Return_Values

   x                  The number of bytes stored in s, not including
                      the null terminating byte. If s is NULL,
                      wcstombs returns the number of bytes required
                      for the multibyte character array.
   (size_t) -1        Indicates an error occurred. The function sets
                      errno to EILSEQ - invalid character sequence,
                      or a wide-character code does not correspond
                      to a valid character.