GNU C Library (libc) Programming Guide - The Elegant and Fast Way

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7.6.2 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.