The setlocale() function is used to set or query the program's current
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.
for all of the locale.
for regular expression matching (it determines the meaning
of range expressions and equivalence classes) and string collation.
for regular expression matching, character classification,
conversion, case-sensitive comparison, and wide character functions.
for localizable natural-language messages.
for monetary formatting.
for number formatting (such as the decimal point and the
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
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 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
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.
A successful call to setlocale() returns an opaque 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.
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.