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### 9.1 `setlocale`, `localeconv`—select or query locale

Synopsis

    #include <locale.h>
    char *setlocale(int category, const char *locale);
    lconv *localeconv(void);

    char *_setlocale_r(void *reent,
        int category, const char *locale);
    lconv *_localeconv_r(void *reent);

Description
`setlocale` is the facility defined by ANSI C to condition the execution environment for international collating and formatting information; `localeconv` reports on the settings of the current locale.

This is a minimal implementation, supporting only the required `"POSIX"` and `"C"` values for locale; strings representing other locales are not honored unless _MB_CAPABLE is defined.

If _MB_CAPABLE is defined, POSIX locale strings are allowed, following the form

language[_TERRITORY][.charset][@modifier]

`"language"` is a two character string per ISO 639, or, if not available for a given language, a three character string per ISO 639-3. `"TERRITORY"` is a country code per ISO 3166. For `"charset"` and `"modifier"` see below.

Additionally to the POSIX specifier, the following extension is supported for backward compatibility with older implementations using newlib: `"C-charset"`. Instead of `"C-"`, you can also specify `"C."`. Both variations allow to specify language neutral locales while using other charsets than ASCII, for instance `"C.UTF-8"`, which keeps all settings as in the C locale, but uses the UTF-8 charset.

The following charsets are recognized: `"UTF-8"`, `"JIS"`, `"EUCJP"`, `"SJIS"`, `"KOI8-R"`, `"KOI8-U"`, `"GEORGIAN-PS"`, `"PT154"`, `"TIS-620"`, `"ISO-8859-x"` with 1 <= x <= 16, or `"CPxxx"` with xxx in [437, 720, 737, 775, 850, 852, 855, 857, 858, 862, 866, 874, 932, 1125, 1250, 1251, 1252, 1253, 1254, 1255, 1256, 1257, 1258].

Charsets are case insensitive. For instance, `"EUCJP"` and `"eucJP"` are equivalent. Charset names with dashes can also be written without dashes, as in `"UTF8"`, `"iso88591"` or `"koi8r"`. `"EUCJP"` and `"EUCKR"` are also recognized with dash, `"EUC-JP"` and `"EUC-KR"`.

Full support for all of the above charsets requires that newlib has been build with multibyte support and support for all ISO and Windows Codepage. Otherwise all singlebyte charsets are simply mapped to ASCII. Right now, only newlib for Cygwin is built with full charset support by default. Under Cygwin, this implementation additionally supports the charsets `"GBK"`, `"GB2312"`, `"eucCN"`, `"eucKR"`, and `"Big5"`. Cygwin does not support `"JIS"`.

Cygwin additionally supports locales from the file /usr/share/locale/locale.alias.

(`""` is also accepted; if given, the settings are read from the corresponding LC_* environment variables and $LANG according to POSIX rules.)

This implementation also supports the modifier `"cjknarrow"`, which affects how the functions `wcwidth` and `wcswidth` handle characters from the "CJK Ambiguous Width" category of characters described at http://www.unicode.org/reports/tr11/#Ambiguous. These characters have a width of 1 for singlebyte charsets and a width of 2 for multibyte charsets other than UTF-8. For UTF-8, their width depends on the language specifier: it is 2 for `"zh"` (Chinese), `"ja"` (Japanese), and `"ko"` (Korean), and 1 for everything else. Specifying `"cjknarrow"` forces a width of 1, independent of charset and language.

If you use `NULL` as the locale argument, `setlocale` returns a pointer to the string representing the current locale. The acceptable values for category are defined in ‘`locale.h`’ as macros beginning with `"LC_"`.

`localeconv` returns a pointer to a structure (also defined in ‘`locale.h`’) describing the locale-specific conventions currently in effect.

`_localeconv_r` and `_setlocale_r` are reentrant versions of `localeconv` and `setlocale` respectively. The extra argument reent is a pointer to a reentrancy structure.

Returns
A successful call to `setlocale` returns a pointer to a string associated with the specified category for the new locale. The string returned by `setlocale` is such that a subsequent call using that string will restore that category (or all categories in case of LC_ALL), to that state. The application shall not modify the string returned which may be overwritten by a subsequent call to `setlocale`. On error, `setlocale` returns `NULL`.

`localeconv` returns a pointer to a structure of type `lconv`, which describes the formatting and collating conventions in effect (in this implementation, always those of the C locale).

Portability
ANSI C requires `setlocale`, but the only locale required across all implementations is the C locale.

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