How to use ZeroMQ in an GTK/QT/Clutter application?

Question

In gtk applications all execution is taking place inside the gtk_main function. And other graphical frame works have similar event loops like app.exec for QT and clutter_main for Clutter. However ZeroMQ is based on the assumption that there is an while (1) ... loop that it is inserted into (see for instance here for examples).

How do you combine those two execution strategies?

I am currently wanting to use zeromq in a clutter application written in C, so I would of course like direct answers to that, but please add answers for other variants as well.

Answer 1

The proper way to combine zmq and gtk or clutter is to connect the file-descriptor of the zmq queue to the main event loop. The fd can be retrieved by using

int fd;
size_t sizeof_fd = sizeof(fd);
if(zmq_getsockopt(socket, ZMQ_FD, &fd, &sizeof_fd))
      perror("retrieving zmq fd");

Connecting it to the main loop is the matter of using io_add_watch:

GIOChannel* channel = g_io_channel_unix_new(fd);    
g_io_add_watch(channel, G_IO_IN|G_IO_ERR|G_IO_HUP, callback_func, NULL);

In the callback function, it is necessary to first check if there is really stuff to read, before reading. Otherwise, the function might block waiting for IO.

gboolean callback_func(GIOChannel *source, GIOCondition condition,gpointer data)
{
    uint32_t status;
    size_t sizeof_status = sizeof(status);   

    while (1){
         if (zmq_getsockopt(socket, ZMQ_EVENTS, &status, &sizeof_status)) {
             perror("retrieving event status");
             return 0; // this just removes the callback, but probably
                       // different error handling should be implemented
         }
         if (status & ZMQ_POLLIN == 0) {
             break;
         }

         // retrieve one message here
    }
    return 1; // keep the callback active
}

Please note: this is not actually tested, I did a translation from Python+Clutter, which is what I use, but I'm pretty sure that it'll work. For reference, below is full Python+Clutter code which actually works.

import sys
from gi.repository import Clutter, GObject
import zmq

def Stage():
    "A Stage with a red spinning rectangle"
    stage = Clutter.Stage()

    stage.set_size(400, 400)
    rect = Clutter.Rectangle()
    color = Clutter.Color()
    color.from_string('red')
    rect.set_color(color)
    rect.set_size(100, 100)
    rect.set_position(150, 150)

    timeline = Clutter.Timeline.new(3000)
    timeline.set_loop(True)

    alpha = Clutter.Alpha.new_full(timeline, Clutter.AnimationMode.EASE_IN_OUT_SINE)
    rotate_behaviour = Clutter.BehaviourRotate.new(
        alpha, 
        Clutter.RotateAxis.Z_AXIS,
        Clutter.RotateDirection.CW,
        0.0, 359.0)
    rotate_behaviour.apply(rect)
    timeline.start()
    stage.add_actor(rect)

    stage.show_all()
    stage.connect('destroy', lambda stage: Clutter.main_quit())
    return stage, rotate_behaviour

def Socket(address):
    ctx = zmq.Context()
    sock = ctx.socket(zmq.SUB)
    sock.setsockopt(zmq.SUBSCRIBE, "")
    sock.connect(address)
    return sock

def zmq_callback(queue, condition, sock):
    print 'zmq_callback', queue, condition, sock

    while sock.getsockopt(zmq.EVENTS) & zmq.POLLIN:
        observed = sock.recv()
        print observed

    return True

def main():
    res, args = Clutter.init(sys.argv)
    if res != Clutter.InitError.SUCCESS:
        return 1

    stage, rotate_behaviour = Stage()

    sock = Socket(sys.argv[2])
    zmq_fd = sock.getsockopt(zmq.FD)
    GObject.io_add_watch(zmq_fd,
                         GObject.IO_IN|GObject.IO_ERR|GObject.IO_HUP,
                         zmq_callback, sock)

    return Clutter.main()

if __name__ == '__main__':
    sys.exit(main())