ANSI 3.1.3.4 The current locale used to convert multibyte characters into corresponding wide characters (codes) for a wide character constant
The current locale is the “C” locale by default. It can be changed with the setlocale library routine. The LC_CTYPE category of the current locale sets the current working code page, which determines correspondence and conversion between the multibyte and wide-character sets. The mbstowcs, wcstombs, mbtowc, and wctomb library routines provide direct mappings between the multibyte and wide-character sets. Also, many of the stream routines, such as the print, scan, get, and put families, automatically provide mappings between these two character sets.