ZeroMQ C++ req-rep termination error - c++

Question

Here is ZeroMq C++ code for simple request - reply where both exchange messages alternatively.
But error occures when messages are only sent continuously ....
Reply code :
include zmq.hpp
include string
include iostream
include unistd.h
include ctime
int main () {
// Prepare our context and socket
zmq::context_t context (1);
zmq::socket_t socket (context, ZMQ_REP);
socket.bind ("tcp://*:5557");
zmq::message_t reply (5);
memcpy ((void *) reply.data (), "World", 5);
zmq::message_t request;
// Wait for next request from client
socket.recv (&request);
///////////////// """observe_line_1"""
std::cout << "Received Hello" << std::endl;
while (true)
{
// Send reply back to client
socket.send (reply);
}
return 0;
}
Request code :
include zmq.hpp
include string
include iostream
include ctime
int main ()
{
zmq::context_t context (1);
zmq::socket_t socket (context, ZMQ_REQ);
socket.connect ("tcp://localhost:5557");
zmq::message_t request (6);
memcpy ((void *) request.data (), "Hello", 5);
zmq::message_t reply;
socket.send(request);
//////////////////////////////////////////"""observe_line_2"""
for (int request_nbr = 0; request_nbr != 1000; request_nbr++)
{
socket.recv (&reply);
std::cout << "Received World " << request_nbr << std::endl;
}
return 0;
}
Output
After executing Reqply then executing request then following error occurs
terminate called after throwing an instance of 'zmq::error_t'
what(): Operation cannot be accomplished in current state
Aborted
Point to be observed
when "observe_line_1" is inside the for loop below it and "observe_line_2" is inside while loop below it and executed then the code is executed succesfully without any errors ????

Answer 1

Sending multiple replies for one request is not allowed by the REQ-REP model.
The REQ-REP socket pair is in lockstep. The client issues zmq_send() and then zmq_recv(), in a loop (or once if that's all it needs). Doing any other sequence (e.g., sending two messages in a row) will result in a return code of -1 from the send or recv call. Similarly, the service issues zmq_recv() and then zmq_send() in that order, as often as it needs to.
reference, alternative socket types, maybe this can help

Answer 2

Actually you can send more than one message in row, as long as you send data with flags=zmq::SNDMORE.