CZMQ : Unable to receive message

问题

I am trying to write a simple program to demonstrate Publisher/Subscriber communication using 'libczmq'. Although I am able to send the message via the 'zmsg_send' api ( or so I presume based on its return value ), I am unable to receive the message via the 'zmsg_recv' (blocking) API, it is probably unable to receive the message.

#include "czmq.h"
int main (void)
{
  int rc;
  const char *ipc_file="ipc://tmp.socket";
  const char *str = "Hello World!!!";

  /*****************************************/
  /* Creating and binding publisher Socket */
  /*****************************************/
  zsock_t *pub_sock = zsock_new(ZMQ_PUB);
  assert(pub_sock!=NULL);
  rc = zsock_bind(pub_sock, ipc_file, NULL);
  assert(rc==0);

  /**************************************************/
  /* Creating and connecting with Subscriber Socket */
  /**************************************************/
  zsock_t *sub_sock = zsock_new(ZMQ_SUB);
  assert(sub_sock);
  rc = zsock_connect(sub_sock, ipc_file, NULL);
  assert(rc==0);

  /***************************************/
  /* Creating messager & Frame instances */
  /*         and sending message         */
  /***************************************/
  zmsg_t *msg = zmsg_new ();
  assert(msg!=NULL);
  zframe_t *frame = zframe_new (str, strlen(str));
  assert(frame!=NULL);
  zmsg_prepend(msg, &frame);
  printf("PUB : frame_count = %u, content_size = %d, msg_ptr = %p\n",
        zmsg_size (msg), zmsg_content_size(msg), msg);
  rc = zmsg_send(&msg, pub_sock);
  assert (rc == 0);
  printf("PUB : Message send successfully...\n");

  /********************************/
  /* Subscriber receiving message */
  /********************************/
  printf("SUB : Reading message...\n");
  msg = zmsg_recv(sub_sock);
  assert(msg!=NULL);
  printf("SUB : frame_count = %u, content_size = %d, msg_ptr = %p\n",
        zmsg_size (msg), zmsg_content_size(msg), msg);
  frame = zmsg_pop(msg);
  assert(frame!=NULL);
  printf("SUB : received in frame = \"%s\"\n", zframe_data (frame));

  zmsg_destroy (&msg);
  zframe_destroy (&frame);
  zsock_destroy (&sub_sock);
  zsock_destroy (&pub_sock);
  return 0;
}

Below is how I built and executed the application.

user@debian:~/progs/czmq$ make
cc -Iczmq/include -ggdb   -c -o pub-sub-test.o pub-sub-test.c
gcc -L./czmq/src/.libs -lzmq -lczmq -lpthread pub-sub-test.o -o ../pub-sub-test
user@debian:~/progs/czmq$ ../pub-sub-test
PUB : frame_count = 1, content_size = 14, msg_ptr = 0x1653b10
PUB : Message send successfully...
SUB : Reading message...

The 'message' is never read by the subscriber, please let me know what I am missing here.

回答1:

By default there is no subscription on a SUB socket. Try using the variant that takes a subscription :

zsock_t* zsock_new_sub (const char *endpoints, const char *subscribe)

Or create the subscription after :

zsock_set_subscribe (void *zsock, const char *subscribe);

The empty string subscribes to everything.

回答2:

There two problems in the above program.

1. The absence of 'zsock_set_subscribe' as @david points out in an earlier comment.
2. The publisher sends the message before subscriber has an opportunity to connect, hence the message would be lost and the subscriber would continue to wait for a message which would never come.

Solution :

1. Adding the 'zsock_set_subscribe' after the subscriber socket is initialized.
2. Initializing the subscriber before the publisher and adding a small delay (say 1 sec) immediately after the publisher' so as to provide enough time for the subscriber to connect and eventually receive the message.

来源：https://stackoverflow.com/questions/39290802/czmq-unable-to-receive-message