#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.
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 , 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 (LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, LC_TIME) 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; its LC_CTYPE part corresponds to the 7-bit ASCII character set.
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", cf. 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() call for locale-dependent information, by using the multi-byte and wide character functions for text processing if MB_CUR_MAX > 1, and by using strcoll(), wcscoll() or strxfrm(), wcsxfrm() to compare strings.