Network diagnostics for ZeroMQ Example

Question

I am trying to implement ZeroMQ to get an application on a Raspberry Pi 3 (Raspbian Stretch) to communicate with an application on a separate machine (in this case Windows 7 64bit OS) linked by a wired or WLAN connection.

I have compiled ZeroMQ with the C library interface on both machines (using Cygwin on Windows) and the Hello World example (which I modified slightly to print the pointer values to assure me that the functions were 'working'). Both machines are connected (in this case via a wired Ethernet link and a router) and the connection is good (I link to RPi from PC via Xrdp or SSH OK).

The problem I have is that the client/server ZeroMQ programs don't appear to be 'seeing' each other even though they do appear to work and my question is: What are the first steps I should take to investigate why this is happening? Are there any commandline or GUI tools that can help me find out what's causing the blockage? (like port activity monitors or something?).

I know very little about networking so consider me a novice in all things sockety/servicey in your reply. The source code on the RPi (server) is:

// ZeroMQ Test Server
// Compile with
// gcc -o zserver zserver.c -lzmq

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

int main (void)
{
    void *context=NULL,*responder=NULL;
    int rc=1;

    //  Socket to talk to clients
    context = zmq_ctx_new ();
printf("Context pointer = %p\n",context);
    responder = zmq_socket (context, ZMQ_REP);
printf("Responder pointer = %p\n",responder);
    rc = zmq_bind (responder, "tcp://*:5555");
printf("rc = %d\n",rc);

    assert (rc == 0);

    while (1) {
        char buffer [10];
        zmq_recv (responder, buffer, 10, 0);
        printf ("Received Hello\n");
        sleep (1);          //  Do some 'work'
        zmq_send (responder, "World", 5, 0);
    }
    return 0;
}

The source code on the PC (Cygwin) client is:

// ZeroMQ Test Client
// Compile with:
// gcc -o zclient zclient.c -L/usr/local/lib -lzmq

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

int main (void)
{
  void *context=NULL,*requester=NULL;

    printf ("Connecting to hello world server\n");
    context = zmq_ctx_new ();
printf("Context pointer = %p\n",context);
    requester = zmq_socket (context, ZMQ_REQ);
printf("Requester pointer = %p\n",requester);
    zmq_connect (requester, "tcp://localhost:5555");

    int request_nbr;
    for (request_nbr = 0; request_nbr != 10; request_nbr++) {
        char buffer [10];
        printf ("Sending Hello %d\n", request_nbr);
        zmq_send (requester, "Hello", 5, 0);
        zmq_recv (requester, buffer, 10, 0);
        printf ("Received World %d\n", request_nbr);
    }
    zmq_close (requester);
    zmq_ctx_destroy (context);
    return 0;
}

On the RPi LXTerminal I run the server and get this:

Context pointer = 0xefe308
Responder pointer = 0xf00e08
rc = 0

and on the Cygwin Bash shell I run the client and get this:

Connecting to hello world server
Context pointer = 0x60005ab90
Requester pointer = 0x60005f890
Sending Hello 0

... and there they both hang - one listening, the other sending but neither responding to each other. Any clue how to start investigating this would be appreciated.

Answer 1

In zmq_connect, you must indicate the IP address of the raspberry (which have executed zmq_bind:

It should have been:

// on PC, remote ip is the raspberry one, the one you use for ssh for instance
rc = zmq_connect(requester, "tcp://<remote ip>:5555");