4.2. Localized Messages with POSIX.1 Native Language Support (NLS)

Contributed by Gábor Kövesdán.

Beyond the basic I18N functions, like supporting various input encodings or supporting national conventions, such as the different decimal separators, at a higher level of I18N, it is possible to localize the messages written to the output by the various programs. A common way of doing this is using the POSIX.1 NLS functions, which are provided as a part of the FreeBSD base system.

4.2.1. Organizing Localized Messages into Catalog Files

POSIX.1 NLS is based on catalog files, which contain the localized messages in the desired encoding. The messages are organized into sets and each message is identified by an integer number in the containing set. The catalog files are conventionally named after the locale they contain localized messages for, followed by the .msg extension. For instance, the Hungarian messages for ISO8859-2 encoding should be stored in a file called hu_HU.ISO8859-2.

These catalog files are common text files that contain the numbered messages. It is possible to write comments by starting the line with a $ sign. Set boundaries are also separated by special comments, where the keyword set must directly follow the $ sign. The set keyword is then followed by the set number. For example:

$set 1

The actual message entries start with the message number and followed by the localized message. The well-known modifiers from printf(3) are accepted:

15 "File not found: %s\n"

The language catalog files have to be compiled into a binary form before they can be opened from the program. This conversion is done with the gencat(1) utility. Its first argument is the filename of the compiled catalog and its further arguments are the input catalogs. The localized messages can also be organized into more catalog files and then all of them can be processed with gencat(1).

4.2.2. Using the Catalog Files from the Source Code

Using the catalog files is simple. To use the related functions, nl_types.h must be included. Before using a catalog, it has to be opened with catopen(3). The function takes two arguments. The first parameter is the name of the installed and compiled catalog. Usually, the name of the program is used, such as grep. This name will be used when looking for the compiled catalog file. The catopen(3) call looks for this file in /usr/share/nls/locale/catname and in /usr/local/share/nls/locale/catname, where locale is the locale set and catname is the catalog name being discussed. The second parameter is a constant, which can have two values:

  • NL_CAT_LOCALE, which means that the used catalog file will be based on LC_MESSAGES.

  • 0, which means that LANG has to be used to open the proper catalog.

The catopen(3) call returns a catalog identifier of type nl_catd. Please refer to the manual page for a list of possible returned error codes.

After opening a catalog catgets(3) can be used to retrieve a message. The first parameter is the catalog identifier returned by catopen(3), the second one is the number of the set, the third one is the number of the messages, and the fourth one is a fallback message, which will be returned if the requested message cannot be retrieved from the catalog file.

After using the catalog file, it must be closed by calling catclose(3), which has one argument, the catalog id.

4.2.3. A Practical Example

The following example will demonstrate an easy solution on how to use NLS catalogs in a flexible way.

The below lines need to be put into a common header file of the program, which is included into all source files where localized messages are necessary:

#ifdef WITHOUT_NLS
#define getstr(n)	 nlsstr[n]
#else
#include <nl_types.h>

extern nl_catd		 catalog;
#define getstr(n)	 catgets(catalog, 1, n, nlsstr[n])
#endif

extern char		*nlsstr[];

Next, put these lines into the global declaration part of the main source file:

#ifndef WITHOUT_NLS
#include <nl_types.h>
nl_catd	 catalog;
#endif

/*
 * Default messages to use when NLS is disabled or no catalog
 * is found.
 */
char    *nlsstr[] = {
        "",
/* 1*/  "some random message",
/* 2*/  "some other message"
};

Next come the real code snippets, which open, read, and close the catalog:

#ifndef WITHOUT_NLS
	catalog = catopen("myapp", NL_CAT_LOCALE);
#endif

...

printf(getstr(1));

...

#ifndef WITHOUT_NLS
	catclose(catalog);
#endif

4.2.3.1. Reducing Strings to Localize

There is a good way of reducing the strings that need to be localized by using libc error messages. This is also useful to just avoid duplication and provide consistent error messages for the common errors that can be encountered by a great many of programs.

First, here is an example that does not use libc error messages:

#include <err.h>
...
if (!S_ISDIR(st.st_mode))
	errx(1, "argument is not a directory");
	  

This can be transformed to print an error message by reading errno and printing an error message accordingly:

#include <err.h>
#include <errno.h>
...
if (!S_ISDIR(st.st_mode)) {
	errno = ENOTDIR;
	err(1, NULL);
}
	  

In this example, the custom string is eliminated, thus translators will have less work when localizing the program and users will see the usual Not a directory error message when they encounter this error. This message will probably seem more familiar to them. Please note that it was necessary to include errno.h in order to directly access errno.

It is worth to note that there are cases when errno is set automatically by a preceding call, so it is not necessary to set it explicitly:

#include <err.h>
...
if ((p = malloc(size)) == NULL)
	err(1, NULL);
	  

4.2.4. Making use of bsd.nls.mk

Using the catalog files requires few repeatable steps, such as compiling the catalogs and installing them to the proper location. In order to simplify this process even more, bsd.nls.mk introduces some macros. It is not necessary to include bsd.nls.mk explicitly, it is pulled in from the common Makefiles, such as bsd.prog.mk or bsd.lib.mk.

Usually it is enough to define NLSNAME, which should have the catalog name mentioned as the first argument of catopen(3) and list the catalog files in NLS without their .msg extension. Here is an example, which makes it possible to to disable NLS when used with the code examples before. The WITHOUT_NLS make(1) variable has to be defined in order to build the program without NLS support.

.if !defined(WITHOUT_NLS)
NLS=	es_ES.ISO8859-1
NLS+=	hu_HU.ISO8859-2
NLS+=	pt_BR.ISO8859-1
.else
CFLAGS+=	-DWITHOUT_NLS
.endif

Conventionally, the catalog files are placed under the nls subdirectory and this is the default behaviour of bsd.nls.mk. It is possible, though to override the location of the catalogs with the NLSSRCDIR make(1) variable. The default name of the precompiled catalog files also follow the naming convention mentioned before. It can be overridden by setting the NLSNAME variable. There are other options to fine tune the processing of the catalog files but usually it is not needed, thus they are not described here. For further information on bsd.nls.mk, please refer to the file itself, it is short and easy to understand.

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