#### 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)

> Preliminary: | MT-Safe locale | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> 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.