x                  The number of array elements modified or
                       required, not included any terminating zero
                       code. The array will not be zero-terminated
                       if the value returned is n. If pwcs is the
                       NULL pointer, mbstowcs returns the number
                       of elements required for the wide-character
                       array.
    (size_t) -1        Indicates that an error occurred. The function
                       sets errno to EILSEQ - Invalid character
                       detected.