ZeroMQ SUB never receives messages - c++

Question

I am having trouble with a PUB/SUB in ZeroMQ.
After connecting everything, publisher publishes all messages ( socket's send message returns true ) but the SUB never receives them and blocks forever on .recv() function.
Here is the code I am using:
void startPublisher()
{
zmq::context_t zmq_context(1);
zmq::socket_t zmq_socket(zmq_context, ZMQ_PUB);
zmq_socket.bind("tcp://127.0.0.1:58951");
zmq::message_t msg(3);
memcpy(msg.data(), "abc", 3);
for(int i = 0; i < 10; i++)
zmq_socket.send(msg); // <-- always true
}
void startSubscriber()
{
zmq::context_t zmq_context(1);
zmq::socket_t zmq_socket(zmq_context, ZMQ_SUB);
zmq_socket.connect("tcp://127.0.0.1:58951");
zmq_socket.setsockopt(ZMQ_SUBSCRIBE, "", 0); // allow all messages
zmq::message_t msg(3);
zmq_socket.recv(&msg); // <-- blocks forever (message never received?)
}
Please note that I am runing these 2 functions in two different threads, starting SUB thread first, waiting for some time and then starting publisher thread ( also tried other way around with publisher sending messages in an endless-loop, but didn't work ).
What am I doing wrong here?

Answer 1

Based on your example, the following code works for me.
The problem is that the PUB / SUB pattern is a slow joiner, meaning you need to wait a while after binding the PUB socket and before sending any message.
#include <thread>
#include <zmq.hpp>
#include <iostream>
#include <unistd.h>
void startPublisher()
{
zmq::context_t zmq_context(1);
zmq::socket_t zmq_socket(zmq_context, ZMQ_PUB);
zmq_socket.bind("tcp://127.0.0.1:58951");
usleep(100000); // Sending message too fast after connexion will result in dropped message
zmq::message_t msg(3);
for(int i = 0; i < 10; i++) {
memcpy(msg.data(), "abc", 3);
zmq_socket.send(msg); // <-- always true
msg.rebuild(3);
usleep(1); // Temporisation between message; not necessary
}
}
volatile bool run = false;
void startSubscriber()
{
zmq::context_t zmq_context(1);
zmq::socket_t zmq_socket(zmq_context, ZMQ_SUB);
zmq_socket.connect("tcp://127.0.0.1:58951");
std::string TOPIC = "";
zmq_socket.setsockopt(ZMQ_SUBSCRIBE, TOPIC.c_str(), TOPIC.length()); // allow all messages
zmq_socket.setsockopt(ZMQ_RCVTIMEO, 1000); // Timeout to get out of the while loop
while(run) {
zmq::message_t msg;
int rc = zmq_socket.recv(&msg); // Works fine
if(rc) // Do no print trace when recv return from timeout
std::cout << std::string(static_cast<char*>(msg.data()), msg.size()) << std::endl;
}
}
int main() {
run = true;
std::thread t_sub(startSubscriber);
sleep(1); // Slow joiner in ZMQ PUB/SUB pattern
std::thread t_pub(startPublisher);
t_pub.join();
sleep(1);
run = false;
t_sub.join();
}