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setlocale - set the current locale.
#include <locale.h> char *setlocale(int category, const char * locale);
The setlocale() function is used to set or query the program's current 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. LC_ALL for all of the locale. LC_COLLATE for regular expression matching (it determines the meaning of range expressions and equivalence classes) and string collation. LC_CTYPE for regular expression matching, character classification, conversion, case- sensitive comparison, and wide character functions. LC_MESSAGES for localizable natural-language messages. LC_MONETARY for monetary formatting. LC_NUMERIC for number formatting (such as the decimal point and the thousands separator). LC_TIME for time and date formatting. 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. 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(), wstrcoll() or strxfrm(), wstrxfrm() to compare strings.
A successful call to setlocale() returns a string that corresponds to the locale set. This string may be allocated in static storage. The string returned is such that a subsequent call with that string and its associated category will restore that part of the process's locale. The return value is NULL if the request cannot be honored.
ANSI C, POSIX.1
Linux (that is, GNU libc) supports the portable locales "C" and "POSIX". In the good old days there used to be support for the European Latin-1 "ISO-8859-1" locale (e.g. in libc-4.5.21 and libc-4.6.27), and the Russian "KOI-8" (more precisely, "koi-8r") locale (e.g. in libc-4.6.27), so that having an environment variable LC_CTYPE=ISO-8859-1 sufficed to make isprint() return the right answer. These days non-English speaking Europeans have to work a bit harder, and must install actual locale files.