def recvmsg(socket, maxSize=8192, cmsgSize=4096, flags=0):
    """
    Receive a message on a socket.

    @param socket: The socket to receive the message on.
    @type socket: L{socket.socket}

    @param maxSize: The maximum number of bytes to receive from the socket using
        the datagram or stream mechanism. The default maximum is 8192.
    @type maxSize: L{int}

    @param cmsgSize: The maximum number of bytes to receive from the socket
        outside of the normal datagram or stream mechanism. The default maximum
        is 4096.
    @type cmsgSize: L{int}

    @param flags: Flags to affect how the message is sent.  See the C{MSG_}
        constants in the sendmsg(2) manual page. By default no flags are set.
    @type flags: L{int}

    @return: A named 3-tuple of the bytes received using the datagram/stream
        mechanism, a L{list} of L{tuple}s giving ancillary received data, and
        flags as an L{int} describing the data received.
    """
    if _PY3:
        # In Twisted's sendmsg.c, the csmg_space is defined as:
        #     int cmsg_size = 4096;
        #     cmsg_space = CMSG_SPACE(cmsg_size);
        # Since the default in Python 3's socket is 0, we need to define our
        # own default of 4096. -hawkie
        data, ancillary, flags = socket.recvmsg(
            maxSize, CMSG_SPACE(cmsgSize), flags)[0:3]
    else:
        data, flags, ancillary = recv1msg(
            socket.fileno(), flags, maxSize, cmsgSize)

    return RecievedMessage(data=data, ancillary=ancillary, flags=flags)

    Receive a message on a socket.

    @param socket: The socket to receive the message on.
    @type socket: L{socket.socket}

    @param maxSize: The maximum number of bytes to receive from the socket using
        the datagram or stream mechanism. The default maximum is 8192.
    @type maxSize: L{int}

    @param cmsgSize: The maximum number of bytes to receive from the socket
        outside of the normal datagram or stream mechanism. The default maximum
        is 4096.
    @type cmsgSize: L{int}

    @param flags: Flags to affect how the message is sent.  See the C{MSG_}
        constants in the sendmsg(2) manual page. By default no flags are set.
    @type flags: L{int}

    @return: A named 3-tuple of the bytes received using the datagram/stream
        mechanism, a L{list} of L{tuple}s giving ancillary received data, and
        flags as an L{int} describing the data received.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/sendmsg.py