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11. The Programmer's View

One aim of the current message catalog implementation provided by GNU gettext was to use the system's 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 did 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 they decided only to include an example of an interface. The major Unix vendors are split in the usage of the two most important specifications: X/Open's catgets vs. Uniforum's gettext interface. We'll describe them both and later explain our solution of this dilemma.


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11.1 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 I and some others do…)

But we must not forget one point: after all the trouble with transferring the rights on Unix(tm) they at last came to X/Open, the very same who published this specification. 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).


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11.1.1 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 be const char *, but the standard is published in 1988, one year before ANSI C.

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

catclose (catd);

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


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11.1.2 Problems with the catgets Interface?!

Now that this description seemed to be really easy -- where are the problems 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 a 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 easy to manage.


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11.2 About gettext

The definition of the gettext interface comes from a Uniforum proposal. It was submitted there by Sun, who had implemented the gettext function in SunOS 4, around 1990. Nowadays, the gettext interface is specified by the OpenI18N standard.

The main point 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 with so many tasks, especially the unique key handling. Of course here also a unique key is needed, but this key is the message itself (how long or short it is). See 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.


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11.2.1 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 a 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 returns 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 it is not available, the argument itself is returned. If the argument is NULL the result is undefined.

One thing which should come into mind is that no explicit dependency to the used domain is given. The current value of the domain 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.


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11.2.2 Solving Ambiguities

While this single name domain works well 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 subject to discussion during 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. Another case are messages from a library, as these have to be independent of the current domain set by the application.

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 category 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.


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11.2.3 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 bindtextdomains second argument (or the default directory), followed by the name of the locale, the locale category, 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 name of the locale category which is designated by LC_category. For gettext and dgettext this LC_category is always LC_MESSAGES.(3) The name of the locale category is determined through setlocale (LC_category, NULL). (4) When using the function dcgettext, you can specify the locale category through the third argument.


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11.2.4 How to specify the output character set gettext uses

gettext not only looks up a translation in a message catalog. It also converts the translation on the fly to the desired output character set. This is useful if the user is working in a different character set than the translator who created the message catalog, because it avoids distributing variants of message catalogs which differ only in the character set.

The output character set is, by default, the value of nl_langinfo (CODESET), which depends on the LC_CTYPE part of the current locale. But programs which store strings in a locale independent way (e.g. UTF-8) can request that gettext and related functions return the translations in that encoding, by use of the bind_textdomain_codeset function.

Note that the msgid argument to gettext is not subject to character set conversion. Also, when gettext does not find a translation for msgid, it returns msgid unchanged - independently of the current output character set. It is therefore recommended that all msgids be US-ASCII strings.

Function: char * bind_textdomain_codeset (const char *domainname, const char *codeset)

The bind_textdomain_codeset function can be used to specify the output character set for message catalogs for domain domainname. The codeset argument must be a valid codeset name which can be used for the iconv_open function, or a null pointer.

If the codeset parameter is the null pointer, bind_textdomain_codeset returns the currently selected codeset for the domain with the name domainname. It returns NULL if no codeset has yet been selected.

The bind_textdomain_codeset function can be used several times. If used multiple times with the same domainname argument, the later call overrides the settings made by the earlier one.

The bind_textdomain_codeset function returns a pointer to a string containing the name of the selected codeset. 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 bind_textdomain_codeset, the return value is NULL and the global variable errno is set accordingly.


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11.2.5 Using contexts for solving ambiguities

One place where the gettext functions, if used normally, have big problems 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 this kind of problems with the gettext functions.

Contexts can be added to strings to be translated. A context dependent translation lookup is when a translation for a given string is searched, that is limited to a given context. The translation for the same string in a different context can be different. The different translations of the same string in different contexts can be stored in the in the same MO file, and can be edited by the translator in the same PO file.

The `gettext.h' include file contains the lookup macros for strings with contexts. They are implemented as thin macros and inline functions over the functions from <libintl.h>.

const char *pgettext (const char *msgctxt, const char *msgid);

In a call of this macro, msgctxt and msgid must be string literals. The macro returns the translation of msgid, restricted to the context given by msgctxt.

The msgctxt string is visible in the PO file to the translator. You should try to make it somehow canonical and never changing. Because every time you change an msgctxt, the translator will have to review the translation of msgid.

Finding a canonical msgctxt string that doesn't change over time can be hard. But you shouldn't use the file name or class name containing the pgettext call - because it is a common development task to rename a file or a class, and it shouldn't cause translator work. Also you shouldn't use a comment in the form of a complete English sentence as msgctxt - because orthography or grammar changes are often applied to such sentences, and again, it shouldn't force the translator to do a review.

The `p' in `pgettext' stands for "particular": pgettext fetches a particular translation of the msgid.

const char *dpgettext (const char *domain_name,
                       const char *msgctxt, const char *msgid);
const char *dcpgettext (const char *domain_name,
                        const char *msgctxt, const char *msgid,
                        int category);

These are generalizations of pgettext. They behave similarly to dgettext and dcgettext, respectively. The domain_name argument defines the translation domain. The category argument allows to use another locale category than LC_MESSAGES.

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.

What distinguishes the two places is the menu path from the menu root to the particular menu entries:

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

The context is thus the menu path without its last part. So, the calls look like this:

pgettext ("Menu|", "File")
pgettext ("Menu|", "Printer")
pgettext ("Menu|File|", "Open")
pgettext ("Menu|File|", "New")
pgettext ("Menu|Printer|", "Select")
pgettext ("Menu|Printer|", "Open")
pgettext ("Menu|Printer|", "Connect")

Whether or not to use the `|' character at the end of the context is a matter of style.

For more complex cases, where the msgctxt or msgid are not string literals, more general macros are available:

const char *pgettext_expr (const char *msgctxt, const char *msgid);
const char *dpgettext_expr (const char *domain_name,
                            const char *msgctxt, const char *msgid);
const char *dcpgettext_expr (const char *domain_name,
                             const char *msgctxt, const char *msgid,
                             int category);

Here msgctxt and msgid can be arbitrary string-valued expressions. These macros are more general. But in the case that both argument expressions are string literals, the macros without the `_expr' suffix are more efficient.


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11.2.6 Additional functions for plural forms

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 complaints 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. For example, Rafal Maszkowski < Adres poczty elektronicznej jest chroniony przed robotami spamującymi. W przeglądarce musi być włączona obsługa JavaScript, żeby go zobaczyć. > 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).

There are two things which can differ between languages (and even inside language families);

  • The form how plural forms are built differs. This is a problem with languages 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 a 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 use 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.

In the English singular case, the number - always 1 - can be replaced with "one":

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

This works because the `printf' function discards excess arguments that are not consumed by the format string.

If this function is meant to yield a format string that takes two or more arguments, you can not use it like this:

printf (ngettext ("%d file removed from directory %s",
                  "%d files removed from directory %s",
                  n, dir),
        n);

because in many languages the translators want to replace the `%d' with an explicit word in the singular case, just like "one" in English, and C format strings cannot consume the second argument but skip the first argument. Instead, you have to reorder the arguments so that `n' comes last:

printf (ngettext ("%$2d file removed from directory %$1s",
                  "%$2d files removed from directory %$1s",
                  dir, n),
        n);

See C Format Strings for details about this argument reordering syntax.

When you know that the value of n is within a given range, you can specify it as a comment directed to the xgettext tool. This information may help translators to use more adequate translations. Like this:

if (days > 7 && days < 14)
  /* xgettext: range: 1..6 */
  printf (ngettext ("one week and one day", "one week and %d days",
                    days - 7),
          days - 7);

It is also possible to use this function when the strings don't contain a cardinal number:

puts (ngettext ("Delete the selected file?",
                "Delete the selected files?",
                n));

In this case the number n is only used to choose the plural form.

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.

Now, how do these functions solve the problem of the plural forms? 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 information about the plural form selection has to be stored in the header entry of the PO file (the one with the empty msgid string). The plural form information looks like this:

Plural-Forms: 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 numbers are allowed, numbers must be decimal, and the only variable allowed is n. Spaces are allowed in the expression, but backslash-newlines are not; in the examples below the backslash-newlines are present for formatting purposes only. 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).(5)

Only one form:

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

Plural-Forms: nplurals=1; plural=0;

Languages with this property include:

Asian family

Japanese, Vietnamese, Korean

Two forms, singular used for one only

This is the form used in most existing programs since it is what English is using. A header entry would look like this:

Plural-Forms: 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

English, German, Dutch, Swedish, Danish, Norwegian, Faroese

Romanic family

Spanish, Portuguese, Italian, Bulgarian

Latin/Greek family

Greek

Finno-Ugric family

Finnish, Estonian

Semitic family

Hebrew

Artificial

Esperanto

Other languages using the same header entry are:

Finno-Ugric family

Hungarian

Turkic/Altaic family

Turkish

Hungarian does not appear to have a plural if you look at sentences involving cardinal numbers. For example, "1 apple" is "1 alma", and "123 apples" is "123 alma". But when the number is not explicit, the distinction between singular and plural exists: "the apple" is "az alma", and "the apples" is "az almák". Since ngettext has to support both types of sentences, it is classified here, under "two forms".

The same holds for Turkish: "1 apple" is "1 elma", and "123 apples" is "123 elma". But when the number is omitted, the distinction between singular and plural exists: "the apple" is "elma", and "the apples" is "elmalar".

Two forms, singular used for zero and one

Exceptional case in the language family. The header entry would be:

Plural-Forms: nplurals=2; plural=n>1;

Languages with this property include:

Romanic family

Brazilian Portuguese, French

Three forms, special case for zero

The header entry would be:

Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n != 0 ? 1 : 2;

Languages with this property include:

Baltic family

Latvian

Three forms, special cases for one and two

The header entry would be:

Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;

Languages with this property include:

Celtic

Gaeilge (Irish)

Three forms, special case for numbers ending in 00 or [2-9][0-9]

The header entry would be:

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

Languages with this property include:

Romanic family

Romanian

Three forms, special case for numbers ending in 1[2-9]

The header entry would look like this:

Plural-Forms: nplurals=3; \
    plural=n%10==1 && n%100!=11 ? 0 : \
           n%10>=2 && (n%100<10 || n%100>=20) ? 1 : 2;

Languages with this property include:

Baltic family

Lithuanian

Three forms, special cases for numbers ending in 1 and 2, 3, 4, except those ending in 1[1-4]

The header entry would look like this:

Plural-Forms: nplurals=3; \
    plural=n%10==1 && n%100!=11 ? 0 : \
           n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;

Languages with this property include:

Slavic family

Russian, Ukrainian, Serbian, Croatian

Three forms, special cases for 1 and 2, 3, 4

The header entry would look like this:

Plural-Forms: nplurals=3; \
    plural=(n==1) ? 0 : (n>=2 && n<=4) ? 1 : 2;

Languages with this property include:

Slavic family

Czech, Slovak

Three forms, special case for one and some numbers ending in 2, 3, or 4

The header entry would look like this:

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

Languages with this property include:

Slavic family

Polish

Four forms, special case for one and all numbers ending in 02, 03, or 04

The header entry would look like this:

Plural-Forms: nplurals=4; \
    plural=n%100==1 ? 0 : n%100==2 ? 1 : n%100==3 || n%100==4 ? 2 : 3;

Languages with this property include:

Slavic family

Slovenian

You might now ask, ngettext handles only numbers n of type `unsigned long'. What about larger integer types? What about negative numbers? What about floating-point numbers?

About larger integer types, such as `uintmax_t' or `unsigned long long': they can be handled by reducing the value to a range that fits in an `unsigned long'. Simply casting the value to `unsigned long' would not do the right thing, since it would treat ULONG_MAX + 1 like zero, ULONG_MAX + 2 like singular, and the like. Here you can exploit the fact that all mentioned plural form formulas eventually become periodic, with a period that is a divisor of 100 (or 1000 or 1000000). So, when you reduce a large value to another one in the range [1000000, 1999999] that ends in the same 6 decimal digits, you can assume that it will lead to the same plural form selection. This code does this:

#include <inttypes.h>
uintmax_t nbytes = ...;
printf (ngettext ("The file has %"PRIuMAX" byte.",
                  "The file has %"PRIuMAX" bytes.",
                  (nbytes > ULONG_MAX
                   ? (nbytes % 1000000) + 1000000
                   : nbytes)),
        nbytes);

Negative and floating-point values usually represent physical entities for which singular and plural don't clearly apply. In such cases, there is no need to use ngettext; a simple gettext call with a form suitable for all values will do. For example:

printf (gettext ("Time elapsed: %.3f seconds"),
        num_milliseconds * 0.001);

Even if num_milliseconds happens to be a multiple of 1000, the output

Time elapsed: 1.000 seconds

is acceptable in English, and similarly for other languages.

The translators' perspective regarding plural forms is explained in Translating plural forms.


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11.2.7 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 does it lead to legible code.

For this reason, GNU gettext caches previous translation results. When the same translation is requested twice, with no new message catalogs being loaded in between, gettext will, the second time, find the result through a single cache lookup.


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11.3 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). One usually defines gettext_noop as a no-op macro. So you should consider the following code for your project:

#define gettext_noop(String) String
#define N_(String) gettext_noop (String)

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.

catgets allows same original entry to have different translations, but gettext has another, scalable approach for solving ambiguities of this kind: See section Solving Ambiguities.


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11.4 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).


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11.5 Being a gettext grok

NOTE: This documentation section is outdated and needs to be revised.

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.

    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 category
    4. LANG

    Afterwards the path is constructed using the found value and the translation file is loaded if available.

    What happens 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'. You don't need to know what this is for. But it can be used to detect whether a gettext implementation is GNU gettext and not non-GNU system's native gettext implementation.


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11.6 Temporary Notes for the Programmers Chapter

NOTE: This documentation section is outdated and needs to be revised.


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11.6.1 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.


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11.6.2 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.


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11.6.3 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 libintl 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 libintl + non-catgets routines. When they port their software to other platforms, they're now going to have to include the front-end (libintl) code plus the back-end code (the non-catgets access routines) with their software instead of just including the libintl 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 libintl 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.


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11.6.4 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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