If the character is n or fewer bytes, the mbtowc function
    converts the multibyte character pointed to by s to its wide-
    character equivalent. If the character is invalid or greater than
    n bytes, the function returns -1 to indicate an error.

    If pwc is a NULL pointer and s is not a null pointer, the
    function determines the number of bytes that constitute the
    multibyte character pointed to by s (regardless of the value
    of n).

    This function is affected by the LC_CTYPE category of the
    program's current locale.