test.c

#include <zmq.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main (void)
{
    int rc;
    void *context = zmq_init(1);
    zmq_msg_t msg;
    zmq_msg_init(&msg);
    void *rep = zmq_socket(context, ZMQ_REP);
    void *req = zmq_socket(context, ZMQ_REQ);
    zmq_bind(rep, "tcp://127.0.0.1:5555");
    zmq_connect(req, "tcp://127.0.0.1:5555");
    printf("rep.send\n");
    rc = zmq_send(rep, &msg, 0);
    // proper error
    if(rc) printf("%d: %s\n", rc, zmq_strerror(zmq_errno()));
    // try again, and it doesn't raise
    printf("rep.send\n");
    // this will block:
    rc = zmq_send(rep, &msg, 0);
    if(rc) printf("%d: %s\n", rc, zmq_strerror(zmq_errno()));
    
    // the same is true for REQ.recv
    printf("req.recv\n");
    rc = zmq_recv(req, &msg, 0);
    if(rc) printf("%d: %s\n", rc, zmq_strerror(zmq_errno()));
    printf("req.recv\n");
    rc = zmq_recv(req, &msg, 0);
    if(rc) printf("%d: %s\n", rc, zmq_strerror(zmq_errno()));
    
    zmq_close(req);
    zmq_close(rep);
    zmq_term(context);
    return 0;
}