super basic listening zmq server with polling

zmq-server.cpp

//
//  Hello World server in C++ w/ polling
//  Binds REP socket to tcp://*:5555
//  Expects "Hello" from client, replies with "World"
//
#include <zmq.hpp>
#include <string>
#include <iostream>
#ifndef _WIN32
#include <unistd.h>
#else
#include <windows.h>

#define sleep(n)    Sleep(n)
#endif

int main () {
    //  Prepare our context and socket
    zmq::context_t context (1);
    zmq::socket_t socket (context, ZMQ_REP);
    socket.bind ("tcp://*:5555");

    // Setup Polling - you can add multiple sockets here if you 
    // want to handle multiple sockets and then check them in serial below
    zmq::pollitem_t items [] = {
     { socket, 0, ZMQ_POLLIN, 0 }
    };
        
    zmq::message_t request;
    while (true) {
        
        // Poll for incoming message event waiting 1sec. 
        // If you have a message to process this call returns immediately
        // Timeout lets you do other things (like update a gui) and then go back to listening
        // If you're doing this on the client side you might want to give up and throw and error message
        zmq::poll (&items[0], 1, 1000);  
        
        if (items[0].revents & ZMQ_POLLIN) {   
          //  Got a message! process it.
          request.rebuild();
          
          // Assuming single part message. If not you'd have to get all parts here
          // zmq guarantees all parts arrive before flagging a message is there. 
          socket.recv (&request); 
          std::cout << "Received Hello" << std::endl;
  
          //  Do some 'work'
          sleep(1);
  
          //  Send reply back to client
          // You have to send a reply or the client will never be able to send another response
          zmq::message_t reply (5);
          memcpy ((void *) reply.data (), "World", 5);
          socket.send (reply);
        }
        
        // Maybe do other things before going back to listening?
    }
    return 0;
}