ZeroMQ IPC across several instances of a program

I am having some problems with inter process communication in ZMQ between several instances of a program

• I am using Linux OS
• I am using zeromq/cppzmq, header-only C++ binding for libzmq

If I run two instances of this application (say on a terminal), I provide one with an argument to be a listener, then providing the other with an argument to be a sender. The listener never receives a message. I have tried TCP and IPC to no avail.

#include <zmq.hpp>#include <string>#include <iostream>int ListenMessage();int SendMessage(std::string str);zmq::context_t global_zmq_context(1);int main(int argc, char* argv[] ) {    std::string str = "Hello World";    if (atoi(argv[1]) == 0) ListenMessage();    else SendMessage(str);    zmq_ctx_destroy(& global_zmq_context);    return 0;}int SendMessage(std::string str) {    assert(global_zmq_context);    std::cout << "Sending \n";    zmq::socket_t publisher(global_zmq_context, ZMQ_PUB);    assert(publisher);    int linger = 0;    int rc = zmq_setsockopt(publisher, ZMQ_LINGER, &linger, sizeof(linger));    assert(rc==0);    rc = zmq_connect(publisher, "tcp://127.0.0.1:4506");    if (rc == -1) {        printf ("E: connect failed: %s\n", strerror (errno));        return -1;    }    zmq::message_t message(static_cast<const void*> (str.data()), str.size());    rc = publisher.send(message);    if (rc == -1) {        printf ("E: send failed: %s\n", strerror (errno));        return -1;    }    return 0;}int ListenMessage() {    assert(global_zmq_context);    std::cout << "Listening \n";    zmq::socket_t subscriber(global_zmq_context, ZMQ_SUB);    assert(subscriber);    int rc = zmq_setsockopt(subscriber, ZMQ_SUBSCRIBE, "", 0);    assert(rc==0);    int linger = 0;    rc = zmq_setsockopt(subscriber, ZMQ_LINGER, &linger, sizeof(linger));    assert(rc==0);    rc = zmq_bind(subscriber, "tcp://127.0.0.1:4506");    if (rc == -1) {        printf ("E: bind failed: %s\n", strerror (errno));        return -1;    }    std::vector<zmq::pollitem_t> p = {{subscriber, 0, ZMQ_POLLIN, 0}};    while (true) {        zmq::message_t rx_msg;        // when timeout (the third argument here) is -1,        // then block until ready to receive        std::cout << "Still Listening before poll \n";        zmq::poll(p.data(), 1, -1);        std::cout << "Found an item \n"; // not reaching        if (p[0].revents & ZMQ_POLLIN) {            // received something on the first (only) socket            subscriber.recv(&rx_msg);            std::string rx_str;            rx_str.assign(static_cast<char *>(rx_msg.data()), rx_msg.size());            std::cout << "Received: "<< rx_str << std::endl;        }    }    return 0;}

This code will work if I running one instance of the program with two threads

    std::thread t_sub(ListenMessage);    sleep(1); // Slow joiner in ZMQ PUB/SUB pattern    std::thread t_pub(SendMessage str);    t_pub.join();    t_sub.join();

But I am wondering why when running two instances of the program the code above won't work?

Thanks for your help!