event_add()

这个函数主要完成了下面几件事：

1.将event注册到event_base的I/O多路复用要监听的事件中

2.将event注册到event_base的已注册事件链表中

3.如果传入了超时时间，则删除旧的超时时间，重新设置，并将event添加到event_base的小根堆中；

如果没有传入超时时间，则不会添加到小根堆中。

只有步骤1成功，才会执行步骤2和3；否则什么都没做，直接返回，保证不会改变event的状态。

从中还可以看到，将event添加到已注册事件链表、添加到小根堆、从活跃事件链表移除、从小根堆中移除，都是通过两个函数完成的：event_queue_insert()、event_queue_remove()

int
event_add(struct event *ev, const struct timeval *tv)
{
    struct event_base *base = ev->ev_base;    //event所属的event_base
    const struct eventop *evsel = base->evsel;    //event_base的I/O多路复用机制
    void *evbase = base->evbase;    //event_base的I/O多路复用机制
    int res = 0;
    //DEBUG log.h
    event_debug((
         "event_add: event: %p, %s%s%scall %p",
         ev,
         ev->ev_events & EV_READ ? "EV_READ " : " ",
         ev->ev_events & EV_WRITE ? "EV_WRITE " : " ",
         tv ? "EV_TIMEOUT " : " ",
         ev->ev_callback));
    assert(!(ev->ev_flags & ~EVLIST_ALL));
    /*
     * prepare for timeout insertion further below, if we get a
     * failure on any step, we should not change any state.
     */
    //如果传入了超时时间并且event不再time小根堆上，则在小根堆上预留一个位置
    //以保证如果后面有步骤失败，不会改变初始状态，保证是个原子操作
    if (tv != NULL && !(ev->ev_flags & EVLIST_TIMEOUT)) {
        if (min_heap_reserve(&base->timeheap,    //min_heap.h
            1 + min_heap_size(&base->timeheap)) == -1)
            return (-1);  /* ENOMEM == errno */
    }
    //如果event不在已注册链表或活跃链表中，
    //则调用evsel->add()注册event事件到I/O多路复用监听的事件上
    if ((ev->ev_events & (EV_READ|EV_WRITE|EV_SIGNAL)) &&
        !(ev->ev_flags & (EVLIST_INSERTED|EVLIST_ACTIVE))) {
        res = evsel->add(evbase, ev);    //将event注册到监听事件上
        //注册监听事件成功，则将event注册到已注册事件链表上
        if (res != -1)
            event_queue_insert(base, ev, EVLIST_INSERTED);  //插入  
    }
    /* 
     * we should change the timout state only if the previous event
     * addition succeeded.
     */
    //前面操作都成功情况下，才能执行下面步骤
    //改变超时状态
    if (res != -1 && tv != NULL) {
        struct timeval now;
        /* 
         * we already reserved memory above for the case where we
         * are not replacing an exisiting timeout.
         */
        //EVLIST_TIMEOUT表明event已在定时器堆中
        //则删除旧的定时器
        if (ev->ev_flags & EVLIST_TIMEOUT)
            event_queue_remove(base, ev, EVLIST_TIMEOUT);  //移除
        /* Check if it is active due to a timeout.  Rescheduling
         * this timeout before the callback can be executed
         * removes it from the active list. */
        //如果事件是由于超时而变成活跃事件
        //则从活跃事件链表中删除
        if ((ev->ev_flags & EVLIST_ACTIVE) &&
            (ev->ev_res & EV_TIMEOUT)) {
            /* See if we are just active executing this
             * event in a loop
             */
            if (ev->ev_ncalls && ev->ev_pncalls) {
                /* Abort loop */
                *ev->ev_pncalls = 0;  //调用次数清0
            }
            //从活跃事件链表移除
            event_queue_remove(base, ev, EVLIST_ACTIVE);  //移除
        }
        gettime(base, &now);
        evutil_timeradd(&now, tv, &ev->ev_timeout);    //为event添加超时时间
        event_debug((
             "event_add: timeout in %ld seconds, call %p",
             tv->tv_sec, ev->ev_callback));
        //将event插入到小根堆中
        event_queue_insert(base, ev, EVLIST_TIMEOUT);  //插入
    }
    return (res);
}

event_queue_insert()

该函数根据不同的输入队列，即不同的事件，在不同的队列中插入，并增加相应的事件计数，更新event状态；

EVLIST_INSERTED：在已注册事件链表event_base.eventqueue插入

EVLIST_ACTIVE：根据event优先级，在活跃事件链表event_base.activequeues[event.ev_pri]插入

EVLIST_TIMEOUT：在小根堆event_base.timeheap中插入

void
event_queue_insert(struct event_base *base, struct event *ev, int queue)
{
    //如果event已经在活跃链表中，则返回；否则，出错
    if (ev->ev_flags & queue) {
        /* Double insertion is possible for active events */
        if (queue & EVLIST_ACTIVE)
            return;
        event_errx(1, "%s: %p(fd %d) already on queue %x", __func__,
               ev, ev->ev_fd, queue);
    }
    if (~ev->ev_flags & EVLIST_INTERNAL)
        base->event_count++;  //增加注册事件数
    ev->ev_flags |= queue;  //改变event状态
    switch (queue) {  //根据不同的输入参数队列，选择在不同的事件集合中插入
    case EVLIST_INSERTED:  //I/O或Signal事件
        TAILQ_INSERT_TAIL(&base->eventqueue, ev, ev_next);  //在已注册事件链表插入
        break;
    case EVLIST_ACTIVE:  //活跃事件
        base->event_count_active++;  //增加活跃事件数
        TAILQ_INSERT_TAIL(base->activequeues[ev->ev_pri],  //在活跃事件链表插入
            ev,ev_active_next);
        break;
    case EVLIST_TIMEOUT: {  //定时器事件
        min_heap_push(&base->timeheap, ev);  //在小根堆插入
        break;
    }
    default:
        event_errx(1, "%s: unknown queue %x", __func__, queue);
    }
}

event_queue_remove()

和event_queue_insert()相对应，这个函数主要根据不同的输入参数，从不同的事件集合中删除事件。

void
event_queue_remove(struct event_base *base, struct event *ev, int queue)
{
    if (!(ev->ev_flags & queue))
        event_errx(1, "%s: %p(fd %d) not on queue %x", __func__,
               ev, ev->ev_fd, queue);
    if (~ev->ev_flags & EVLIST_INTERNAL)
        base->event_count--;
    ev->ev_flags &= ~queue;
    switch (queue) {
    case EVLIST_INSERTED:  //I/O、Signal事件
        TAILQ_REMOVE(&base->eventqueue, ev, ev_next);
        break;
    case EVLIST_ACTIVE:  //活跃事件
        base->event_count_active--;
        TAILQ_REMOVE(base->activequeues[ev->ev_pri],
            ev, ev_active_next);
        break;
    case EVLIST_TIMEOUT:  //定时器事件
        min_heap_erase(&base->timeheap, ev);
        break;
    default:
        event_errx(1, "%s: unknown queue %x", __func__, queue);
    }
}

event_del()

libevent还提供了event_del()这个函数，该函数从直接删除event事件，该函数就是主要通过调用event_queue_remove()函数完成删除的功能。

另外，该函数还将event从I/O多路复用监听的事件中删除。

int
event_del(struct event *ev)
{
    struct event_base *base;
    const struct eventop *evsel;
    void *evbase;
    event_debug(("event_del: %p, callback %p",
         ev, ev->ev_callback));
    /* An event without a base has not been added */
    if (ev->ev_base == NULL)
        return (-1);
    base = ev->ev_base;
    evsel = base->evsel;
    evbase = base->evbase;
    assert(!(ev->ev_flags & ~EVLIST_ALL));
    /* See if we are just active executing this event in a loop */
    //计数清0
    if (ev->ev_ncalls && ev->ev_pncalls) {
        /* Abort loop */
        *ev->ev_pncalls = 0;
    }
    //根据event不同的状态，从相应的event集合中删除
    if (ev->ev_flags & EVLIST_TIMEOUT)
        event_queue_remove(base, ev, EVLIST_TIMEOUT);
    if (ev->ev_flags & EVLIST_ACTIVE)
        event_queue_remove(base, ev, EVLIST_ACTIVE);
    if (ev->ev_flags & EVLIST_INSERTED) {
        event_queue_remove(base, ev, EVLIST_INSERTED);
        return (evsel->del(evbase, ev));  //从I/O多路复用监听的事件中删除
    }
    return (0);
}