Return_Values

   0                  Successful completion.
   -1                 Indicates an error. Neither of the group IDs
                      is changed, and errno is set to one of the
                      following values:

                      o  EINVAL - The value of the rgid or egid
                         argument is invalid or out-of-range.

                      o  EPERM - The process does not have the
                         IMPERSONATE privilege, and a change
                         other than changing the real group ID to
                         the saved set-group-ID, or changing the
                         effective group ID to the real group ID or
                         the saved group ID, was requested.