nng to pynng pub/sub. No messages recieved by client

Question

I have the following server (C++):

#include <nng/nng.h>
#include <nng/protocol/pubsub0/pub.h>
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <unistd.h>
void
fatal(const char *func, int rv)
{
        fprintf(stderr, "%s: %s\n", func, nng_strerror(rv));
}
int main(int argc, char* argv[]){
    nng_socket sock;
    int rv;
    std::string url = "tcp://0.0.0.0:" + std::to_string(5563);
    if ((rv = nng_pub0_open(&sock)) != 0) {
            fatal("nng_pub0_open", rv);
    }
    if ((rv = nng_listen(sock, url.c_str(), NULL, 0)) < 0) {
            fatal("nng_listen", rv);
    }
    while(1){
        std::string msg = std::string("msg");
        //if ((rv = nng_send(sock, (void*)frame, bpp * nImgW * nImgH, 0)) != 0) {
        if ((rv = nng_send(sock, (void*)msg.c_str(), 3, 0)) != 0) {
            fatal("nng_send", rv);
        }else{
            std::cout << "Frame Sent... "<< std::endl;
        } 
        sleep(1);
    }
}

And the following client (python):

import pynng
from pynng import Pub0, Sub0, Timeout
cam_path = "tcp://127.0.0.1:5563"
with Sub0(dial=cam_path,recv_timeout=2000, topics=b'') as sub:
    sub.recv_max_size = 0 #recieve msg of any size
    while(1):
        try:
            print("waiting for msg")
            img = sub.recv()
            print(img)
        except pynng.exceptions.Timeout as e:
            print('Timed out, retrying...')

I dont understand why no messages ever arrive to the client. I have set topics and recv_max_size but still no messages arrives at the client. What am I doing wrong here now?

Answer 1

Q : "What am I doing wrong here now?"

You happened to blindly assume that things happen in a very way your code does not attempt to validate. Old assembler wolves used to state # ASSUME NOTHING before they ever started a code :o)

Pieter Hintjens', the famous AMQP / ZeroMQ evangelisation Guru (an older brother of the nanomsg / nng ) uses explicit POSACK-s from assert-s :

if ( (  aRetCODE = nng_pub0_open( &sock ) ) != 0 ) {
        fatal( "nng_pub0_open",
                aRetCODE );
}
assert( aRetCODE == 0 && "EXC: Pub0 socket failed to instantiate" );

if ( (  aRetCODE = nng_listen( sock, url.c_str(), NULL, 0 ) ) < 0 ) {
        fatal( "nng_listen",
                aRetCODE );
}
assert( aRetCODE == 0 && "EXC: Pub0 socket failed to .listen()" );

Python has also a similar assert-tool for explicit POSACK-checks, so worth polishing the code-robustness using them as Pieter Hintjens has made Martin Sustrik to do so wherever you read the API :o)

assert rc == 0, "INF: some condition was not met for ( %r )" % someVariableNAME

Happy using nng / pynng for your Projects!