ZeroMQ

Signaling Between Threads (PAIR Sockets)

When you start making multithreaded applications with ØMQ, you’ll encounter the question of how to coordinate your threads. Though you might be tempted to insert “sleep” statements, or use multithreading techniques such as semaphores or mutexes, the only mechanism that you should use is ØMQ messages. Remember the story of the drunkards and the beer.

Let’s make three threads that signal each other when they are ready (Figure 2-13). In Example 2-12, we use PAIR sockets over the inproc transport.

Example 2-12. Multithreaded relay (mtrelay.c)

//
//  Multithreaded relay
//
#include "zhelpers.h"
#include <pthread.h>

static void *
step1 (void *context) {
    //  Connect to step2 and tell it we're ready
    void *xmitter = zmq_socket (context, ZMQ_PAIR);
    zmq_connect (xmitter, "inproc://step2");
    printf ("Step 1 ready, signaling step 2\n");
    s_send (xmitter, "READY");
    zmq_close (xmitter);

    return NULL;
}

static void *
step2 (void *context) {
    //  Bind inproc socket before starting step1
    void *receiver = zmq_socket (context, ZMQ_PAIR);
    zmq_bind (receiver, "inproc://step2");
    pthread_t thread;
    pthread_create (&thread, NULL, step1, context);

    //  Wait for signal and pass it on
    char *string = s_recv (receiver);
    free (string);
    zmq_close (receiver);

    //  Connect to step3 and tell it we're ready
    void *xmitter = zmq_socket (context, ZMQ_PAIR);
    zmq_connect (xmitter, "inproc://step3");
    printf ("Step 2 ready, signaling step 3\n");
    s_send (xmitter, "READY");
    zmq_close (xmitter);