libevent kqueue doesn't work on fd returned from zmq_getsockopt()

Question

ORIGINAL POST: I'm writing a service in C programming using libevent and zmq. Msg is pushed from python code to C service using PUSH-PULL pattern.

fd received from zmq socket:

void *receiver = zmq_socket (base.zmq_ctx, ZMQ_PULL);
zmq_connect (receiver, "tcp://localhost:5557");
int fd=0;
size_t fd_len = sizeof(fd);
zmq_getsockopt (receiver, ZMQ_FD, &fd, &fd_len);

Using Libevent, event registered with fd for persistent read

struct event *read_data_on_zmq =event_new(base.evbase, fd, EV_READ | EV_PERSIST , read_data_on_zmq_cb,receiver);
event_add(read_data_on_zmq,NULL);
event_base_dispatch(base.evbase);

On the callback method I'm doing a non-blocking receive

void read_data_on_zmq_cb(evutil_socket_t fd, short what, void *arg)
{
    char *msg = calloc(1024,sizeof(char));
    int size = zmq_recv (receiver, msg, 255, ZMQ_DONTWAIT);
    if (size != -1) 
    {
        puts ("is size is not -1");
        printf("msg = %s\n",msg); 
    }
}

In the python code I'm continuously sending message to the socket.

import zmq
import time

c=zmq.Context()
s=c.socket(zmq.PUSH)
s.bind('tcp://127.0.0.1:5557')
while(True):
    s.send("abc")
    time.sleep(2)

The problem is I'm only able to receive the message once, after that the event callback never gets hit. If I do zmq_connect inside the read_data_on_zmq_cb after zmq_recv, then it works fine, but I guess that is redundant and not the correct way to do it. What is the problem here?

EDIT1: In addition to checking ZMQ_EVENTS after doing zmq_recv(), you need to fetch all the message because zmq is EDGE triggered. A great explanation about EDGE trigered notification is here http://funcptr.net/2012/09/10/zeromq---edge-triggered-notification/ so ultimately my event callback would look like

void read_data_on_zmq_cb(evutil_socket_t fd, short what, void *arg)
{
    unsigned int     zmq_events;
    size_t           zmq_events_size  = sizeof(zmq_events);
    char *msg=NULL;

    zmq_getsockopt (receiver, ZMQ_EVENTS, &zmq_events, &zmq_events_size);

    while(zmq_events & ZMQ_POLLIN)
    {
        msg = calloc(1024,sizeof(char));
        int size = zmq_recv (receiver, msg, 255, ZMQ_DONTWAIT);
        if (size != -1) {
        #ifdef DEBUG    
            printf("msg = %s\n",msg);
        #endif
            //return msg;
        }
        zmq_getsockopt (receiver, ZMQ_EVENTS, &zmq_events, &zmq_events_size);
    }
}

Answer 1

PLEASE read my EDIT1 for complete answer with code. The problem was: I didn't recheck ZMQ_EVENTS after doing zmq_recv(), as the state of the socket changes at that time.

So calling

zmq_getsockopt (receiver, ZMQ_EVENTS, &fd, &fd_size);

after zmq_recv() solved my problem.