Question

Code Connected Volume 1 – page 47 have an example about how to receive multipart message:

while (1) {
    zmq_msg_t message;
    zmq_msg_init(&message);
    zmq_msg_recv(&message, socket, 0);
    // Process the message frame
    ...
    zmq_msg_close(&message);
    if (!zmq_msg_more(&message))
        break;
}

Is this correct? Shouldn’t we use zmq_msg_close() after zmq_msg_more()?

Answer 1

The API reference for zmq_msg_more() and zmq_msg_recv() (ZeroMQ 3.2.2 Stable) both contain examples showing that zmq_msg_close() is called after zmq_msg_more. As far as I know the API docs do not specifically state anything to contradict this, thus the example from Code Connected seems wrong. The documentation for zmq_msg_close() states that the actual memory release may be postponed by underlaying layers, implicating that the zmq_msg_more() operation may succeed but it still looks wrong to call it after closing the message.

Example from zmq_msg_more() API documentation (3.2.2) (edited slightly for readability):

zmq_msg_t part;
while (true) 
{
    //  Create an empty ØMQ message to hold the message part
    int rc = zmq_msg_init (&part);
    assert (rc == 0);

    //  Block until a message is available to be received from socket
    rc = zmq_recvmsg (socket, &part, 0);
    assert (rc != -1);

    if (zmq_msg_more (&part))
        fprintf (stderr, "more\n");
    else 
    {
        fprintf (stderr, "end\n");
        break;
    }
    zmq_msg_close (part); 
}

However, looking in the ZeroMq Guide regarding Multi-Part Messages, that example actually checks for more messages after closing the message, but that is achieved by checking the socket using zmq_getsockopt(), without using any references to the message. I suspect the Code Connected examples simply used that example and changed from zmq_getsockopt() to zmq_msg_more() (probably incorrecly so).

Example from ZeroMq Guide (multi-part messages):

while (1) 
{
    zmq_msg_t message;
    zmq_msg_init (&message);
    zmq_msg_recv (&message, socket, 0);
    //  Process the message frame
    zmq_msg_close (&message);
    int more;
    size_t more_size = sizeof (more);
    zmq_getsockopt (socket, ZMQ_RCVMORE, &more, &more_size);
    if (!more)
        break;      //  Last message frame
}