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Apache C++ Standard Library User's Guide

24.1 The C Locale

All the culture and language dependencies discussed in the previous section need to be represented in an operating system. This information is usually represented in a kind of language table, called a locale.

The X/Open consortium has standardized a variety of services for Native Language Support (NLS) in the programming language C. This standard is commonly known as XPG4. Internationalization services as well as localization support are included in X/Open's Native Language Support. The description below is based on this standard.

According to XPG4, the C locale is composed of several categories, which are given in Table 24.

Table 24: Categories of the C locale 

Category Content


Rules and symbols for numbers


Values for date and time information


Rules and symbols for monetary information


Character classification and case conversion


Collation sequence


Formats and values of messages

The external representation of a C locale is usually a file in UNIX. Other operating systems may use other representations. The external representation is transformed into an internal memory representation by calling the function setlocale(), as shown in Figure 4:

Figure 4: Transformation of a C locale from external to internal representation

Inside a program, the C locale is represented by one or more global data structures. The C library provides a set of functions that use information from those global data structures to adapt their behavior to local conventions. Examples of these functions and the information they cover are listed in Table 25:

Table 25: C locale functions and the information they cover 

C Locale Function Information Covered

setlocale(), ...

Locale initialization and

language information

isalpha(), isupper(), isdigit(), ...

Character classification

strftime(), ...

Date and time functions


Monetary functions

printf(), scanf(), ...

Number parsing and formatting

strcoll(), wcscoll(), ...

String collation

mblen(), mbtowc(), wctomb(), ...

Multibyte functions

catopen(), catgets(), catclose()

Message retrieval

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