---
name: "setlocale"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/setlocale"
---

# setlocale(3)

natural language formatting for C

## NAME

**setlocale** – natural language formatting for C

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<locale.h\>**

*char* *\**\

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

## DESCRIPTION

The **setlocale**() function sets the C library's notion of natural language formatting style for particular sets of routines. Each such style is called a ‘locale’ and is invoked using an appropriate name passed as a C string.

The **setlocale**() function recognizes several categories of routines. These are the categories and the sets of routines they select:

- LC\_ALL

  Set the entire locale generically.

- LC\_COLLATE

  Set a locale for string collation routines. This controls alphabetic ordering in **strcoll**() and **strxfrm**().

- LC\_CTYPE

  Set a locale for the [ctype(3)](/ctype/3.md) and [multibyte(3)](/multibyte/3.md) functions. This controls recognition of upper and lower case, alphabetic or non-alphabetic characters, and so on.

- LC\_MESSAGES

  Set a locale for message catalogs, see [catopen(3)](/catopen/3.md) function.

- LC\_MONETARY

  Set a locale for formatting monetary values; this affects the **localeconv**() function.

- LC\_NUMERIC

  Set a locale for formatting numbers. This controls the formatting of decimal points in input and output of floating point numbers in functions such as **printf**() and **scanf**(), as well as values returned by **localeconv**().

- LC\_TIME

  Set a locale for formatting dates and times using the **strftime**() function.

- LANG

  Sets the generic locale category for native language, local customs and coded character set in the absence of more specific locale variables.

Only three locales are defined by default, the empty string "" which denotes the native environment, and the "C" and "POSIX" locales, which denote the C language environment. A *locale* argument of NULL causes **setlocale**() to return the current locale.

The option **-a** to the [locale(1)](/locale/1.md) command can be used to display all further possible names for the *locale* argument that are recognized. Specifying any unrecognized value for *locale* makes **setlocale**() fail.

By default, C programs start in the "C" locale.

The only function in the library that sets the locale is **setlocale**(); the locale is never changed as a side effect of some other routine.

## RETURN VALUES

Upon successful completion, **setlocale**() returns the string associated with the specified *category* for the requested *locale*. The **setlocale**() function returns NULL and fails to change the locale if the given combination of *category* and *locale* makes no sense.

## FILES

- *$PATH\_LOCALE/**locale/category*

  - */usr/share/locale/**locale/category*

  locale file for the locale *locale* and the category *category*.

## EXAMPLES

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");
```

When a process is started, its current locale is set to the C or POSIX locale. An internationalized program that depends on locale data not defined in the C or POSIX locale must invoke the setlocale subroutine in the following manner before using any of the locale-specific information:

```
    setlocale(LC_ALL, "");
```

## ERRORS

No errors are defined.

## SEE ALSO

[locale(1)](/locale/1.md), [localedef(1)](/localedef/1.md), [catopen(3)](/catopen/3.md), [ctype(3)](/ctype/3.md), [localeconv(3)](/localeconv/3.md), [multibyte(3)](/multibyte/3.md), [strcoll(3)](/strcoll/3.md), [strxfrm(3)](/strxfrm/3.md), [euc(5)](/euc/5.md), [utf8(5)](/utf8/5.md), [environ(7)](/environ/7.md)

## STANDARDS

The **setlocale**() function conforms to ISO/IEC 9899:1999 (“ISO C99”).

## HISTORY

The **setlocale**() function first appeared in 4.4BSD.