btowc(3) -- Linux man page
NAMEbtowc - convert single byte to wide character
#include <wchar.h> wint_t btowc(int c);
DESCRIPTIONThe btowc function converts c, interpreted as a multibyte sequence of length 1, starting in the initial shift state, to a wide character and returns it. If c is EOF or not a valid multibyte sequence of length 1, the btowc function returns WEOF.
RETURN VALUEThe btowc function returns the wide character converted from the single byte c. If c is EOF or not a valid multibyte sequence of length 1, it returns WEOF.
CONFORMING TOISO/ANSI C, UNIX98
NOTESThe behaviour of btowc depends on the LC_CTYPE category of the current locale.
This function should never be used. It does not work for encodings which have state, and unnecessarily treats single bytes differently from multibyte sequences. Use the function mbtowc instead.