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[Libevent-users] Trivial Example of Multi-Thread Issue

Hi guys,

  I’ve been trying to track down a bug with multi-thread use of libevent.   I’m only using the timeout feature of libevent (no fd’s).  I’ve dumbed down the issue to a simple example which is copied in below.    This code deadlocks waiting for the event to occur.


[ Psuedo Code ]

<thread 1: main>

* kicks of thread 2

* waits for other thread to alloc a base+event

* adds event

* waits for callback to occur (which never happens)

* exits

<thread 2>

* alloc’s event base + event

* dispatches


Any idea’s why this doesn’t work?   And yes I’ve called evthread_use_pthreads ;^)


I’ve tried this on a Fedora 15 and OSX box with the same results.  Although I tried with the package managed versions I also tried and got the same results with the trunk of the git tree, release-2.1.3-alpha-44-g197abd8 (on the Fedora box only).





Build the following with “gcc -Wall -o event-test main.c -lrt -levent -levent_pthreads”


#include <stdio.h>

#include <event.h>

#include <pthread.h>

#include <errno.h>

#include <string.h>

#include <unistd.h>

#include <semaphore.h>

#include <event2/thread.h>

#include <fcntl.h>


struct event_base* eb = NULL;

struct event *ev = NULL;

sem_t *sem;

const struct timeval tv = {

    .tv_sec = 2,

    .tv_usec = 0




void cb_sem_post(evutil_socket_t fd, short val, void *arg)


    printf("Running %s\n", __func__);





void *new_thread(void *arg)


    printf("Running %s\n", __func__);


    /* initialize libevent */

    if(evthread_use_pthreads()) {

        fprintf(stderr, "evthread_use_pthreads failed: %s\n", strerror(errno));

        return 0;


    eb = event_base_new();

    ev = evtimer_new(eb, cb_sem_post, 0);


        fprintf(stderr, "evtimer_new failed: %s\n", strerror(errno));


    sem_post(sem);    /* ok free to use ev & eb from other thread */



    return 0;




int main(int argc, const char * argv[])


    pthread_t thread;


    printf("Running %s\n", __func__);


    /* initialize a semaphore */

    sem = sem_open("my_sem", O_CREAT, 0777, 0);

    if(sem == SEM_FAILED) {

        fprintf(stderr, "sem_open failed: %s\n", strerror(errno));

        return 1;



    if(pthread_create(&thread, NULL, new_thread, 0)) {

        fprintf(stderr, "%s\n", strerror(errno));

        return 1;



    sem_wait(sem);     /* ev has been allocated */

    //usleep(3000000);   /* wait till other thread good and sleeping */


    /* add event that will never expire */

    if(event_add(ev, &tv))

        fprintf(stderr, "%s\n", strerror(errno));


    printf("waiting for callback that never occurs\n");





    /* clean up */

    event_base_loopexit(eb, NULL);

    if(ev) event_free(ev);

    if(eb) event_base_free(eb);

    pthread_join(thread, NULL);



    return 0;




Program output:


Running main

Running new_thread

[debug] event_add: event: 0xb6f00618 (fd 7), EV_READ   call 0xb788a7d3

[debug] Epoll ADD(1) on fd 7 okay. [old events were 0; read change was 1; write change was 0]

[debug] event_base_loop: no events registered.

[debug] event_add: event: 0xb6f00908 (fd -1),   EV_TIMEOUT call 0x80489e0

[debug] event_add: event 0xb6f00908, timeout in 2 seconds 0 useconds, call 0x80489e0

waiting for callback that never occurs