libevent事件（三）---event_add和epoll

libevent事件（三）---event_add和epoll_add

libevent将定时事件、信号、描述符事件都集成到event中，并且分别维护了相关的链表。
因此，event_add和event_delete主要是操作链表，并且调用底层的api。

1. event_add函数

首先是超时事件，timer事件用一个小根堆来表示，这个堆中的最小时间可以用来select/poll/epoll参数的最大值．
另外，添加事件分为两个内容，首先会调用底层的add回调，它有可能是epoll_ctl()，然后会将该event添加到注册事件的链表中

int
event_add(struct event *ev, const struct timeval *tv)
{
    struct event_base *base = ev->ev_base;//event_base
    const struct eventop *evsel = base->evsel;//evsel为底层的操作
    void *evbase = base->evbase;
    int res = 0;
    //为timer事件分配空间，min_heap_reserve的操作实际上是realloc
    if (tv != NULL && !(ev->ev_flags & EVLIST_TIMEOUT)) {
        if (min_heap_reserve(&base->timeheap,
            1 + min_heap_size(&base->timeheap)) == -1)
            return (-1);  /* ENOMEM == errno */
    }//reserve
    //添加事件
    if ((ev->ev_events & (EV_READ|EV_WRITE|EV_SIGNAL)) &&
        !(ev->ev_flags & (EVLIST_INSERTED|EVLIST_ACTIVE))) {
        res = evsel->add(evbase, ev);//底层回调函数
        if (res != -1)
            event_queue_insert(base, ev, EVLIST_INSERTED);//event插入队列
    }
    /* 
     * we should change the timout state only if the previous event
     * addition succeeded.
     */
    if (res != -1 && tv != NULL) {
   //当上一步操作成功时，如果是timer事件将调整timer堆
        struct timeval now;
        /* 
         * we already reserved memory above for the case where we
         * are not replacing an exisiting timeout.
         */
         //表明event已经在timer堆中了，删除旧的
        if (ev->ev_flags & EVLIST_TIMEOUT)
            event_queue_remove(base, ev, EVLIST_TIMEOUT);
        /* Check if it is active due to a timeout.  Rescheduling
         * this timeout before the callback can be executed
         * removes it from the active list. */
        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;
            }
            event_queue_remove(base, ev, EVLIST_ACTIVE);
        }
        gettime(base, &now);//计算时间并插入到timer的小根堆中
        evutil_timeradd(&now, tv, &ev->ev_timeout);//宏定义：timeout= now + tv
        event_queue_insert(base, ev, EVLIST_TIMEOUT);
    }
    return (res);
}

epoll_add函数

上文提到，相同的接口会调用特定的回调，以epoll为例，将调用封装的epoll_add

static int
epoll_add(void *arg, struct event *ev)
{
    struct epollop *epollop = arg;
    struct epoll_event epev = {
   0, {
   0}};
    struct evepoll *evep;
    int fd, op, events;
    if (ev->ev_events & EV_SIGNAL)
        return (evsignal_add(ev));
    fd = ev->ev_fd;
    if (fd >= epollop->nfds) {
        /* Extent the file descriptor array as necessary */
        if (epoll_recalc(ev->ev_base, epollop, fd) == -1)
            return (-1);
    }
    evep = &epollop->fds[fd];
    op = EPOLL_CTL_ADD;
    events = 0;
    if (evep->evread != NULL) {
        events |= EPOLLIN;
        op = EPOLL_CTL_MOD;
    }
    if (evep->evwrite != NULL) {
        events |= EPOLLOUT;
        op = EPOLL_CTL_MOD;
    }
    if (ev->ev_events & EV_READ)
        events |= EPOLLIN;
    if (ev->ev_events & EV_WRITE)
        events |= EPOLLOUT;
    epev.data.fd = fd;
    epev.events = events;
    if (epoll_ctl(epollop->epfd, op, ev->ev_fd, &epev) == -1)
            return (-1);
    /* Update events responsible */
    if (ev->ev_events & EV_READ)
        evep->evread = ev;
    if (ev->ev_events & EV_WRITE)
        evep->evwrite = ev;
    return (0);
}

首先来看传进来的两个参数，在event_add中有如下调用evsel->add(evbase,ev)，这个时候就明白evbase的作用了，在event_base_new调用中，
有base->evbase=base->evsel->init(base)，回调函数将最终调用epoll_init创建一个struct epollop结构体，并返回指针，为什么要这么做？
本质上来说，还是为了可移植性，对于select也会有struct selectop这样的结构体

再看epoll_add函数，将封装的struct event和struct epollop结构体，转化为epoll认识的struct epoll_event中