Locales and Internationalization
********************************

Different countries and cultures have varying conventions for how to communicate.  These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken.

Internationalization of software means programming it to be able to adapt to the user's favorite conventions.  In ISO C, internationalization works by means of locales.  Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables).

All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user.

What Effects a Locale Has
=========================

Each locale specifies conventions for several purposes, including the following:

  * What multibyte character sequences are valid, and how they are interpreted (see Character Set Handling).

  * Classification of which characters in the local character set are considered alphabetic, and upper- and lower-case conversion conventions (see Character Handling).

  * The collating sequence for the local language and character set (see Collation Functions).

  * Formatting of numbers and currency amounts (see General Numeric).

  * Formatting of dates and times (see Formatting Calendar Time).

  * What language to use for output, including error messages (see Message Translation).

  * What language to use for user answers to yes-or-no questions (see Yes-or-No Questions).

  * What language to use for more complex user input. (The C library doesn't yet help you implement this.)

Some aspects of adapting to the specified locale are handled automatically by the library subroutines.  For example, all your program needs to do in order to use the collating sequence of the chosen locale is to use strcoll or strxfrm to compare strings.

Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages.  The only way you can support output in the user's favorite language is to program this more or less by hand.  The C library provides functions to handle translations for multiple languages easily.

This chapter discusses the mechanism by which you can modify the current locale.  The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions.

Choosing a Locale
=================

The simplest way for the user to choose a locale is to set the environment variable LANG.  This specifies a single locale to use for all purposes.  For example, a user could specify a hypothetical locale named espana-castellano to use the standard conventions of most of Spain.

The set of locales supported depends on the operating system you are using, and so do their names.  We can't make any promises about what locales will exist, except for one standard locale called C or POSIX.  Later we will describe how to construct locales.

A user also has the option of specifying different locales for different purposes---in effect, choosing a mixture of multiple locales.

For example, the user might specify the locale espana-castellano for most purposes, but specify the locale usa-english for currency formatting.  This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars.

Note that both locales espana-castellano and usa-english, like all locales, would include conventions for all of the purposes to which locales apply.  However, the user can choose to use each locale for a particular subset of those purposes.

Categories of Activities that Locales Affect
============================================

The purposes that locales serve are grouped into categories, so that a user or a program can choose the locale for each category independently.  Here is a table of categories; each name is both an environment variable that a user can set, and a macro name that you can use as an argument to setlocale.

LC_COLLATE
     This category applies to collation of strings (functions strcoll and strxfrm); see Collation Functions.

LC_CTYPE
     This category applies to classification and conversion of characters, and to multibyte and wide characters; see Character Handling, and Character Set Handling.

LC_MONETARY
     This category applies to formatting monetary values; see General Numeric.

LC_NUMERIC
     This category applies to formatting numeric values that are not monetary; see General Numeric.

LC_TIME
     This category applies to formatting date and time values; see Formatting Calendar Time.

LC_MESSAGES
     This category applies to selecting the language used in the user interface for message translation (see The Uniforum approach; see Message catalogs a la X/Open)  and contains regular expressions for affirmative and negative responses.

LC_ALL
     This is not an environment variable; it is only a macro that you can use with setlocale to set a single locale for all purposes.  Setting this environment variable overwrites all selections by the other LC_* variables or LANG.

LANG
     If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above.

When developing the message translation functions it was felt that the functionality provided by the variables above is not sufficient.  For example, it should be possible to specify more than one locale name. Take a Swedish user who better speaks German than English, and a program whose messages are output in English by default.  It should be possible to specify that the first choice of language is Swedish, the second German, and if this also fails to use English.  This is possible with the variable LANGUAGE.  For further description of this GNU extension see Using gettextized software.

How Programs Set the Locale
===========================

A C program inherits its locale environment variables when it starts up. This happens automatically.  However, these variables do not automatically control the locale used by the library functions, because ISO C says that all programs start by default in the standard C locale.  To use the locales specified by the environment, you must call setlocale.  Call it as follows:

     setlocale (LC_ALL, "");

to select a locale based on the user choice of the appropriate environment variables.

You can also use setlocale to specify a particular locale, for general use or for a specific category.

The symbols in this section are defined in the header file locale.h.

 -- Function: char * setlocale (int CATEGORY, const char *LOCALE)
     The function setlocale sets the current locale for category CATEGORY to LOCALE.  A list of all the locales the system provides can be created by running

            locale -a

     If CATEGORY is LC_ALL, this specifies the locale for all purposes.  The other possible values of CATEGORY specify an single purpose (see Locale Categories).

     You can also use this function to find out the current locale by passing a null pointer as the LOCALE argument.  In this case, setlocale returns a string that is the name of the locale currently selected for category CATEGORY.

     The string returned by setlocale can be overwritten by subsequent calls, so you should make a copy of the string (see Copying and Concatenation) if you want to save it past any further calls to setlocale.  (The standard library is guaranteed never to call setlocale itself.)

     You should not modify the string returned by setlocale.  It might be the same string that was passed as an argument in a previous call to setlocale.  One requirement is that the CATEGORY must be the same in the call the string was returned and the one when the string is passed in as LOCALE parameter.

     When you read the current locale for category LC_ALL, the value encodes the entire combination of selected locales for all categories. In this case, the value is not just a single locale name.  In fact, we don't make any promises about what it looks like.  But if you specify the same ``locale name'' with LC_ALL in a subsequent call to setlocale, it restores the same combination of locale selections.

     To be sure you can use the returned string encoding the currently selected locale at a later time, you must make a copy of the string.  It is not guaranteed that the returned pointer remains valid over time.

     When the LOCALE argument is not a null pointer, the string returned by setlocale reflects the newly-modified locale.

     If you specify an empty string for LOCALE, this means to read the appropriate environment variable and use its value to select the locale for CATEGORY.

     If a nonempty string is given for LOCALE, then the locale of that name is used if possible.

     If you specify an invalid locale name, setlocale returns a null pointer and leaves the current locale unchanged.

Here is an example showing how you might use setlocale to temporarily switch to a new locale.

     #include <stddef.h>
     #include <locale.h>
     #include <stdlib.h>
     #include <string.h>

     void
     with_other_locale (char *new_locale,
                        void (*subroutine) (int),
                        int argument)
     {
       char *old_locale, *saved_locale;

       /* Get the name of the current locale.  */
       old_locale = setlocale (LC_ALL, NULL);

       /* Copy the name so it won't be clobbered by setlocale. */
       saved_locale = strdup (old_locale);
       if (saved_locale == NULL)
         fatal ("Out of memory");

       /* Now change the locale and do some stuff with it. */
       setlocale (LC_ALL, new_locale);
       (*subroutine) (argument);

       /* Restore the original locale. */
       setlocale (LC_ALL, saved_locale);
       free (saved_locale);
     }

Portability Note: Some ISO C systems may define additional locale categories, and future versions of the library will do so.  For portability, assume that any symbol beginning with LC_ might be defined in locale.h.

Standard Locales
================

The only locale names you can count on finding on all operating systems are these three standard ones:

"C"
     This is the standard C locale.  The attributes and behavior it provides are specified in the ISO C standard.  When your program starts up, it initially uses this locale by default.

"POSIX"
     This is the standard POSIX locale.  Currently, it is an alias for the standard C locale.

""
     The empty name says to select a locale based on environment variables. See Locale Categories.

Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library).  It is also possible for the user to create private locales.  All this will be discussed later when describing the tool to do so.

If your program needs to use something other than the C locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name.  Remember, different machines might have different sets of locales installed.

Accessing Locale Information
============================

There are several ways to access locale information.  The simplest way is to let the C library itself do the work.  Several of the functions in this library implicitly access the locale data, and use what information is provided by the currently selected locale.  This is how the locale model is meant to work normally.

As an example take the strftime function, which is meant to nicely format date and time information (see Formatting Calendar Time). Part of the standard information contained in the LC_TIME category is the names of the months.  Instead of requiring the programmer to take care of providing the translations the strftime function does this all by itself.  %A in the format string is replaced by the appropriate weekday name of the locale currently selected by LC_TIME.  This is an easy example, and wherever possible functions do things automatically in this way.

But there are quite often situations when there is simply no function to perform the task, or it is simply not possible to do the work automatically.  For these cases it is necessary to access the information in the locale directly.  To do this the C library provides two functions: localeconv and nl_langinfo.  The former is part of ISO C and therefore portable, but has a brain-damaged interface.  The second is part of the Unix interface and is portable in as far as the system follows the Unix standards.

localeconv: It is portable but ...
----------------------------------

Together with the setlocale function the ISO C people invented the localeconv function.  It is a masterpiece of poor design.  It is expensive to use, not extendable, and not generally usable as it provides access to only LC_MONETARY and LC_NUMERIC related information.  Nevertheless, if it is applicable to a given situation it should be used since it is very portable.  The function strfmon formats monetary amounts according to the selected locale using this information.

 -- Function: struct lconv * localeconv (void)
     The localeconv function returns a pointer to a structure whose components contain information about how numeric and monetary values should be formatted in the current locale.

     You should not modify the structure or its contents.  The structure might be overwritten by subsequent calls to localeconv, or by calls to setlocale, but no other function in the library overwrites this value.

 -- Data Type: struct lconv
     localeconv's return value is of this data type.  Its elements are described in the following subsections.

If a member of the structure struct lconv has type char, and the value is CHAR_MAX, it means that the current locale has no value for that parameter.

Generic Numeric Formatting Parameters
.....................................

These are the standard members of struct lconv; there may be others.

char *decimal_point
char *mon_decimal_point
     These are the decimal-point separators used in formatting non-monetary and monetary quantities, respectively.  In the C locale, the value of decimal_point is ".", and the value of mon_decimal_point is "".

char *thousands_sep
char *mon_thousands_sep
     These are the separators used to delimit groups of digits to the left of the decimal point in formatting non-monetary and monetary quantities, respectively.  In the C locale, both members have a value of "" (the empty string).

char *grouping
char *mon_grouping
     These are strings that specify how to group the digits to the left of the decimal point.  grouping applies to non-monetary quantities and mon_grouping applies to monetary quantities.  Use either thousands_sep or mon_thousands_sep to separate the digit groups.

     Each member of these strings is to be interpreted as an integer value of type char.  Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point.)  The last member is either 0, in which case the previous member is used over and over again for all the remaining groups, or CHAR_MAX, in which case there is no more grouping---or, put another way, any remaining digits form one large group without separators.

     For example, if grouping is "\04\03\02", the correct grouping for the number 123456787654321 is 12, 34, 56, 78, 765, 4321.  This uses a group of 4 digits at the end, preceded by a group of 3 digits, preceded by groups of 2 digits (as many as needed).  With a separator of ,, the number would be printed as 12,34,56,78,765,4321.

     A value of "\03" indicates repeated groups of three digits, as normally used in the U.S.

     In the standard C locale, both grouping and mon_grouping have a value of "".  This value specifies no grouping at all.

char int_frac_digits
char frac_digits
     These are small integers indicating how many fractional digits (to the right of the decimal point) should be displayed in a monetary value in international and local formats, respectively.  (Most often, both members have the same value.)

     In the standard C locale, both of these members have the value CHAR_MAX, meaning ``unspecified''.  The ISO standard doesn't say what to do when you find this value; we recommend printing no fractional digits.  (This locale also specifies the empty string for mon_decimal_point, so printing any fractional digits would be confusing!)

Printing the Currency Symbol
............................

These members of the struct lconv structure specify how to print the symbol to identify a monetary value---the international analog of $ for US dollars.

Each country has two standard currency symbols.  The local currency symbol is used commonly within the country, while the international currency symbol is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously.

For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars.  But when the context is known to be Canada, there is no need to make this explicit---dollar amounts are implicitly assumed to be in Canadian dollars.

char *currency_symbol
     The local currency symbol for the selected locale.

     In the standard C locale, this member has a value of "" (the empty string), meaning ``unspecified''.  The ISO standard doesn't say what to do when you find this value; we recommend you simply print the empty string as you would print any other string pointed to by this variable.

char *int_curr_symbol
     The international currency symbol for the selected locale.

     The value of int_curr_symbol should normally consist of a three-letter abbreviation determined by the international standard ISO 4217 Codes for the Representation of Currency and Funds, followed by a one-character separator (often a space).

     In the standard C locale, this member has a value of "" (the empty string), meaning ``unspecified''.  We recommend you simply print the empty string as you would print any other string pointed to by this variable.

char p_cs_precedes
char n_cs_precedes
char int_p_cs_precedes
char int_n_cs_precedes
     These members are 1 if the currency_symbol or int_curr_symbol strings should precede the value of a monetary amount, or 0 if the strings should follow the value.  The p_cs_precedes and int_p_cs_precedes members apply to positive amounts (or zero), and the n_cs_precedes and int_n_cs_precedes members apply to negative amounts.

     In the standard C locale, all of these members have a value of CHAR_MAX, meaning ``unspecified''.  The ISO standard doesn't say what to do when you find this value.  We recommend printing the currency symbol before the amount, which is right for most countries. In other words, treat all nonzero values alike in these members.

     The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.

char p_sep_by_space
char n_sep_by_space
char int_p_sep_by_space
char int_n_sep_by_space
     These members are 1 if a space should appear between the currency_symbol or int_curr_symbol strings and the amount, or 0 if no space should appear.  The p_sep_by_space and int_p_sep_by_space members apply to positive amounts (or zero), and the n_sep_by_space and int_n_sep_by_space members apply to negative amounts.

     In the standard C locale, all of these members have a value of CHAR_MAX, meaning ``unspecified''.  The ISO standard doesn't say what you should do when you find this value; we suggest you treat it as 1 (print a space).  In other words, treat all nonzero values alike in these members.

     The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.  There is one specialty with the int_curr_symbol, though.  Since all legal values contain a space at the end the string one either printf this space (if the currency symbol must appear in front and must be separated) or one has to avoid printing this character at all (especially when at the end of the string).

Printing the Sign of a Monetary Amount
......................................

These members of the struct lconv structure specify how to print the sign (if any) of a monetary value.

char *positive_sign
char *negative_sign
     These are strings used to indicate positive (or zero) and negative monetary quantities, respectively.

     In the standard C locale, both of these members have a value of "" (the empty string), meaning ``unspecified''.

     The ISO standard doesn't say what to do when you find this value; we recommend printing positive_sign as you find it, even if it is empty.  For a negative value, print negative_sign as you find it unless both it and positive_sign are empty, in which case print - instead.  (Failing to indicate the sign at all seems rather unreasonable.)

char p_sign_posn
char n_sign_posn
char int_p_sign_posn
char int_n_sign_posn
     These members are small integers that indicate how to position the sign for nonnegative and negative monetary quantities, respectively.  (The string used by the sign is what was specified with positive_sign or negative_sign.)  The possible values are as follows:

     0
          The currency symbol and quantity should be surrounded by parentheses.

     1
          Print the sign string before the quantity and currency symbol.

     2
          Print the sign string after the quantity and currency symbol.

     3
          Print the sign string right before the currency symbol.

     4
          Print the sign string right after the currency symbol.

     CHAR_MAX
          ``Unspecified''.  Both members have this value in the standard C locale.

     The ISO standard doesn't say what you should do when the value is CHAR_MAX.  We recommend you print the sign after the currency symbol.

     The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.

Pinpoint Access to Locale Data
------------------------------

When writing the X/Open Portability Guide the authors realized that the localeconv function is not enough to provide reasonable access to locale information.  The information which was meant to be available in the locale (as later specified in the POSIX.1 standard) requires more ways to access it.  Therefore the nl_langinfo function was introduced.

 -- Function: char * nl_langinfo (nl_item ITEM)
     The nl_langinfo function can be used to access individual elements of the locale categories.  Unlike the localeconv function, which returns all the information, nl_langinfo lets the caller select what information it requires.  This is very fast and it is not a problem to call this function multiple times.

     A second advantage is that in addition to the numeric and monetary formatting information, information from the LC_TIME and LC_MESSAGES categories is available.

     The type nl_type is defined in nl_types.h.  The argument ITEM is a numeric value defined in the header langinfo.h. The X/Open standard defines the following values:

     CODESET
          nl_langinfo returns a string with the name of the coded character set used in the selected locale.

     ABDAY_1
     ABDAY_2
     ABDAY_3
     ABDAY_4
     ABDAY_5
     ABDAY_6
     ABDAY_7
          nl_langinfo returns the abbreviated weekday name.  ABDAY_1 corresponds to Sunday.

     DAY_1
     DAY_2
     DAY_3
     DAY_4
     DAY_5
     DAY_6
     DAY_7
          Similar to ABDAY_1 etc., but here the return value is the unabbreviated weekday name.

     ABMON_1
     ABMON_2
     ABMON_3
     ABMON_4
     ABMON_5
     ABMON_6
     ABMON_7
     ABMON_8
     ABMON_9
     ABMON_10
     ABMON_11
     ABMON_12
          The return value is abbreviated name of the month.  ABMON_1 corresponds to January.

     MON_1
     MON_2
     MON_3
     MON_4
     MON_5
     MON_6
     MON_7
     MON_8
     MON_9
     MON_10
     MON_11
     MON_12
          Similar to ABMON_1 etc., but here the month names are not abbreviated. Here the first value MON_1 also corresponds to January.

     AM_STR
     PM_STR
          The return values are strings which can be used in the representation of time as an hour from 1 to 12 plus an am/pm specifier.

          Note that in locales which do not use this time representation these strings might be empty, in which case the am/pm format cannot be used at all.

     D_T_FMT
          The return value can be used as a format string for strftime to represent time and date in a locale-specific way.

     D_FMT
          The return value can be used as a format string for strftime to represent a date in a locale-specific way.

     T_FMT
          The return value can be used as a format string for strftime to represent time in a locale-specific way.

     T_FMT_AMPM
          The return value can be used as a format string for strftime to represent time in the am/pm format.

          Note that if the am/pm format does not make any sense for the selected locale, the return value might be the same as the one for T_FMT.

     ERA
          The return value represents the era used in the current locale.

          Most locales do not define this value.  An example of a locale which does define this value is the Japanese one.  In Japan, the traditional representation of dates includes the name of the era corresponding to the then-emperor's reign.

          Normally it should not be necessary to use this value directly. Specifying the E modifier in their format strings causes the strftime functions to use this information.  The format of the returned string is not specified, and therefore you should not assume knowledge of it on different systems.

     ERA_YEAR
          The return value gives the year in the relevant era of the locale. As for ERA it should not be necessary to use this value directly.

     ERA_D_T_FMT
          This return value can be used as a format string for strftime to represent dates and times in a locale-specific era-based way.

     ERA_D_FMT
          This return value can be used as a format string for strftime to represent a date in a locale-specific era-based way.

     ERA_T_FMT
          This return value can be used as a format string for strftime to represent time in a locale-specific era-based way.

     ALT_DIGITS
          The return value is a representation of up to 100 values used to represent the values 0 to 99.  As for ERA this value is not intended to be used directly, but instead indirectly through the strftime function.  When the modifier O is used in a format which would otherwise use numerals to represent hours, minutes, seconds, weekdays, months, or weeks, the appropriate value for the locale is used instead.

     INT_CURR_SYMBOL
          The same as the value returned by localeconv in the int_curr_symbol element of the struct lconv.

     CURRENCY_SYMBOL
     CRNCYSTR
          The same as the value returned by localeconv in the currency_symbol element of the struct lconv.

          CRNCYSTR is a deprecated alias still required by Unix98.

     MON_DECIMAL_POINT
          The same as the value returned by localeconv in the mon_decimal_point element of the struct lconv.

     MON_THOUSANDS_SEP
          The same as the value returned by localeconv in the mon_thousands_sep element of the struct lconv.

     MON_GROUPING
          The same as the value returned by localeconv in the mon_grouping element of the struct lconv.

     POSITIVE_SIGN
          The same as the value returned by localeconv in the positive_sign element of the struct lconv.

     NEGATIVE_SIGN
          The same as the value returned by localeconv in the negative_sign element of the struct lconv.

     INT_FRAC_DIGITS
          The same as the value returned by localeconv in the int_frac_digits element of the struct lconv.

     FRAC_DIGITS
          The same as the value returned by localeconv in the frac_digits element of the struct lconv.

     P_CS_PRECEDES
          The same as the value returned by localeconv in the p_cs_precedes element of the struct lconv.

     P_SEP_BY_SPACE
          The same as the value returned by localeconv in the p_sep_by_space element of the struct lconv.

     N_CS_PRECEDES
          The same as the value returned by localeconv in the n_cs_precedes element of the struct lconv.

     N_SEP_BY_SPACE
          The same as the value returned by localeconv in the n_sep_by_space element of the struct lconv.

     P_SIGN_POSN
          The same as the value returned by localeconv in the p_sign_posn element of the struct lconv.

     N_SIGN_POSN
          The same as the value returned by localeconv in the n_sign_posn element of the struct lconv.

     INT_P_CS_PRECEDES
          The same as the value returned by localeconv in the int_p_cs_precedes element of the struct lconv.

     INT_P_SEP_BY_SPACE
          The same as the value returned by localeconv in the int_p_sep_by_space element of the struct lconv.

     INT_N_CS_PRECEDES
          The same as the value returned by localeconv in the int_n_cs_precedes element of the struct lconv.

     INT_N_SEP_BY_SPACE
          The same as the value returned by localeconv in the int_n_sep_by_space element of the struct lconv.

     INT_P_SIGN_POSN
          The same as the value returned by localeconv in the int_p_sign_posn element of the struct lconv.

     INT_N_SIGN_POSN
          The same as the value returned by localeconv in the int_n_sign_posn element of the struct lconv.

     DECIMAL_POINT
     RADIXCHAR
          The same as the value returned by localeconv in the decimal_point element of the struct lconv.

          The name RADIXCHAR is a deprecated alias still used in Unix98.

     THOUSANDS_SEP
     THOUSEP
          The same as the value returned by localeconv in the thousands_sep element of the struct lconv.

          The name THOUSEP is a deprecated alias still used in Unix98.

     GROUPING
          The same as the value returned by localeconv in the grouping element of the struct lconv.

     YESEXPR
          The return value is a regular expression which can be used with the regex function to recognize a positive response to a yes/no question.  The GNU C library provides the rpmatch function for easier handling in applications.

     NOEXPR
          The return value is a regular expression which can be used with the regex function to recognize a negative response to a yes/no question.

     YESSTR
          The return value is a locale-specific translation of the positive response to a yes/no question.

          Using this value is deprecated since it is a very special case of message translation, and is better handled by the message translation functions (see Message Translation).

          The use of this symbol is deprecated.  Instead message translation should be used.

     NOSTR
          The return value is a locale-specific translation of the negative response to a yes/no question.  What is said for YESSTR is also true here.

          The use of this symbol is deprecated.  Instead message translation should be used.

     The file langinfo.h defines a lot more symbols but none of them is official.  Using them is not portable, and the format of the return values might change.  Therefore we recommended you not use them.

     Note that the return value for any valid argument can be used for in all situations (with the possible exception of the am/pm time formatting codes).  If the user has not selected any locale for the appropriate category, nl_langinfo returns the information from the "C" locale.  It is therefore possible to use this function as shown in the example below.

     If the argument ITEM is not valid, a pointer to an empty string is returned.

An example of nl_langinfo usage is a function which has to print a given date and time in a locale-specific way.  At first one might think that, since strftime internally uses the locale information, writing something like the following is enough:

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, "%X %D", tp);
     }

The format contains no weekday or month names and therefore is internationally usable.  Wrong!  The output produced is something like "hh:mm:ss MM/DD/YY".  This format is only recognizable in the USA.  Other countries use different formats.  Therefore the function should be rewritten like this:

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, nl_langinfo (D_T_FMT), tp);
     }

Now it uses the date and time format of the locale selected when the program runs.  If the user selects the locale correctly there should never be a misunderstanding over the time and date format.

A dedicated function to format numbers
======================================

We have seen that the structure returned by localeconv as well as the values given to nl_langinfo allow you to retrieve the various pieces of locale-specific information to format numbers and monetary amounts.  We have also seen that the underlying rules are quite complex.

Therefore the X/Open standards introduce a function which uses such locale information, making it easier for the user to format numbers according to these rules.

 -- Function: ssize_t strfmon (char *S, size_t MAXSIZE, const char *FORMAT, ...)
     The strfmon function is similar to the strftime function in that it takes a buffer, its size, a format string, and values to write into the buffer as text in a form specified by the format string.  Like strftime, the function also returns the number of bytes written into the buffer.

     There are two differences: strfmon can take more than one argument, and, of course, the format specification is different.  Like strftime, the format string consists of normal text, which is output as is, and format specifiers, which are indicated by a %. Immediately after the %, you can optionally specify various flags and formatting information before the main formatting character, in a similar way to printf:

       * Immediately following the % there can be one or more of the following flags:

          =F
               The single byte character F is used for this field as the numeric fill character.  By default this character is a space character. Filling with this character is only performed if a left precision is specified.  It is not just to fill to the given field width.

          ^
               The number is printed without grouping the digits according to the rules of the current locale.  By default grouping is enabled.

          +, (
               At most one of these flags can be used.  They select which format to represent the sign of a currency amount.  By default, and if + is given, the locale equivalent of +/- is used.  If ( is given, negative amounts are enclosed in parentheses.  The exact format is determined by the values of the LC_MONETARY category of the locale selected at program runtime.

          !
               The output will not contain the currency symbol.

          -
               The output will be formatted left-justified instead of right-justified if it does not fill the entire field width.

     The next part of a specification is an optional field width.  If no width is specified 0 is taken.  During output, the function first determines how much space is required.  If it requires at least as many characters as given by the field width, it is output using as much space as necessary.  Otherwise, it is extended to use the full width by filling with the space character.  The presence or absence of the - flag determines the side at which such padding occurs.  If present, the spaces are added at the right making the output left-justified, and vice versa.

     So far the format looks familiar, being similar to the printf and strftime formats.  However, the next two optional fields introduce something new.  The first one is a # character followed by a decimal digit string.  The value of the digit string specifies the number of digit positions to the left of the decimal point (or equivalent).  This does not include the grouping character when the ^ flag is not given.  If the space needed to print the number does not fill the whole width, the field is padded at the left side with the fill character, which can be selected using the = flag and by default is a space.  For example, if the field width is selected as 6 and the number is 123, the fill character is * the result will be ***123.

     The second optional field starts with a . (period) and consists of another decimal digit string.  Its value describes the number of characters printed after the decimal point.  The default is selected from the current locale (frac_digits, int_frac_digits, see General Numeric).  If the exact representation needs more digits than given by the field width, the displayed value is rounded.  If the number of fractional digits is selected to be zero, no decimal point is printed.

     As a GNU extension, the strfmon implementation in the GNU libc allows an optional L next as a format modifier.  If this modifier is given, the argument is expected to be a long double instead of a double value.

     Finally, the last component is a format specifier.  There are three specifiers defined:

     i
          Use the locale's rules for formatting an international currency value.

     n
          Use the locale's rules for formatting a national currency value.

     %
          Place a % in the output.  There must be no flag, width specifier or modifier given, only %% is allowed.

     As for printf, the function reads the format string from left to right and uses the values passed to the function following the format string.  The values are expected to be either of type double or long double, depending on the presence of the modifier L.  The result is stored in the buffer pointed to by S.  At most MAXSIZE characters are stored.

     The return value of the function is the number of characters stored in S, including the terminating NULL byte.  If the number of characters stored would exceed MAXSIZE, the function returns -1 and the content of the buffer S is unspecified.  In this case errno is set to E2BIG.

A few examples should make clear how the function works.  It is assumed that all the following pieces of code are executed in a program which uses the USA locale (en_US).  The simplest form of the format is this:

     strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678);

The output produced is

     "@$123.45@-$567.89@$12,345.68@"

We can notice several things here.  First, the widths of the output numbers are different.  We have not specified a width in the format string, and so this is no wonder.  Second, the third number is printed using thousands separators.  The thousands separator for the en_US locale is a comma.  The number is also rounded. .678 is rounded to .68 since the format does not specify a precision and the default value in the locale is 2.  Finally, note that the national currency symbol is printed since %n was used, not i.  The next example shows how we can align the output.

     strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678);

The output this time is:

     "@    $123.45@   -$567.89@ $12,345.68@"

Two things stand out.  Firstly, all fields have the same width (eleven characters) since this is the width given in the format and since no number required more characters to be printed.  The second important point is that the fill character is not used.  This is correct since the white space was not used to achieve a precision given by a # modifier, but instead to fill to the given width.  The difference becomes obvious if we now add a width specification.

     strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@",
              123.45, -567.89, 12345.678);

The output is

     "@ $***123.45@-$***567.89@ $12,456.68@"

Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character.  Note that although the width is selected to be 5 and 123.45 has three digits left of the decimal point, the space is filled with three asterisks.  This is correct since, as explained above, the width does not include the positions used to store thousands separators.  One last example should explain the remaining functionality.

     strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@",
              123.45, -567.89, 12345.678);

This rather complex format string produces the following output:

     "@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @"

The most noticeable change is the alternative way of representing negative numbers.  In financial circles this is often done using parentheses, and this is what the ( flag selected.  The fill character is now 0.  Note that this 0 character is not regarded as a numeric zero, and therefore the first and second numbers are not printed using a thousands separator.  Since we used the format specifier i instead of n, the international form of the currency symbol is used.  This is a four letter string, in this case "USD ".  The last point is that since the precision right of the decimal point is selected to be three, the first and second numbers are printed with an extra zero at the end and the third number is printed without rounding.

Yes-or-No Questions
===================

Some non GUI programs ask a yes-or-no question.  If the messages (especially the questions) are translated into foreign languages, be sure that you localize the answers too.  It would be very bad habit to ask a question in one language and request the answer in another, often English.

The GNU C library contains rpmatch to give applications easy access to the corresponding locale definitions.

 -- Function: int rpmatch (const char *RESPONSE)
     The function rpmatch checks the string in RESPONSE whether or not it is a correct yes-or-no answer and if yes, which one.  The check uses the YESEXPR and NOEXPR data in the LC_MESSAGES category of the currently selected locale.  The return value is as follows:

     1
          The user entered an affirmative answer.

     0
          The user entered a negative answer.

     -1
          The answer matched neither the YESEXPR nor the NOEXPR regular expression.

     This function is not standardized but available beside in GNU libc at least also in the IBM AIX library.

This function would normally be used like this:

       ...
       /* Use a safe default.  */
       _Bool doit = false;

       fputs (gettext ("Do you really want to do this? "), stdout);
       fflush (stdout);
       /* Prepare the getline call.  */
       line = NULL;
       len = 0;
       while (getline (&line, &len, stdout) >= 0)
         {
           /* Check the response.  */
           int res = rpmatch (line);
           if (res >= 0)
             {
               /* We got a definitive answer.  */
               if (res > 0)
                 doit = true;
               break;
             }
         }
       /* Free what getline allocated.  */
       free (line);

Note that the loop continues until an read error is detected or until a definitive (positive or negative) answer is read.