectively.

     If the start of the line is a number the message number is obvious.  It is an error if the same message number already appeared for this set.

     If the leading token was an identifier the message number gets automatically assigned.  The value is the current maximum messages number for this set plus one.  It is an error if the identifier was already used for a message in this set.  It is OK to reuse the identifier for a message in another thread.  How to use the symbolic identifiers will be explained below (see Common Usage).  There is one limitation with the identifier: it must not be Set.  The reason will be explained below.

     The text of the messages can contain escape characters.  The usual bunch of characters known from the ISO C language are recognized (\n, \t, \v, \b, \r, \f, \\, and \NNN, where NNN is the octal coding of a character code).

Important: The handling of identifiers instead of numbers for the set and messages is a GNU extension.  Systems strictly following the X/Open specification do not have this feature.  An example for a message catalog file is this:

     $ This is a leading comment.
     $quote "

     $set SetOne
     1 Message with ID 1.
     two "   Message with ID \"two\", which gets the value 2 assigned"

     $set SetTwo
     $ Since the last set got the number 1 assigned this set has number 2.
     4000 "The numbers can be arbitrary, they need not start at one."

This small example shows various aspects:

  * Lines 1 and 9 are comments since they start with $ followed by a whitespace.

  * The quoting character is set to ".  Otherwise the quotes in the message definition would have to be left away and in this case the message with the identifier two would loose its leading whitespace.

  * Mixing numbered messages with message having symbolic names is no problem and the numbering happens automatically.

While this file format is pretty easy it is not the best possible for use in a running program.  The catopen function would have to parser the file and handle syntactic errors gracefully.  This is not so easy and the whole process is pretty slow.  Therefore the catgets functions expect the data in another more compact and ready-to-use file format.  There is a special program gencat which is explained in detail in the next section.

Files in this other format are not human readable.  To be easy to use by programs it is a binary file.  But the format is byte order independent so translation files can be shared by systems of arbitrary architecture (as long as they use the GNU C Library).

Details about the binary file format are not important to know since these files are always created by the gencat program.  The sources of the GNU C Library also provide the sources for the gencat program and so the interested reader can look through these source files to learn about the file format.

Generate Message Catalogs files
-------------------------------

The gencat program is specified in the X/Open standard and the GNU implementation follows this specification and so processes all correctly formed input files.  Additionally some extension are implemented which help to work in a more reasonable way with the catgets functions.

The gencat program can be invoked in two ways:

     `gencat [OPTION]... [OUTPUT-FILE [INPUT-FILE]...]`

This is the interface defined in the X/Open standard.  If no INPUT-FILE parameter is given input will be read from standard input.  Multiple input files will be read as if they are concatenated. If OUTPUT-FILE is also missing, the output will be written to standard output.  To provide the interface one is used to from other programs a second interface is provided.

     `gencat [OPTION]... -o OUTPUT-FILE [INPUT-FILE]...`

The option -o is used to specify the output file and all file arguments are used as input files.

Beside this one can use - or /dev/stdin for INPUT-FILE to denote the standard input.  Corresponding one can use - and /dev/stdout for OUTPUT-FILE to denote standard output.  Using - as a file name is allowed in X/Open while using the device names is a GNU extension.

The gencat program works by concatenating all input files and then merge the resulting collection of message sets with a possibly existing output file.  This is done by removing all messages with set/message number tuples matching any of the generated messages from the output file and then adding all the new messages.  To regenerate a catalog file while ignoring the old contents therefore requires to remove the output file if it exists.  If the output is written to standard output no merging takes place.

The following table shows the options understood by the gencat program.  The X/Open standard does not specify any option for the program so all of these are GNU extensions.

-V
--version
     Print the version information and exit.

-h
--help
     Print a usage message listing all available options, then exit successfully.

--new
     Do never merge the new messages from the input files with the old content of the output files.  The old content of the output file is discarded.

-H
--header=name
     This option is used to emit the symbolic names given to sets and messages in the input files for use in the program.  Details about how to use this are given in the next section.  The NAME parameter to this option specifies the name of the output file.  It will contain a number of C preprocessor #defines to associate a name with a number.

     Please note that the generated file only contains the symbols from the input files.  If the output is merged with the previous content of the output file the possibly existing symbols from the file(s) which generated the old output files are not in the generated header file.

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.

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.

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.)

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.

The Uniforum approach to Message Translation
============================================

Sun Microsystems tried to standardize a different approach to message translation in the Uniforum group.  There never was a real standard defined but still the interface was used in Sun's operation systems. Since this approach fits better in the development process of free software it is also used throughout the GNU package and the GNU gettext package provides support for this outside the GNU C Library.

The code of the libintl from GNU gettext is the same as the code in the GNU C Library.  So the documentation in the GNU gettext manual is also valid for the functionality here.  The following text will describe the library functions in detail.  But the numerous helper programs are not described in this manual.  Instead people should read the GNU gettext manual (see GNU gettext utilities (Native Language Support Library and Tools)). We will only give a short overview.

Though the catgets functions are available by default on more systems the gettext interface is at least as portable as the former.  The GNU gettext package can be used wherever the functions are not available.

The gettext family of functions
-------------------------------

The paradigms underlying the gettext approach to message translations is different from that of the catgets functions the basic functionally is equivalent.  There are functions of the following categories:

What has to be done to translate a message?
...........................................

The gettext functions have a very simple interface.  The most basic function just takes the string which shall be translated as the argument and it returns the translation.  This is fundamentally different from the catgets approach where an extra key is necessary and the original string is only used for the error case.

If the string which has to be translated is the only argument this of course means the string itself is the key.  I.e., the translation will be selected based on the original string.  The message catalogs must therefore contain the original strings plus one translation for any such string.  The task of the gettext function is it to compare the argument string with the available strings in the catalog and return the appropriate translation.  Of course this process is optimized so that this process is not more expensive than an access using an atomic key like in catgets.

The gettext approach has some advantages but also some disadvantages.  Please see the GNU gettext manual for a detailed discussion of the pros and cons.

All the definitions and declarations for gettext can be found in the libintl.h header file.  On systems where these functions are not part of the C library they can be found in a separate library named libintl.a (or accordingly different for shared libraries).

 -- Function: char * gettext (const char *MSGID)
     The gettext function searches the currently selected message catalogs for a string which is equal to MSGID.  If there is such a string available it is returned.  Otherwise the argument string MSGID is returned.

     Please note that all though the return value is char * the returned string must not be changed.  This broken type results from the history of the function and does not reflect the way the function should be used.

     Please note that above we wrote ``message catalogs'' (plural).  This is a specialty of the GNU implementation of these functions and we will say more about this when we talk about the ways message catalogs are selected (see Locating gettext catalog).

     The gettext function does not modify the value of the global ERRNO variable.  This is necessary to make it possible to write something like

            printf (gettext ("Operation failed: %m\n"));

     Here the ERRNO value is used in the printf function while processing the %m format element and if the gettext function would change this value (it is called before printf is called) we would get a wrong message.

     So there is no easy way to detect a missing message catalog beside comparing the argument string with the result.  But it is normally the task of the user to react on missing catalogs.  The program cannot guess when a message catalog is really necessary since for a user who s peaks the language the program was developed in does not need any translation.

The remaining two functions to access the message catalog add some functionality to select a message catalog which is not the default one. This is important if parts of the program are developed independently. Every part can have its own message catalog and all of them can be used at the same time.  The C library itself is an example: internally it uses the gettext functions but since it must not depend on a currently selected default message catalog it must specify all ambiguous information.

 -- Function: char * dgettext (const char *DOMAINNAME, const char *MSGID)
     The dgettext functions acts just like the gettext function.  It only takes an additional first argument DOMAINNAME which guides the selection of the message catalogs which are searched for the translation.  If the DOMAINNAME parameter is the null pointer the dgettext function is exactly equivalent to gettext since the default value for the domain name is used.

     As for gettext the return value type is char * which is an anachronism.  The returned string must never be modified.

 -- Function: char * dcgettext (const char *DOMAINNAME, const char *MSGID, int CATEGORY)
     The dcgettext adds another argument to those which dgettext takes.  This argument CATEGORY specifies the last piece of information needed to localize the message catalog.  I.e., the domain name and the locale category exactly specify which message catalog has to be used (relative to a given directory, see below).

     The dgettext function can be expressed in terms of dcgettext by using

          dcgettext (domain, string, LC_MESSAGES)

     instead of

          dgettext (domain, string)

     This also shows which values are expected for the third parameter.  One has to use the available selectors for the categories available in locale.h.  Normally the available values are LC_CTYPE, LC_COLLATE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and LC_TIME.  Please note that LC_ALL must not be used and even though the names might suggest this, there is no relation to the environments variables of this name.

     The dcgettext function is only implemented for compatibility with other systems which have gettext functions.  There is not really any situation where it is necessary (or useful) to use a different value but LC_MESSAGES in for the CATEGORY parameter.  We are dealing with messages here and any other choice can only be irritating.

     As for gettext the return value type is char * which is an anachronism.  The returned string must never be modified.

When using the three functions above in a program it is a frequent case that the MSGID argument is a constant string.  So it is worth to optimize this case.  Thinking shortly about this one will realize that as long as no new message catalog is loaded the translation of a message will not change.  I.e., the algorithm to determine the translation is deterministic.

Exactly this is what the optimizations implemented in the libintl.h header will use.  Whenever a program is compiler with the GNU C compiler, optimization is selected and the MSGID argument to gettext, dgettext or dcgettext is a constant string the actual function call will only be done the first time the message is used and then always only if any new message catalog was loaded and so the result of the translation lookup might be different.  See the libintl.h header file for details.  For the user it is only important to know that the result is always the same, independent of the compiler or compiler options in use.

How to determine which catalog to be used
.........................................

The functions to retrieve the translations for a given message have a remarkable simple interface.  But to provide the user of the program still the opportunity to select exactly the translation s/he wants and also to provide the programmer the possibility to influence the way to locate the search for catalogs files there is a quite complicated underlying mechanism which controls all this.  The code is complicated the use is easy.

Basically we have two different tasks to perform which can also be performed by the catgets functions:

 1. Locate the set of message catalogs.  There are a number of files for different languages and which all belong to the package.  Usually they are all stored in the filesystem below a certain directory.

     There can be arbitrary many packages installed and they can follow different guidelines for the placement of their files.

 2. Relative to the location specified by the package the actual translation files must be searched, based on the wishes of the user.  I.e., for each language the user selects the program should be able to locate the appropriate file.

This is the functionality required by the specifications for gettext and this is also what the catgets functions are able to do.  But there are some problems unresolved:

  * The language to be used can be specified in several different ways. There is no generally accepted standard for this and the user always expects the program understand what s/he means.  E.g., to select the German translation one could write de, german, or deutsch and the program should always react the same.

  * Sometimes the specification of the user is too detailed.  If s/he, e.g., specifies de_DE.ISO-8859-1 which means German, spoken in Germany, coded using the ISO 8859-1 character set there is the possibility that a message catalog matching this exactly is not available.  But there could be a catalog matching de and if the character set used on the machine is always ISO 8859-1 there is no reason why this later message catalog should not be used.  (We call this message inheritance.)

  * If a catalog for a wanted language is not available it is not always the second best choice to fall back on the language of the developer and simply not translate any message.  Instead a user might be better able to read the messages in another language and so the user of the program should be able to define an precedence order of languages.

We can divide the configuration actions in two parts: the one is performed by the programmer, the other by the user.  We will start with the functions the programmer can use since the user configuration will be based on this.

As the functions described in the last sections already mention separate sets of messages can be selected by a domain name.  This is a simple string which should be unique for each program part with uses a separate domain.  It is possible to use in one program arbitrary many domains at the same time.  E.g., the GNU C Library itself uses a domain named libc while the program using the C Library could use a domain named foo.  The important point is that at any time exactly one domain is active.  This is controlled with the following function.

 -- Function: char * textdomain (const char *DOMAINNAME)
     The textdomain function sets the default domain, which is used in all future gettext calls, to DOMAINNAME.  Please note that dgettext and dcgettext calls are not influenced if the DOMAINNAME parameter of these functions is not the null pointer.

     Before the first call to textdomain the default domain is messages.  This is the name specified in the specification of the gettext API.  This name is as good as any other name.  No program should ever really use a domain with this name since this can only lead to problems.

     The function returns the value which is from now on taken as the default domain.  If the system went out of memory the returned value is NULL and the global variable ERRNO is set to ENOMEM. Despite the return value type being char * the return string must not be changed.  It is allocated internally by the textdomain function.

     If the DOMAINNAME parameter is the null pointer no new default domain is set.  Instead the currently selected default domain is returned.

     If the DOMAINNAME parameter is the empty string the default domain is reset to its initial value, the domain with the name messages. This possibility is questionable to use since the domain messages really never should be used.

 -- Function: char * bindtextdomain (const char *DOMAINNAME, const char *DIRNAME)
     The bindtextdomain function can be used to specify the directory which contains the message catalogs for domain DOMAINNAME for the different languages.  To be correct, this is the directory where the hierarchy of directories is expected.  Details are explained below.

     For the programmer it is important to note that the translations which come with the program have be placed in a directory hierarchy starting at, say, /foo/bar.  Then the program should make a bindtextdomain call to bind the domain for the current program to this directory.  So it is made sure the catalogs are found.  A correctly running program does not depend on the user setting an environment variable.

     The bindtextdomain function can be used several times and if the DOMAINNAME argument is different the previously bounded domains will not be overwritten.

     If the program which wish to use bindtextdomain at some point of time use the chdir function to change the current working directory it is important that the DIRNAME strings ought to be an absolute pathname.  Otherwise the addressed directory might vary with the time.

     If the DIRNAME parameter is the null pointer bindtextdomain returns the currently selected directory for the domain with the name DOMAINNAME.

     The bindtextdomain function returns a pointer to a string containing the name of the selected directory name.  The string is allocated internally in the function and must not be changed by the user.  If the system went out of core during the execution of bindtextdomain the return value is NULL and the global variable ERRNO is set accordingly.

Additional functions for more complicated situations
....................................................

The functions of the gettext family described so far (and all the catgets functions as well) have one problem in the real world which have been neglected completely in all existing approaches.  What is meant here is the handling of plural forms.

Looking through Unix source code before the time anybody thought about internationalization (and, sadly, even afterwards) one can often find code similar to the following:

        printf ("%d file%s deleted", n, n == 1 ? "" : "s");

After the first complains from people internationalizing the code people either completely avoided formulations like this or used strings like "file(s)".  Both look unnatural and should be avoided.  First tries to solve the problem correctly looked like this:

        if (n == 1)
          printf ("%d file deleted", n);
        else
          printf ("%d files deleted", n);

But this does not solve the problem.  It helps languages where the plural form of a noun is not simply constructed by adding an `s' but that is all.  Once again people fell into the trap of believing the rules their language is using are universal.  But the handling of plural forms differs widely between the language families.  There are two things we can differ between (and even inside language families);

  * The form how plural forms are build differs.  This is a problem with language which have many irregularities.  German, for instance, is a drastic case.  Though English and German are part of the same language family (Germanic), the almost regular forming of plural noun forms (appending an `s') is hardly found in German.

  * The number of plural forms differ.  This is somewhat surprising for those who only have experiences with Romanic and Germanic languages since here the number is the same (there are two).

     But other language families have only one form or many forms.  More information on this in an extra section.

The consequence of this is that application writers should not try to solve the problem in their code.  This would be localization since it is only usable for certain, hardcoded language environments.  Instead the extended gettext interface should be used.

These extra functions are taking instead of the one key string two strings and an numerical argument.  The idea behind this is that using the numerical argument and the first string as a key, the implementation can select using rules specified by the translator the right plural form.  The two string arguments then will be used to provide a return value in case no message catalog is found (similar to the normal gettext behavior).  In this case the rules for Germanic language is used and it is assumed that the first string argument is the singular form, the second the plural form.

This has the consequence that programs without language catalogs can display the correct strings only if the program itself is written using a Germanic language.  This is a limitation but since the GNU C library (as well as the GNU gettext package) are written as part of the GNU package and the coding standards for the GNU project require program being written in English, this solution nevertheless fulfills its purpose.

 -- Function: char * ngettext (const char *MSGID1, const char *MSGID2, unsigned long int N)
     The ngettext function is similar to the gettext function as it finds the message catalogs in the same way.  But it takes two extra arguments.  The MSGID1 parameter must contain the singular form of the string to be converted.  It is also used as the key for the search in the catalog.  The MSGID2 parameter is the plural form. The parameter N is used to determine the plural form.  If no message catalog is found MSGID1 is returned if n == 1, otherwise msgid2.

     An example for the us of this function is:

            printf (ngettext ("%d file removed", "%d files removed", n), n);

     Please note that the numeric value N has to be passed to the printf function as well.  It is not sufficient to pass it only to ngettext.

 -- Function: char * dngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N)
     The dngettext is similar to the dgettext function in the way the message catalog is selected.  The difference is that it takes two extra parameter to provide the correct plural form.  These two parameters are handled in the same way ngettext handles them.

 -- Function: char * dcngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N, int CATEGORY)
     The dcngettext is similar to the dcgettext function in the way the message catalog is selected.  The difference is that it takes two extra parameter to provide the correct plural form.  These two parameters are handled in the same way ngettext handles them.

The problem of plural forms
...........................

A description of the problem can be found at the beginning of the last section.  Now there is the question how to solve it.  Without the input of linguists (which was not available) it was not possible to determine whether there are only a few different forms in which plural forms are formed or whether the number can increase with every new supported language.

Therefore the solution implemented is to allow the translator to specify the rules of how to select the plural form.  Since the formula varies with every language this is the only viable solution except for hardcoding the information in the code (which still would require the possibility of extensions to not prevent the use of new languages).  The details are explained in the GNU gettext manual.  Here only a a bit of information is provided.

The information about the plural form selection has to be stored in the header entry (the one with the empty (msgid string).  There should be something like:

       nplurals=2; plural=n == 1 ? 0 : 1

The nplurals value must be a decimal number which specifies how many different plural forms exist for this language.  The string following plural is an expression which is using the C language syntax.  Exceptions are that no negative number are allowed, numbers must be decimal, and the only variable allowed is n.  This expression will be evaluated whenever one of the functions ngettext, dngettext, or dcngettext is called.  The numeric value passed to these functions is then substituted for all uses of the variable n in the expression.  The resulting value then must be greater or equal to zero and smaller than the value given as the value of nplurals.

The following rules are known at this point.  The language with families are listed.  But this does not necessarily mean the information can be generalized for the whole family (as can be easily seen in the table below). (Additions are welcome.  Send appropriate information to bug-glibc-manual@gnu.org.)

Only one form:
     Some languages only require one single form.  There is no distinction between the singular and plural form.  And appropriate header entry would look like this:

          nplurals=1; plural=0

     Languages with this property include:

     Finno-Ugric family
          Hungarian

     Asian family
          Japanese

     Turkic/Altaic family
          Turkish

Two forms, singular used for one only
     This is the form used in most existing programs sine it is what English is using.  A header entry would look like this:

          nplurals=2; plural=n != 1

     (Note: this uses the feature of C expressions that boolean expressions have to value zero or one.)

     Languages with this property include:

     Germanic family
          Danish, Dutch, English, German, Norwegian, Swedish

     Finno-Ugric family
          Finnish

     Latin/Greek family
          Greek

     Semitic family
          Hebrew

     Romance family
          Italian, Spanish

     Artificial
          Esperanto

Two forms, singular used for zero and one
     Exceptional case in the language family.  The header entry would be:

          nplurals=2; plural=n>1

     Languages with this property include:

     Romanic family
          French

Three forms, special cases for one and two
     The header entry would be:

          nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2

     Languages with this property include:

     Celtic
          Gaeilge

Three forms, special case for one and all numbers ending in 2, 3, or 4
     The header entry would look like this:

          nplurals=3; plural=n==1 ? 0 : n%10>=2 && n%10<=4 ? 1 : 2

     Languages with this property include:

     Slavic family
          Russian

Three forms, special case for one and some numbers ending in 2, 3, or 4
     The header entry would look like this:

          nplurals=3; plural=n==1 ? 0 : \
            n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2

     (Continuation in the next line is possible.)

     Languages with this property include:

     Slavic family
          Polish

Four forms, special case for one and all numbers ending in 2, 3, or 4
     The header entry would look like this:

          nplurals=4; plural=n==1 ? 0 : n%10==2 ? 1 : n%10==3 || n%10==4 ? 2 : 3

     Languages with this property include:

     Slavic family
          Slovenian

How to use gettext in GUI programs
..................................

One place where the gettext functions if used normally have big programs is within programs with graphical user interfaces (GUIs).  The problem is that many of the strings which have to be translated are very short.  They have to appear in pull-down menus which restricts the length.  But strings which are not containing entire sentences or at least large fragments of a sentence may appear in more than one situation in the program but might have different translations.  This is especially true for the one-word strings which are frequently used in GUI programs.

As a consequence many people say that the gettext approach is wrong and instead catgets should be used which indeed does not have this problem.  But there is a very simple and powerful method to handle these kind of problems with the gettext functions.

As as example consider the following fictional situation.  A GUI program has a menu bar with the following entries:

     +------------+------------+--------------------------------------+
     | File       | Printer    |                                      |
     +------------+------------+--------------------------------------+
     | Open     | | Select   |
     | New      | | Open     |
     +----------+ | Connect  |
                  +----------+

To have the strings File, Printer, Open, New, Select, and Connect translated there has to be at some point in the code a call to a function of the gettext family.  But in two places the string passed into the function would be Open.  The translations might not be the same and therefore we are in the dilemma described above.

One solution to this problem is to artificially enlengthen the strings to make them unambiguous.  But what would the program do if no translation is available?  The enlengthened string is not what should be printed.  So we should use a little bit modified version of the functions.

To enlengthen the strings a uniform method should be used.  E.g., in the example above the strings could be chosen as

     Menu|File
     Menu|Printer
     Menu|File|Open
     Menu|File|New
     Menu|Printer|Select
     Menu|Printer|Open
     Menu|Printer|Connect

Now all the strings are different and if now instead of gettext the following little wrapper function is used, everything works just fine:

       char *
       sgettext (const char *msgid)
       {
         char *msgval = gettext (msgid);
         if (msgval == msgid)
           msgval = strrchr (msgid, '|') + 1;
         return msgval;
       }

What this little function does is to recognize the case when no translation is available.  This can be done very efficiently by a pointer comparison since the return value is the input value.  If there is no translation we know that the input string is in the format we used for the Menu entries and therefore contains a | character.  We simply search for the last occurrence of this character and return a pointer to the character following it.  That's it!

If one now consistently uses the enlengthened string form and replaces the gettext calls with calls to sgettext (this is normally limited to very few places in the GUI implementation) then it is possible to produce a program which can be internationalized.

With advanced compilers (such as GNU C) one can write the sgettext functions as an inline function or as a macro like this:

     #define sgettext(msgid) \
       ({ const char *__msgid = (msgid);            \
          char *__msgstr = gettext (__msgid);       \
          if (__msgval == __msgid)                  \
            __msgval = strrchr (__msgid, '|') + 1;  \
          __msgval; })

The other gettext functions (dgettext, dcgettext and the ngettext equivalents) can and should have corresponding functions as well which look almost identical, except for the parameters and the call to the underlying function.

Now there is of course the question why such functions do not exist in the GNU C library?  There are two parts of the answer to this question.

  * They are easy to write and therefore can be provided by the project they are used in.  This is not an answer by itself and must be seen together with the second part which is:

  * There is no way the C library can contain a version which can work everywhere.  The problem is the selection of the character to separate the prefix from the actual string in the enlenghtened string.  The examples above used | which is a quite good choice because it resembles a notation frequently used in this context and it also is a character not often used in message strings.

     But what if the character is used in message strings.  Or if the chose character is not available in the character set on the machine one compiles (e.g., | is not required to exist for ISO C; this is why the iso646.h file exists in ISO C programming environments).

There is only one more comment to make left.  The wrapper function above require that the translations strings are not enlengthened themselves. This is only logical.  There is no need to disambiguate the strings (since they are never used as keys for a search) and one also saves quite some memory and disk space by doing this.

User influence on gettext
.........................

The last sections described what the programmer can do to internationalize the messages of the program.  But it is finally up to the user to select the message s/he wants to see.  S/He must understand them.

The POSIX locale model uses the environment variables LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, NUMERIC, and LC_TIME to select the locale which is to be used.  This way the user can influence lots of functions.  As we mentioned above the gettext functions also take advantage of this.

To understand how this happens it is necessary to take a look at the various components of the filename which gets computed to locate a message catalog.  It is composed as follows:

     DIR_NAME/LOCALE/LC_CATEGORY/DOMAIN_NAME.mo

The default value for DIR_NAME is system specific.  It is computed from the value given as the prefix while configuring the C library. This value normally is /usr or /.  For the former the complete DIR_NAME is:

     /usr/share/locale

We can use /usr/share since the .mo files containing the message catalogs are system independent, so all systems can use the same files.  If the program executed the bindtextdomain function for the message domain that is currently handled, the dir_name component is exactly the value which was given to the function as the second parameter.  I.e., bindtextdomain allows overwriting the only system dependent and fixed value to make it possible to address files anywhere in the filesystem.

The CATEGORY is the name of the locale category which was selected in the program code.  For gettext and dgettext this is always LC_MESSAGES, for dcgettext this is selected by the value of the third parameter.  As said above it should be avoided to ever use a category other than LC_MESSAGES.

The LOCALE component is computed based on the category used.  Just like for the setlocale function here comes the user selection into the play.  Some environment variables are examined in a fixed order and the first environment variable set determines the return value of the lookup process.  In detail, for the category LC_xxx the following variables in this order are examined:

LANGUAGE

LC_ALL

LC_xxx

LANG

This looks very familiar.  With the exception of the LANGUAGE environment variable this is exactly the lookup order the setlocale function uses.  But why introducing the LANGUAGE variable?

The reason is that the syntax of the values these variables can have is different to what is expected by the setlocale function.  If we would set LC_ALL to a value following the extended syntax that would mean the setlocale function will never be able to use the value of this variable as well.  An additional variable removes this problem plus we can select the language independently of the locale setting which sometimes is useful.

While for the LC_xxx variables the value should consist of exactly one specification of a locale the LANGUAGE variable's value can consist of a colon separated list of locale names.  The attentive reader will realize that this is the way we manage to implement one of our additional demands above: we want to be able to specify an ordered list of language.

Back to the constructed filename we have only one component missing. The DOMAIN_NAME part is the name which was either registered using the textdomain function or which was given to dgettext or dcgettext as the first parameter.  Now it becomes obvious that a good choice for the domain name in the program code is a string which is closely related to the program/package name.  E.g., for the GNU C Library the domain name is libc.

A limit piece of example code should show how the programmer is supposed to work:

     {
       textdomain ("test-package");
       bindtextdomain ("test-package", "/usr/local/share/locale");
       puts (gettext ("Hello, world!");
     }

At the program start the default domain is messages.  The textdomain call changes this to test-package.  The bindtextdomain call specifies that the message catalogs for the domain test-package can be found below the directory /usr/local/share/locale.

If now the user set in her/his environment the variable LANGUAGE to de the gettext function will try to use the translations from the file

     /usr/local/share/locale/de/LC_MESSAGES/test-package.mo

From the above descriptions it should be clear which component of this filename is determined by which source.

In the above example we assumed that the LANGUAGE environment variable to de.  This might be an appropriate selection but what happens if the user wants to use LC_ALL because of the wider usability and here the required value is de_DE.ISO-8859-1?  We already mentioned above that a situation like this is not infrequent. E.g., a person might prefer reading a dialect and if this is not available fall back on the standard language.

The gettext functions know about situations like this and can handle them gracefully.  The functions recognize the format of the value of the environment variable.  It can split the value is different pieces and by leaving out the only or the other part it can construct new values.  This happens of course in a predictable way.  To understand this one must know the format of the environment variable value.  There are to more or less standardized forms:

X/Open Format
     language[_territory[.codeset]][@modifier]

CEN Format (European Community Standard)
     language[_territory][+audience][+special][,[sponsor][_revision]]

The functions will automatically recognize which format is used.  Less specific locale names will be stripped of in the order of the following list:

 1. revision

 2. sponsor

 3. special

 4. codeset

 5. normalized codeset

 6. territory

 7. audience/modifier

From the last entry one can see that the meaning of the modifier field in the X/Open format and the audience format have the same meaning.  Beside one can see that the language field for obvious reasons never will be dropped.

The only new thing is the normalized codeset entry.  This is another goodie which is introduced to help reducing the chaos which derives from the inability of the people to standardize the names of character sets.  Instead of ISO-8859-1 one can often see 8859-1, 88591, iso8859-1, or iso_8859-1.  The normalized codeset value is generated from the user-provided character set name by applying the following rules:

 1. Remove all characters beside numbers and letters.

 2. Fold letters to lowercase.

 3. If the same only contains digits prepend the string "iso".

So all of the above name will be normalized to iso88591.  This allows the program user much more freely choosing the locale name.

Even this extended functionality still does not help to solve the problem that completely different names can be used to denote the same locale (e.g., de and german).  To be of help in this situation the locale implementation and also the gettext functions know about aliases.

The file /usr/share/locale/locale.alias (replace /usr with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names.  The system manager is free to add new entries to fill her/his own needs.  The selected locale from the environment is compared with the entries in the first column of this file ignoring the case.  If they match the value of the second column is used instead for the further handling.

In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog.  In fact, only catalogs which contain text written using the character set of the system/program can be used (directly; there will come a solution for this some day).  This means for the user that s/he will always have to take care for this.  If in the collection of the message catalogs there are files for the same language but coded using different character sets the user has to be careful.

Programs to handle message catalogs for gettext
-----------------------------------------------

The GNU C Library does not contain the source code for the programs to handle message catalogs for the gettext functions.  As part of the GNU project the GNU gettext package contains everything the developer needs.  The functionality provided by the tools in this package by far exceeds the abilities of the gencat program described above for the catgets functions.

There is a program msgfmt which is the equivalent program to the gencat program.  It generates from the human-readable and -editable form of the message catalog a binary file which can be used by the gettext functions.  But there are several more programs available.

The xgettext program can be used to automatically extract the translatable messages from a source file.  I.e., the programmer need not take care for the translations and the list of messages which have to be translated.  S/He will simply wrap the translatable string in calls to gettext et.al and the rest will be done by xgettext.  This program has a lot of option which help to customize the output or do help to understand the input better.

Other programs help to manage development cycle when new messages appear in the source files or when a new translation of the messages appear. here it should only be noted that using all the tools in GNU gettext it is possible to completely automize the handling of message catalog.  Beside marking the translatable string in the source code and generating the translations the developers do not have anything to do themselves.