libevent EVLOOP_NO_EXIT_ON_EMPTY not working?

Question

I don't understand how the EVLOOP_NO_EXIT_ON_EMPTY flag is supposed to work in version 2.1.x of libevent.

If I don't add any events to my event_base the

event_base_loop(my_base, EVLOOP_NO_EXIT_ON_EMPTY);

call returns immediately which is not at all what I think it's supposed to do.

If I add an event it loops with that pending event until it get's active but then the loop exits which I hoped would not happen.

Goal:

Have a named pipe open and libevent listening for a read. Whenever I

echo "something" > pipe

the registered callback should be called. If the callback has finished the event get's back to pending and the loop waits for another echo.

Here's what I got so far: (error checking omitted)

#include <fcntl.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <event.h>
#include <unistd.h>

#define PIPE "/tmp/ev_pipe"


void cb_func(evutil_socket_t fd, short what, void *arg)
{
  printf("foo\n");
}

int main(void)
{
  /* create & open named pipe */
  mkfifo(PIPE, 0666);
  int socket = open(PIPE, O_RDONLY | O_NONBLOCK);

  /* libevent specific stuff */
  struct event_base *ev_base = event_base_new();
  struct event *ev = event_new(ev_base, (evutil_socket_t) socket, EV_READ, cb_func, NULL);
  event_add(ev, NULL);

  /* loop forever */
  event_base_loop(ev_base, EVLOOP_NO_EXIT_ON_EMPTY);

  printf("a\n");

  /* clean up */
  unlink(PIPE);
  event_base_free(ev_base);
  close(socket);

  return 0;
}

What am I missing? The event loop exits after the first write to the queue :/

Thanks for any help!

Answer 1

The implementation of the feature looks buggy! I had faced the same issue with the 2.1.x version. One way to get around the issue is as @Wizzard has pointed out. Another way of getting around the issue is OR the flag EV_PERSIST to the events argument to the function event_new:

struct event *ev = event_new(ev_base, 
    (evutil_socket_t) socket, EV_READ|EV_PERSIST, cb_func, NULL);

This will prevent the event from being removed. https://github.com/libevent/libevent/blob/master/include/event2/event.h +872

Please be aware that you might get multiple callbacks when there is data on pipe.

Answer 2

Just use event_base_dispatch (ev_base); instead of event_base_loop ()``, it will loop and handle all attached events either until you explicitly delete all attached events or callevent_base_loopbreak ()``` function.