In computing, C localization functions are a group of functions in the C programming language implementing basic localization routines. The functions are used in multilingual programs to adapt to the specific locale. In particular, the way of displaying of numbers and currency can be modified. These settings affect the behaviour of input/output functions in the C Standard Library.

C localization functions and types are defined in locale.h (clocale header in C++).

The localization state is stored globally. This means that in a given program all operations involving a locale can use only one locale at a time. As a result, it is very difficult to implement programs that use more than one locale.

The functions alter the behavior of printf, scanf, strtod and other functions which are often used to write saved data to a file or to other programs. The result is that a saved file in one locale will not be readable in another locale, or not be readable at all due to assumptions such as "numbers don't have commas in them". Most large-scale software forces the locale to "C" (or another fixed value) to work around these problems.