wint_t btowc_l(int c, locale_t loc);
btowc_l function determines whether c constitutes a valid single-byte character in the locale loc. If c is a valid single-byte character, btowc_l returns the wide character representation of that character
btowc_l returns WEOF if c has the value EOF or if (unsigned char)c does not constitute a valid single-byte character in the initial shift state.