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);
}