A running C program has the concept of a current locale, a set of representational customs and configuration. This configuration is divided into distinct locale categories. Each category affects different functions, and can be independently set to different locales. Locales are identified by implementation-defined strings. Categories are identified by symbolic constants.

#include <locale.h>
char *setlocale(int cat, const char *loc);

setlocale can be used to set or get the current locale for a given category. If loc is NULL, it returns a pointer to a null-terminated string that describes the current locale in the category cat.

Otherise, loc must point to a null-terminated string that identifies a locale. cat can then be either LC_ALL, indicating that all categories shall be set to the specified locale, or one of the category symbols beginning with LC_ below. The function attempts to set the locale for the category, and returns NULL on failure. If it succeeds, it returns a null-terminated string which can passed as loc to a future call to setlocale in the same category. restoring the locale to previous state. The result should be saved separately, as other calls to setlocale could overwrite it.

Two locale identifiers are defined. A string "C" identifies a minimal but very portable locale, and applies by default. [Therefore, it has to be compatible with the translation-time locale, right? Otherwise, no string literal containing non-basic characters would make sense.] An empty string "" identifies an environment-defined locale, which often means that values of certain environmental parameters will be used to determine the locale, although the mechanism is implementation-defined.

There are five standard locale categories:

#include <locale.h>
struct lconv *localeconv(void);

<locale.h> defines a structure type struct lconv, and localeconv returns a pointer to such a structure based on the locale categories LC_NUMERIC and LC_MONETARY. The structure has the following members: