def testS():

    # test S
    assert packet.pack('S','abc') == struct.pack('<cI3s', b'S', 3, b'abc')
    assert packet.pack('S','abcd') != struct.pack('<cI3s', b'S', 3, b'abc')

    assert packet.unpack_stream(
        io.BytesIO(struct.pack('<cI3s', b'S', 3, b'abc'))) == ('S', 'abc')

    assert packet.unpack_stream(
        io.BytesIO(struct.pack('<cI3s', b'S', 3, b'abc'))) != ('S', 'abcd')