Locale Categories - The GNU C Library

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### 7.3 Categories of Activities that Locales Affect

The purposes that locales serve are grouped into categories, so that a user or a program can choose the locale for each category independently. Here is a table of categories; each name is both an environment variable that a user can set, and a macro name that you can use as an argument to `setlocale`.

`LC_COLLATE`
  This category applies to collation of strings (functions `strcoll` and `strxfrm`); see Collation Functions.
`LC_CTYPE`
  This category applies to classification and conversion of characters, and to multibyte and wide characters; see Character Handling, and Character Set Handling.
`LC_MONETARY`
  This category applies to formatting monetary values; see General Numeric.
`LC_NUMERIC`
  This category applies to formatting numeric values that are not monetary; see General Numeric.
`LC_TIME`
  This category applies to formatting date and time values; see Formatting Calendar Time.
`LC_MESSAGES`
  This category applies to selecting the language used in the user interface for message translation (see The Uniforum approach; see Message catalogs a la X/Open) and contains regular expressions for affirmative and negative responses.
`LC_ALL`
  This is not an environment variable; it is only a macro that you can use with `setlocale` to set a single locale for all purposes. Setting this environment variable overwrites all selections by the other `LC_*` variables or `LANG`.
`LANG`
  If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above.

When developing the message translation functions it was felt that the functionality provided by the variables above is not sufficient. For example, it should be possible to specify more than one locale name. Take a Swedish user who better speaks German than English, and a program whose messages are output in English by default. It should be possible to specify that the first choice of language is Swedish, the second German, and if this also fails to use English. This is possible with the variable `LANGUAGE`. For further description of this GNU extension see Using gettextized software.