The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


setlocale - set program locale


#include <locale.h>

char *setlocale(int
category, const char *locale);


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

The setlocale() function selects the appropriate piece of the program's locale, as specified by the category and locale arguments, and may be used to change or query the program's entire locale or portions thereof. The value LC_ALL for category names the program's entire locale; other values for category name only a part of the program's locale:

Affects the behavior of regular expressions and the collation functions.
Affects the behavior of regular expressions, character classification, character conversion functions, and wide-character functions.
[CX] [Option Start] Affects what strings are expected by commands and utilities as affirmative or negative responses. [Option End]

[XSI] [Option Start] It also affects what strings are given by commands and utilities as affirmative or negative responses, and the content of messages. [Option End]

Affects the behavior of functions that handle monetary values.
Affects the behavior of functions that handle numeric values.
Affects the behavior of the time conversion functions.

The locale argument is a pointer to a character string containing the required setting of category. The contents of this string are implementation-defined. In addition, the following preset values of locale are defined for all settings of category:

[CX] [Option Start] Specifies the minimal environment for C-language translation called the POSIX locale. If setlocale() is not invoked, the POSIX locale is the default at entry to main(). [Option End]
Equivalent to "POSIX".
Specifies an implementation-defined native environment. [CX] [Option Start]  The determination of the name of the new locale for the specified category depends on the value of the associated environment variables, LC_* and LANG ; see the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 7, Locale and the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Variables. [Option End]
A null pointer
Used to direct setlocale() to query the current internationalized environment and return the name of the locale.

[CX] [Option Start] Setting all of the categories of the locale of the process is similar to successively setting each individual category of the locale of the process, except that all error checking is done before any actions are performed. To set all the categories of the locale of the process, setlocale() is invoked as:

setlocale(LC_ALL, "");

In this case, setlocale() shall first verify that the values of all the environment variables it needs according to the precedence rules (described in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Variables) indicate supported locales. If the value of any of these environment variable searches yields a locale that is not supported (and non-null), setlocale() shall return a null pointer and the locale of the process shall not be changed. If all environment variables name supported locales, setlocale() shall proceed as if it had been called for each category, using the appropriate value from the associated environment variable or from the implementation-defined default if there is no such value. [Option End]

[THR] [Option Start] The locale state is common to all threads within a process. [Option End]


Upon successful completion, setlocale() shall return the string associated with the specified category for the new locale. Otherwise, setlocale() shall return a null pointer and the program's locale is not changed.

A null pointer for locale causes setlocale() to return a pointer to the string associated with the category for the program's current locale. The program's locale shall not be changed.

The string returned by setlocale() is such that a subsequent call with that string and its associated category shall restore that part of the program's locale. The application shall not modify the string returned which may be overwritten by a subsequent call to setlocale().


No errors are defined.

The following sections are informative.




The following code illustrates how a program can initialize the international environment for one language, while selectively modifying the program's locale such that regular expressions and string operations can be applied to text recorded in a different language:

setlocale(LC_ALL, "De");
setlocale(LC_COLLATE, "Fr@dict");

Internationalized programs must call setlocale() to initiate a specific language operation. This can be done by calling setlocale() as follows:

setlocale(LC_ALL, "");

Changing the setting of LC_MESSAGES has no effect on catalogs that have already been opened by calls to catopen().


The ISO C standard defines a collection of functions to support internationalization. One of the most significant aspects of these functions is a facility to set and query the international environment. The international environment is a repository of information that affects the behavior of certain functionality, namely:

  1. Character handling

  2. Collating

  3. Date/time formatting

  4. Numeric editing

  5. Monetary formatting

  6. Messaging

The setlocale() function provides the application developer with the ability to set all or portions, called categories, of the international environment. These categories correspond to the areas of functionality mentioned above. The syntax for setlocale() is as follows:

char *setlocale(int category, const char *locale);

where category is the name of one of following categories, namely:







In addition, a special value called LC_ALL directs setlocale() to set all categories.

There are two primary uses of setlocale():

  1. Querying the international environment to find out what it is set to

  2. Setting the international environment, or locale, to a specific value

The behavior of setlocale() in these two areas is described below. Since it is difficult to describe the behavior in words, examples are used to illustrate the behavior of specific uses.

To query the international environment, setlocale() is invoked with a specific category and the NULL pointer as the locale. The NULL pointer is a special directive to setlocale() that tells it to query rather than set the international environment. The following syntax is used to query the name of the international environment:

    LC_NUMERIC, LC_TIME},(char *) NULL);

The setlocale() function shall return the string corresponding to the current international environment. This value may be used by a subsequent call to setlocale() to reset the international environment to this value. However, it should be noted that the return value from setlocale() may be a pointer to a static area within the function and is not guaranteed to remain unchanged (that is, it may be modified by a subsequent call to setlocale()). Therefore, if the purpose of calling setlocale() is to save the value of the current international environment so it can be changed and reset later, the return value should be copied to an array of char in the calling program.

There are three ways to set the international environment with setlocale():

This usage sets a specific category in the international environment to a specific value corresponding to the value of the string. A specific example is provided below:
setlocale(LC_ALL, "fr_FR.ISO-8859-1");

In this example, all categories of the international environment are set to the locale corresponding to the string "fr_FR.ISO-8859-1", or to the French language as spoken in France using the ISO/IEC 8859-1:1998 standard codeset.

If the string does not correspond to a valid locale, setlocale() shall return a NULL pointer and the international environment is not changed. Otherwise, setlocale() shall return the name of the locale just set.

setlocale(category, "C")
The ISO C standard states that one locale must exist on all conforming implementations. The name of the locale is C and corresponds to a minimal international environment needed to support the C programming language.
setlocale(category, "")
This sets a specific category to an implementation-defined default. This corresponds to the value of the environment variables.




exec(), isalnum(), isalpha(), isblank(), iscntrl(), isdigit(), isgraph(), islower(), isprint(), ispunct(), isspace(), isupper(), iswalnum(), iswalpha(), iswblank(), iswcntrl(), iswctype(), iswdigit(), iswgraph(), iswlower(), iswprint(), iswpunct(), iswspace(), iswupper(), iswxdigit(), isxdigit(), localeconv(), mblen(), mbstowcs() , mbtowc(), nl_langinfo(), printf(), scanf(), setlocale, strcoll(), strerror(), strfmon(), strtod(), strxfrm(), tolower(), toupper(), towlower(), towupper(), wcscoll(), wcstod(), wcstombs(), wcsxfrm(), wctomb(), the Base Definitions volume of IEEE Std 1003.1-2001, <langinfo.h>, <locale.h>


First released in Issue 3.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Threads Extension.

Issue 6

Extensions beyond the ISO C standard are marked.

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/124 is applied, updating the DESCRIPTION to clarify the behavior of:

setlocale(LC_ALL, "");

End of informative text.

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