Return_Values

   x                  The converted value.
   0                  Indicates that the string starts with an
                      unrecognized wide character or that the value
                      for base is invalid. If the string starts with
                      an unrecognized wide character, *endptr is set
                      to nptr. The function sets errno to EINVAL.
   ULONG_MAX          Indicates that the converted value would
                      cause an overflow. The function sets errno
                      to ERANGE.