ZMQ Hello world does not work - c++

Question

I try to run simple ZMQ application ( ROUTER/DEALER ).
I just send a request from DEALER to ROUTER, send it back. But DEALER cannot receive it.
I run it in one process ( ROUTER has its own thread ).
#include <zmq.hpp>
#include <string>
#include <iostream>
#include <thread>
void router()
{
zmq::context_t context(1);
zmq::socket_t socket(context, ZMQ_ROUTER);
socket.bind("tcp://*:5561");
while(1)
{
// Wait for next request from client
zmq::message_t reply;
socket.recv (&reply);
std::cout << "Router: Received request" << std::endl;
// Send reply back to client
std::string string= "example";
zmq::message_t message(string.size());
memcpy (message.data(), string.data(), string.size());
std::cout << "Router: Sending" << std::endl;
socket.send (message);
}
}
int main ()
{
std::thread t{&router};
// Prepare our context and socket
zmq::context_t context (2);
zmq::socket_t socket (context, ZMQ_DEALER);
std::cout << "Dealer: Connecting to hello world server…" << std::endl;
socket.connect ("tcp://127.0.0.1:5561");
for (int i = 0; i != 10; i++)
{
zmq::message_t request (5);
memcpy (request.data (), "Hello", 5);
std::cout << "Dealer: Sending Hello " << i << "…" << std::endl;
socket.send (request);
zmq::message_t reply;
socket.recv (&reply);
std::cout << "Dealer: Received " << i << std::endl;
}
return 0;
}
I have an output:
Dealer: Connecting to hello world server…
Dealer: Sending Hello 0…
Router: Received request
Router: Sending
Router: Received request
Router: Sending

Answer 1

From ZMQ's documentation on socket :
When receiving messages a ZMQ_ROUTER socket shall prepend a message
part containing the identity of the originating peer to the message
before passing it to the application. Messages received are
fair-queued from among all connected peers. When sending messages a
ZMQ_ROUTER socket shall remove the first part of the message and use
it to determine the identity of the peer the message shall be routed
to.
So modify your code to something like this :
#include <zmq.hpp>
#include <string>
#include <iostream>
#include <thread>
#include <unistd.h>
void router()
{
zmq::context_t context(1);
zmq::socket_t socket(context, ZMQ_ROUTER);
socket.bind("tcp://*:5561");
while(1) // Fix that infinite loop or your thread won't join
{
// Wait for next request from client
zmq::message_t id;
socket.recv (&id);
zmq::message_t reply;
socket.recv (&reply);
std::cout << std::string(static_cast<char*>(reply.data()),reply.size()) << std::endl;
std::cout << "Router: Received request" << std::endl;
// Send reply back to client
zmq::message_t copy_id;
copy_id.copy(&id);
std::string string= "example";
zmq::message_t message(string.size());
memcpy (message.data(), string.data(), string.size());
std::cout << "Router: Sending" << std::endl;
socket.send(id, ZMQ_SNDMORE);
socket.send(message);
}
sleep(1);
socket.setsockopt(ZMQ_LINGER, 0);
socket.close();
context.close();
}
int main ()
{
std::thread t{&router};
// Prepare our context and socket
zmq::context_t context (2);
zmq::socket_t socket (context, ZMQ_DEALER);
std::cout << "Dealer: Connecting to hello world server…" << std::endl;
socket.connect ("tcp://127.0.0.1:5561");
for (int i = 0; i != 10; i++)
{
zmq::message_t request (5);
memcpy (request.data (), "Hello", 5);
std::cout << "Dealer: Sending Hello " << i << "…" << std::endl;
socket.send(request);
zmq::message_t reply;
socket.recv(&reply);
std::cout << "Dealer: Received " << i << std::endl;
}
socket.setsockopt(ZMQ_LINGER, 0);
socket.close();
context.close();
t.join();
return 0;
}