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.