Python socket.MSG_CTRUNC Examples

def _read_frame(self, ts_mono_ns: int) -> _media.TimestampedDataFrame:
        while True:
            data, ancdata, msg_flags, _addr = self._sock.recvmsg(self._native_frame_size,
                                                                 self._ancillary_data_buffer_size)
            assert msg_flags & socket.MSG_TRUNC == 0, 'The data buffer is not large enough'
            assert msg_flags & socket.MSG_CTRUNC == 0, 'The ancillary data buffer is not large enough'

            loopback = bool(msg_flags & socket.MSG_CONFIRM)
            ts_system_ns = 0
            for cmsg_level, cmsg_type, cmsg_data in ancdata:
                if cmsg_level == socket.SOL_SOCKET and cmsg_type == _SO_TIMESTAMP:
                    sec, usec = _TIMEVAL_STRUCT.unpack(cmsg_data)
                    ts_system_ns = (sec * 1_000_000 + usec) * 1000
                else:
                    assert False, f'Unexpected ancillary data: {cmsg_level}, {cmsg_type}, {cmsg_data!r}'

            assert ts_system_ns > 0, 'Missing the timestamp; does the driver support timestamping?'
            timestamp = pyuavcan.transport.Timestamp(system_ns=ts_system_ns, monotonic_ns=ts_mono_ns)

            out = SocketCANMedia._parse_native_frame(data, loopback=loopback, timestamp=timestamp)
            if out is not None:
                return out 

def checkHopLimitTruncatedHeader(self, ancbufsize, ignoreflags=0):
        # Receive hop limit into ancbufsize bytes of ancillary data
        # space, which should be too small to contain the ancillary
        # data header (if ancbufsize is None, pass no second argument
        # to recvmsg()).  Check that data is MSG, MSG_CTRUNC is set
        # (unless included in ignoreflags), and no ancillary data is
        # returned.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.misc_event.set()
        args = () if ancbufsize is None else (ancbufsize,)
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), *args)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC,
                        ignore=ignoreflags) 

def checkTruncatedArray(self, ancbuf, maxdata, mindata=0):
        # Check that file descriptor data is truncated to between
        # mindata and maxdata bytes when received with buffer size
        # ancbuf, and that any complete file descriptor numbers are
        # valid.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), ancbuf)
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        if mindata == 0 and ancdata == []:
            return
        self.assertEqual(len(ancdata), 1)
        cmsg_level, cmsg_type, cmsg_data = ancdata[0]
        self.assertEqual(cmsg_level, socket.SOL_SOCKET)
        self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
        self.assertGreaterEqual(len(cmsg_data), mindata)
        self.assertLessEqual(len(cmsg_data), maxdata)
        fds = array.array("i")
        fds.frombytes(cmsg_data[:
                len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
        self.checkFDs(fds) 

def testSingleCmsgTruncInData(self):
        # Test truncation of a control message inside its associated
        # data.  The message may be returned with its data truncated,
        # or not returned at all.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.misc_event.set()
        msg, ancdata, flags, addr = self.doRecvmsg(
            self.serv_sock, len(MSG), socket.CMSG_LEN(SIZEOF_INT) - 1)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        self.assertLessEqual(len(ancdata), 1)
        if ancdata:
            cmsg_level, cmsg_type, cmsg_data = ancdata[0]
            self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
            self.assertEqual(cmsg_type, socket.IPV6_HOPLIMIT)
            self.assertLess(len(cmsg_data), SIZEOF_INT) 

def testSingleCmsgTruncInData(self):
        # Test truncation of a control message inside its associated
        # data.  The message may be returned with its data truncated,
        # or not returned at all.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.misc_event.set()
        msg, ancdata, flags, addr = self.doRecvmsg(
            self.serv_sock, len(MSG), socket.CMSG_LEN(SIZEOF_INT) - 1)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        self.assertLessEqual(len(ancdata), 1)
        if ancdata:
            cmsg_level, cmsg_type, cmsg_data = ancdata[0]
            self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
            self.assertEqual(cmsg_type, socket.IPV6_HOPLIMIT)
            self.assertLess(len(cmsg_data), SIZEOF_INT) 

def checkHopLimitTruncatedHeader(self, ancbufsize, ignoreflags=0):
        # Receive hop limit into ancbufsize bytes of ancillary data
        # space, which should be too small to contain the ancillary
        # data header (if ancbufsize is None, pass no second argument
        # to recvmsg()).  Check that data is MSG, MSG_CTRUNC is set
        # (unless included in ignoreflags), and no ancillary data is
        # returned.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.misc_event.set()
        args = () if ancbufsize is None else (ancbufsize,)
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), *args)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC,
                        ignore=ignoreflags) 

def checkTruncatedArray(self, ancbuf, maxdata, mindata=0):
        # Check that file descriptor data is truncated to between
        # mindata and maxdata bytes when received with buffer size
        # ancbuf, and that any complete file descriptor numbers are
        # valid.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), ancbuf)
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        if mindata == 0 and ancdata == []:
            return
        self.assertEqual(len(ancdata), 1)
        cmsg_level, cmsg_type, cmsg_data = ancdata[0]
        self.assertEqual(cmsg_level, socket.SOL_SOCKET)
        self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
        self.assertGreaterEqual(len(cmsg_data), mindata)
        self.assertLessEqual(len(cmsg_data), maxdata)
        fds = array.array("i")
        fds.frombytes(cmsg_data[:
                len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
        self.checkFDs(fds) 

def checkTruncatedArray(self, ancbuf, maxdata, mindata=0):
        # Check that file descriptor data is truncated to between
        # mindata and maxdata bytes when received with buffer size
        # ancbuf, and that any complete file descriptor numbers are
        # valid.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), ancbuf)
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        if mindata == 0 and ancdata == []:
            return
        self.assertEqual(len(ancdata), 1)
        cmsg_level, cmsg_type, cmsg_data = ancdata[0]
        self.assertEqual(cmsg_level, socket.SOL_SOCKET)
        self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
        self.assertGreaterEqual(len(cmsg_data), mindata)
        self.assertLessEqual(len(cmsg_data), maxdata)
        fds = array.array("i")
        fds.frombytes(cmsg_data[:
                len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
        self.checkFDs(fds) 

def checkHopLimitTruncatedHeader(self, ancbufsize, ignoreflags=0):
        # Receive hop limit into ancbufsize bytes of ancillary data
        # space, which should be too small to contain the ancillary
        # data header (if ancbufsize is None, pass no second argument
        # to recvmsg()).  Check that data is MSG, MSG_CTRUNC is set
        # (unless included in ignoreflags), and no ancillary data is
        # returned.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.misc_event.set()
        args = () if ancbufsize is None else (ancbufsize,)
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), *args)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC,
                        ignore=ignoreflags) 

def testSingleCmsgTruncInData(self):
        # Test truncation of a control message inside its associated
        # data.  The message may be returned with its data truncated,
        # or not returned at all.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.misc_event.set()
        msg, ancdata, flags, addr = self.doRecvmsg(
            self.serv_sock, len(MSG), socket.CMSG_LEN(SIZEOF_INT) - 1)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        self.assertLessEqual(len(ancdata), 1)
        if ancdata:
            cmsg_level, cmsg_type, cmsg_data = ancdata[0]
            self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
            self.assertEqual(cmsg_type, socket.IPV6_HOPLIMIT)
            self.assertLess(len(cmsg_data), SIZEOF_INT) 

def testFDPassSeparateMinSpace(self):
        # Pass two FDs in two separate arrays, receiving them into the
        # minimum space for two arrays.
        self.checkRecvmsgFDs(2,
                             self.doRecvmsg(self.serv_sock, len(MSG),
                                            socket.CMSG_SPACE(SIZEOF_INT) +
                                            socket.CMSG_LEN(SIZEOF_INT)),
                             maxcmsgs=2, ignoreflags=socket.MSG_CTRUNC) 

def testSecomdCmsgTruncInData(self):
        # Test truncation of the second of two control messages inside
        # its associated data.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVTCLASS, 1)
        self.misc_event.set()
        msg, ancdata, flags, addr = self.doRecvmsg(
            self.serv_sock, len(MSG),
            socket.CMSG_SPACE(SIZEOF_INT) + socket.CMSG_LEN(SIZEOF_INT) - 1)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        cmsg_types = {socket.IPV6_TCLASS, socket.IPV6_HOPLIMIT}

        cmsg_level, cmsg_type, cmsg_data = ancdata.pop(0)
        self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
        cmsg_types.remove(cmsg_type)
        self.assertEqual(len(cmsg_data), SIZEOF_INT)
        a = array.array("i")
        a.frombytes(cmsg_data)
        self.assertGreaterEqual(a[0], 0)
        self.assertLessEqual(a[0], 255)

        if ancdata:
            cmsg_level, cmsg_type, cmsg_data = ancdata.pop(0)
            self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
            cmsg_types.remove(cmsg_type)
            self.assertLess(len(cmsg_data), SIZEOF_INT)

        self.assertEqual(ancdata, []) 

def testSecondCmsgTrunc0(self):
        self.checkTruncatedSecondHeader(socket.CMSG_SPACE(SIZEOF_INT),
                                        ignoreflags=socket.MSG_CTRUNC) 

def testCmsgTruncNoBufSize(self):
        # Check that no ancillary data is received when no ancillary
        # buffer size is provided.
        self.checkHopLimitTruncatedHeader(ancbufsize=None,
                                          # BSD seems to set
                                          # MSG_CTRUNC only if an item
                                          # has been partially
                                          # received.
                                          ignoreflags=socket.MSG_CTRUNC) 

def testSingleCmsgTrunc0(self):
        # Check that no ancillary data is received when ancillary
        # buffer size is zero.
        self.checkHopLimitTruncatedHeader(ancbufsize=0,
                                          ignoreflags=socket.MSG_CTRUNC) 

def testSecondCmsgTrunc0(self):
        self.checkTruncatedSecondHeader(socket.CMSG_SPACE(SIZEOF_INT),
                                        ignoreflags=socket.MSG_CTRUNC) 

def testSecomdCmsgTruncInData(self):
        # Test truncation of the second of two control messages inside
        # its associated data.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVTCLASS, 1)
        self.misc_event.set()
        msg, ancdata, flags, addr = self.doRecvmsg(
            self.serv_sock, len(MSG),
            socket.CMSG_SPACE(SIZEOF_INT) + socket.CMSG_LEN(SIZEOF_INT) - 1)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC)

        cmsg_types = {socket.IPV6_TCLASS, socket.IPV6_HOPLIMIT}

        cmsg_level, cmsg_type, cmsg_data = ancdata.pop(0)
        self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
        cmsg_types.remove(cmsg_type)
        self.assertEqual(len(cmsg_data), SIZEOF_INT)
        a = array.array("i")
        a.frombytes(cmsg_data)
        self.assertGreaterEqual(a[0], 0)
        self.assertLessEqual(a[0], 255)

        if ancdata:
            cmsg_level, cmsg_type, cmsg_data = ancdata.pop(0)
            self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
            cmsg_types.remove(cmsg_type)
            self.assertLess(len(cmsg_data), SIZEOF_INT)

        self.assertEqual(ancdata, []) 

def checkRecvmsgFDs(self, numfds, result, maxcmsgs=1, ignoreflags=0):
        # Check that constant MSG was received with numfds file
        # descriptors in a maximum of maxcmsgs control messages (which
        # must contain only complete integers).  By default, check
        # that MSG_CTRUNC is unset, but ignore any flags in
        # ignoreflags.
        msg, ancdata, flags, addr = result
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkunset=socket.MSG_CTRUNC,
                        ignore=ignoreflags)

        self.assertIsInstance(ancdata, list)
        self.assertLessEqual(len(ancdata), maxcmsgs)
        fds = array.array("i")
        for item in ancdata:
            self.assertIsInstance(item, tuple)
            cmsg_level, cmsg_type, cmsg_data = item
            self.assertEqual(cmsg_level, socket.SOL_SOCKET)
            self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
            self.assertIsInstance(cmsg_data, bytes)
            self.assertEqual(len(cmsg_data) % SIZEOF_INT, 0)
            fds.frombytes(cmsg_data)

        self.assertEqual(len(fds), numfds)
        self.checkFDs(fds) 

def testFDPassCMSG_LEN(self):
        # Test using CMSG_LEN() to calculate ancillary buffer size.
        self.checkRecvmsgFDs(1,
                             self.doRecvmsg(self.serv_sock, len(MSG),
                                            socket.CMSG_LEN(4 * SIZEOF_INT)),
                             # RFC 3542 says implementations may set
                             # MSG_CTRUNC if there isn't enough space
                             # for trailing padding.
                             ignoreflags=socket.MSG_CTRUNC) 

def checkHopLimit(self, ancbufsize, maxhop=255, ignoreflags=0):
        # Receive hop limit into ancbufsize bytes of ancillary data
        # space.  Check that data is MSG, ancillary data is not
        # truncated (but ignore any flags in ignoreflags), and hop
        # limit is between 0 and maxhop inclusive.
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.misc_event.set()
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), ancbufsize)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkunset=socket.MSG_CTRUNC,
                        ignore=ignoreflags)

        self.assertEqual(len(ancdata), 1)
        self.assertIsInstance(ancdata[0], tuple)
        cmsg_level, cmsg_type, cmsg_data = ancdata[0]
        self.assertEqual(cmsg_level, socket.IPPROTO_IPV6)
        self.assertEqual(cmsg_type, socket.IPV6_HOPLIMIT)
        self.assertIsInstance(cmsg_data, bytes)
        self.assertEqual(len(cmsg_data), SIZEOF_INT)
        a = array.array("i")
        a.frombytes(cmsg_data)
        self.assertGreaterEqual(a[0], 0)
        self.assertLessEqual(a[0], maxhop) 

def testFDPassEmpty(self):
        # Try to pass an empty FD array.  Can receive either no array
        # or an empty array.
        self.checkRecvmsgFDs(0, self.doRecvmsg(self.serv_sock,
                                               len(MSG), 10240),
                             ignoreflags=socket.MSG_CTRUNC) 

def testFDPassPartialInt(self):
        # Try to pass a truncated FD array.
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), 10240)
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, ignore=socket.MSG_CTRUNC)
        self.assertLessEqual(len(ancdata), 1)
        for cmsg_level, cmsg_type, cmsg_data in ancdata:
            self.assertEqual(cmsg_level, socket.SOL_SOCKET)
            self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
            self.assertLess(len(cmsg_data), SIZEOF_INT) 

def testSingleCmsgTrunc0(self):
        # Check that no ancillary data is received when ancillary
        # buffer size is zero.
        self.checkHopLimitTruncatedHeader(ancbufsize=0,
                                          ignoreflags=socket.MSG_CTRUNC) 

def checkTruncatedHeader(self, result, ignoreflags=0):
        # Check that no ancillary data items are returned when data is
        # truncated inside the cmsghdr structure.
        msg, ancdata, flags, addr = result
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.assertEqual(ancdata, [])
        self.checkFlags(flags, eor=True, checkset=socket.MSG_CTRUNC,
                        ignore=ignoreflags) 

def testCmsgTruncNoBufSize(self):
        # Check that no ancillary data is received when no buffer size
        # is specified.
        self.checkTruncatedHeader(self.doRecvmsg(self.serv_sock, len(MSG)),
                                  # BSD seems to set MSG_CTRUNC only
                                  # if an item has been partially
                                  # received.
                                  ignoreflags=socket.MSG_CTRUNC) 

def testCmsgTruncNoBufSize(self):
        # Check that no ancillary data is received when no ancillary
        # buffer size is provided.
        self.checkHopLimitTruncatedHeader(ancbufsize=None,
                                          # BSD seems to set
                                          # MSG_CTRUNC only if an item
                                          # has been partially
                                          # received.
                                          ignoreflags=socket.MSG_CTRUNC) 

def testCmsgTrunc0(self):
        # Check that no ancillary data is received when buffer size is 0.
        self.checkTruncatedHeader(self.doRecvmsg(self.serv_sock, len(MSG), 0),
                                  ignoreflags=socket.MSG_CTRUNC) 

def checkTrafficClassAndHopLimit(self, ancbufsize, maxhop=255,
                                     ignoreflags=0):
        # Receive traffic class and hop limit into ancbufsize bytes of
        # ancillary data space.  Check that data is MSG, ancillary
        # data is not truncated (but ignore any flags in ignoreflags),
        # and traffic class and hop limit are in range (hop limit no
        # more than maxhop).
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVHOPLIMIT, 1)
        self.serv_sock.setsockopt(socket.IPPROTO_IPV6,
                                  socket.IPV6_RECVTCLASS, 1)
        self.misc_event.set()
        msg, ancdata, flags, addr = self.doRecvmsg(self.serv_sock,
                                                   len(MSG), ancbufsize)

        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkunset=socket.MSG_CTRUNC,
                        ignore=ignoreflags)
        self.assertEqual(len(ancdata), 2)
        ancmap = self.ancillaryMapping(ancdata)

        tcdata = ancmap[(socket.IPPROTO_IPV6, socket.IPV6_TCLASS)]
        self.assertEqual(len(tcdata), SIZEOF_INT)
        a = array.array("i")
        a.frombytes(tcdata)
        self.assertGreaterEqual(a[0], 0)
        self.assertLessEqual(a[0], 255)

        hldata = ancmap[(socket.IPPROTO_IPV6, socket.IPV6_HOPLIMIT)]
        self.assertEqual(len(hldata), SIZEOF_INT)
        a = array.array("i")
        a.frombytes(hldata)
        self.assertGreaterEqual(a[0], 0)
        self.assertLessEqual(a[0], maxhop) 

def testSecondCmsgTrunc0(self):
        self.checkTruncatedSecondHeader(socket.CMSG_SPACE(SIZEOF_INT),
                                        ignoreflags=socket.MSG_CTRUNC) 

def checkRecvmsgFDs(self, numfds, result, maxcmsgs=1, ignoreflags=0):
        # Check that constant MSG was received with numfds file
        # descriptors in a maximum of maxcmsgs control messages (which
        # must contain only complete integers).  By default, check
        # that MSG_CTRUNC is unset, but ignore any flags in
        # ignoreflags.
        msg, ancdata, flags, addr = result
        self.assertEqual(msg, MSG)
        self.checkRecvmsgAddress(addr, self.cli_addr)
        self.checkFlags(flags, eor=True, checkunset=socket.MSG_CTRUNC,
                        ignore=ignoreflags)

        self.assertIsInstance(ancdata, list)
        self.assertLessEqual(len(ancdata), maxcmsgs)
        fds = array.array("i")
        for item in ancdata:
            self.assertIsInstance(item, tuple)
            cmsg_level, cmsg_type, cmsg_data = item
            self.assertEqual(cmsg_level, socket.SOL_SOCKET)
            self.assertEqual(cmsg_type, socket.SCM_RIGHTS)
            self.assertIsInstance(cmsg_data, bytes)
            self.assertEqual(len(cmsg_data) % SIZEOF_INT, 0)
            fds.frombytes(cmsg_data)

        self.assertEqual(len(fds), numfds)
        self.checkFDs(fds)