C++ ZMQ Pub and Sub not connecting - c++

Question

I am currently working on a project that requires me to connect two terminals via ZMQ sockets, and my current solution does so via the PUB-SUB Socket functionality. However, when I run the programs, while the publisher sends the messages, the subscriber never receives any of the messages. I've tried changing the IP address between them, and trying to "brute force send" message between the sub and the pub, but to no avail.
Reduced form of the code:
Server.cpp:
#include <zmq.h>
const char* C_TO_S = "tcp://127.0.0.1:5557";
const char* S_TO_C = "tcp://127.0.0.1:5558";
int main() {
zmq::context_t context(1);
zmq::socket_t pub(context, ZMQ_PUB);
zmq::socket_t sub(context, ZMQ_SUB);
int sndhwm = 0;
sub.connect(C_TO_S);
pub.bind(S_TO_C);
sub.setsockopt(ZMQ_SUBSCRIBE, &sndhwm, sizeof(sndhwm));
//cout << C_TO_S << endl;
while(true) {
zmq::message_t rx_msg;
sub.recv(&rx_msg);
cout << "b\n";
// other code goes here
}
}
Client.cpp:
#incldue <zmq.h>
const char* C_TO_S = "tcp://127.0.0.1:5557";
const char* S_TO_C = "tcp://127.0.0.1:5558";
void network_thread() {
zmq::context_t context(1);
zmq::socket_t pub(context, ZMQ_PUB);
zmq::socket_t sub(context, ZMQ_SUB);
int sndhwm = 0;
sub.connect(S_TO_C);
pub.connect(C_TO_S);
sub.setsockopt(ZMQ_SUBSCRIBE, &sndhwm, sizeof(sndhwm));
while (true) {
cout << pub.send("a", strlen("a"), 0);
cout << "AA\n";
}
// Other code that doesn't matter
}
The main in Client.cpp calls network_thread in a separate thread, and then spams the publisher to send the message "a" to the server. However, the server does not get any message from the client. If the server got any message, it would print out "b", however it never does that. I also know that the publisher is sending messages because it prints out "1" upon execution.
Also, assume that the client subscriber and the server publisher has a purpose. While they don't work atm either, a fix to the other set should translate into a fix of those.
I have tried changing the port, spamming send messages, etc. Nothing resulted in the server receiving any messages.
Any help would be appreciated, thank you.

Answer 1

You set a message filter option on the SUB socket. This means that you will only receive messages that begin with the bytes set by the filter.
This code:
int sndhwm = 0;
sub.setsockopt(ZMQ_SUBSCRIBE, &sndhwm, sizeof(sndhwm));
Sets the filter to sizeof(sndhwm) bytes with value 0x00. But your message does not begin with this number of 0x00 bytes. Hence the message is ignored by the SUB socket.
You should remove the setsockopt call.
If your intent was to clear the message filter, you can do this with:
sub.setsockopt(ZMQ_SUBSCRIBE, nullptr, 0);