#include <locale.h> char *setlocale(int category, const char *locale);
If locale is not NULL, the program's current locale is modified according to the arguments. The argument category determines which parts of the program's current locale should be modified.
| Category | Governs |
| LC_ALL | All of the locale |
| LC_ADDRESS |
Formatting of addresses and
geography-related items (*) |
| LC_COLLATE | String collation |
| LC_CTYPE | Character classification |
| LC_IDENTIFICATION | Metadata describing the locale (*) |
| LC_MEASUREMENT |
Settings related to measurements
(metric versus US customary) (*) |
| LC_MESSAGES | Localizable natural-language messages |
| LC_MONETARY | Formatting of monetary values |
| LC_NAME | Formatting of salutations for persons (*) |
| LC_NUMERIC | Formatting of nonmonetary numeric values |
| LC_PAPER | Settings related to the standard paper size (*) |
| LC_TELEPHONE | Formats to be used with telephone services (*) |
| LC_TIME | Formatting of date and time values |
The categories marked with an asterisk in the above table are GNU extensions. For further information on these locale categories, see locale(7).
The argument locale is a pointer to a character string containing the required setting of category. Such a string is either a well-known constant like "C" or "da_DK" (see below), or an opaque string that was returned by another call of setlocale().
If locale is an empty string, "", each part of the locale that should be modified is set according to the environment variables. The details are implementation-dependent. For glibc, first (regardless of category), the environment variable LC_ALL is inspected, next the environment variable with the same name as the category (see the table above), and finally the environment variable LANG. The first existing environment variable is used. If its value is not a valid locale specification, the locale is unchanged, and setlocale() returns NULL.
The locale C or POSIX is a portable locale; it exists on all conforming systems.
A locale name is typically of the form language[_territory][.codeset][@modifier], where language is an ISO 639 language code, territory is an ISO 3166 country code, and codeset is a character set or encoding identifier like ISO-8859-1 or UTF-8. For a list of all supported locales, try "locale -a" (see locale(1)).
If locale is NULL, the current locale is only queried, not modified.
On startup of the main program, the portable C locale is selected as default. A program may be made portable to all locales by calling:
setlocale(LC_ALL, "");
after program initialization, by using the values returned from a localeconv(3) call for locale-dependent information, by using the multibyte and wide character functions for text processing if MB_CUR_MAX > 1, and by using strcoll(3), wcscoll(3) or strxfrm(3), wcsxfrm(3) to compare strings.
| Interface | Attribute | Value |
| setlocale() | Thread safety | MT-Unsafe const:locale env |
The C standards specify only the categories LC_ALL, LC_COLLATE, LC_CTYPE, LC_MONETARY, LC_NUMERIC, and LC_TIME. POSIX.1 adds LC_MESSAGES. The remaining categories are GNU extensions.