libevent事件（三）---event_add和epoll_add

今天药忘吃喽~ 2022-06-17 08:09 162阅读 0赞

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