libevent的事件机制

1. 事件初始化

从上一篇文章《libevent是怎么选择底层实现的》可以看出来，调用event_base_new()函数就是初始化好底层实现，给event_base结构体中evsel赋值，evsel是一个eventop结构体，我们再来看下：

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;
};

可以看到结构体里面都是回调函数，给evsel赋值的时候，相应的回调函数就会被注册好，以epoll为例：

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
};

把该结构体赋给evsel，相应的epoll的处理函数就注册到了回调函数中，当有相应的事件发生时，我们就调用相应的回调函数。

同时在evsel赋值的时候，就会调用init回调函数进行初始化。

2. 事件主循环

在event.c中有个event_base_dispatch函数，它就是事件主循环，里面会调用event_base_loop，event_base_loop实现如下：

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);
}

可以看到，这是个死循环，除了某些特殊情况下会跳出循环外，其他时候会一直循环调用回调函数中dispatch函数进行处理。

3. 事件处理

关于事件处理，我们还是跟流程，同样以epoll为例，在epoll.c的epoll_dispatch函数中，有如下代码：

    //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);
    }

可以看到，所有的文件描述符变化最后都交给了evmap_io_active_去处理，evmap_io_active_函数实现如下：

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);
    }
}

里面调用了event_active_nolock_，接下来流程如下：

event_active_nolock_ --> event_callback_activate_nolock_ --> event_queue_insert_active

这样就将事件插入到了已激活事件队列中。

然后回到主事件循环中，看如下代码：

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);
}

我们可以看到调用完回调函数dispatch以后，如果存在激活事件，就会调用event_process_active函数，event_process_active调用event_process_active_single_queue函数，event_process_active_single_queue函数会调用事件上绑定的回调函数进行事件处理。