def test_syscallErrorWithControlMessage(self):
        """
        The behavior when the underlying C{sendmsg} call fails is the same
        whether L{send1msg} is passed ancillary data or not.
        """
        with open(devnull) as probe:
            fd = probe.fileno()
        exc = self.assertRaises(
            error, send1msg, fd, "hello, world", 0, [(0, 0, "0123")])
        self.assertEqual(exc.args[0], errno.EBADF)

        The behavior when the underlying C{sendmsg} call fails is the same
        whether L{send1msg} is passed ancillary data or not.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/test/test_sendmsg.py