RETURN VALUES
If mbchar is NULL, the mbtowc function returns nonzero if shift states are supported, zero otherwise. Otherwise, if mbchar is not a null pointer, mbtowc either returns 0 if mbchar represents the null wide character, or returns the number of bytes processed in mbchar, or returns -1 if no multibyte character could be recognized or converted. In this case, mbtowc s internal conversion state is undefined.
ERRORS
The mbtowc function will fail if: