libevent的事件机制

struct eventop {
    /** The name of this backend. */
    const char *name;
    /** Function to set up an event_base to use this backend.  It should
     * create a new structure holding whatever information is needed to
     * run the backend, and return it.  The returned pointer will get
     * stored by event_init into the event_base.evbase field.  On failure,
     * this function should return NULL. */
    void *(*init)(struct event_base *);
    /** Enable reading/writing on a given fd or signal.  'events' will be
     * the events that we're trying to enable: one or more of EV_READ,
     * EV_WRITE, EV_SIGNAL, and EV_ET.  'old' will be those events that
     * were enabled on this fd previously.  'fdinfo' will be a structure
     * associated with the fd by the evmap; its size is defined by the
     * fdinfo field below.  It will be set to 0 the first time the fd is
     * added.  The function should return 0 on success and -1 on error.
     */
    int (*add)(struct event_base *, evutil_socket_t fd, short old, short events, void *fdinfo);
    /** As "add", except 'events' contains the events we mean to disable. */
    int (*del)(struct event_base *, evutil_socket_t fd, short old, short events, void *fdinfo);
    /** Function to implement the core of an event loop.  It must see which
        added events are ready, and cause event_active to be called for each
        active event (usually via event_io_active or such).  It should
        return 0 on success and -1 on error.
     */
    int (*dispatch)(struct event_base *, struct timeval *);
    /** Function to clean up and free our data from the event_base. */
    void (*dealloc)(struct event_base *);
    /** Flag: set if we need to reinitialize the event base after we fork.
     */
    int need_reinit;
    /** Bit-array of supported event_method_features that this backend can
     * provide. */
    enum event_method_feature features;
    /** Length of the extra information we should record for each fd that
        has one or more active events.  This information is recorded
        as part of the evmap entry for each fd, and passed as an argument
        to the add and del functions above.
     */
    size_t fdinfo_len;
};

const struct eventop epollops = {
    "epoll",
    epoll_init,
    epoll_nochangelist_add,
    epoll_nochangelist_del,
    epoll_dispatch,
    epoll_dealloc,
    1, /* need reinit */
    EV_FEATURE_ET|EV_FEATURE_O1|EV_FEATURE_EARLY_CLOSE,
    0
};

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);

        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);
}

    //里面会调用epoll_wait函数
    res = epoll_wait(epollop->epfd, events, epollop->nevents, timeout);

    EVBASE_ACQUIRE_LOCK(base, th_base_lock);

    if (res == -1) {
        if (errno != EINTR) {
            event_warn("epoll_wait");
            return (-1);
        }

        return (0);
    }

    event_debug(("%s: epoll_wait reports %d", __func__, res));
    EVUTIL_ASSERT(res <= epollop->nevents);

    for (i = 0; i < res; i++) {
        int what = events[i].events;
        short ev = 0;
#ifdef USING_TIMERFD
        if (events[i].data.fd == epollop->timerfd)
            continue;
#endif

        if (what & (EPOLLHUP|EPOLLERR)) {
            ev = EV_READ | EV_WRITE;
        } else {
            if (what & EPOLLIN)
                ev |= EV_READ;
            if (what & EPOLLOUT)
                ev |= EV_WRITE;
            if (what & EPOLLRDHUP)
                ev |= EV_CLOSED;
        }

        if (!ev)
            continue;

        evmap_io_active_(base, events[i].data.fd, ev | EV_ET);
    }

void
evmap_io_active_(struct event_base *base, evutil_socket_t fd, short events)
{
    struct event_io_map *io = &base->io;
    struct evmap_io *ctx;
    struct event *ev;

#ifndef EVMAP_USE_HT
    if (fd < 0 || fd >= io->nentries)
        return;
#endif
    //获得一个evmap_io
    GET_IO_SLOT(ctx, io, fd, evmap_io);

    if (NULL == ctx)
        return;
    LIST_FOREACH(ev, &ctx->events, ev_io_next) {
        if (ev->ev_events & events)
            event_active_nolock_(ev, ev->ev_events & events, 1);
    }
}

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);

        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);
}