int
event_base_loop(struct event_base *base, int flags)
{
    const struct eventop *evsel = base->evsel;
    struct timeval tv;
    struct timeval *tv_p;
    int res, done, retval = 0;

    /* Grab the lock.  We will release it inside evsel.dispatch, and again
     * as we invoke user callbacks. */
    EVBASE_ACQUIRE_LOCK(base, th_base_lock);

    if (base->running_loop) {
        event_warnx("%s: reentrant invocation.  Only one event_base_loop"
            " can run on each event_base at once.", __func__);
        EVBASE_RELEASE_LOCK(base, th_base_lock);
        return -1;
    }

    base->running_loop = 1;

    clear_time_cache(base);

    if (base->sig.ev_signal_added && base->sig.ev_n_signals_added)
        evsig_set_base_(base);

    done = 0;

#ifndef EVENT__DISABLE_THREAD_SUPPORT
    base->th_owner_id = EVTHREAD_GET_ID();
#endif

    base->event_gotterm = base->event_break = 0;

    while (!done) {
        base->event_continue = 0;
        base->n_deferreds_queued = 0;

        /* Terminate the loop if we have been asked to */
        if (base->event_gotterm) {
            break;
        }

        if (base->event_break) {
            break;
        }

        tv_p = &tv;
        if (!N_ACTIVE_CALLBACKS(base) && !(flags & EVLOOP_NONBLOCK)) {
            timeout_next(base, &tv_p);
        } else {
            /*
             * if we have active events, we just poll new events
             * without waiting.
             */
            evutil_timerclear(&tv);
        }

        /* If we have no events, we just exit */
        if (0==(flags&EVLOOP_NO_EXIT_ON_EMPTY) &&
            !event_haveevents(base) && !N_ACTIVE_CALLBACKS(base)) {
            event_debug(("%s: no events registered.", __func__));
            retval = 1;
            goto done;
        }

        event_queue_make_later_events_active(base);

        clear_time_cache(base);
        
        //这个dispatch是根据不同版本，系统等选择的，这里以epoll_dispatch为例
        res = evsel->dispatch(base, tv_p);

        if (res == -1) {
            event_debug(("%s: dispatch returned unsuccessfully.",
                __func__));
            retval = -1;
            goto done;
        }

        update_time_cache(base);

        timeout_process(base);

        if (N_ACTIVE_CALLBACKS(base)) {
            int n = event_process_active(base);
            if ((flags & EVLOOP_ONCE)
                && N_ACTIVE_CALLBACKS(base) == 0
                && n != 0)
                done = 1;
        } else if (flags & EVLOOP_NONBLOCK)
            done = 1;
    }
    event_debug(("%s: asked to terminate loop.", __func__));

done:
    clear_time_cache(base);
    base->running_loop = 0;

    EVBASE_RELEASE_LOCK(base, th_base_lock);

    return (retval);
}

根据操作系统类型，版本得到不同的dispatch函数

libevent 之 event_base_loop_不同版本

event_base_loop函数主要流程

libevent 之 event_base_loop_#ifndef_02