1. 定时器是怎么实现的

在之前的文章里我们讲过，libevent最后处理都是在event_base_loop调用了相应的dispatch函数，定时器也是在dispatch函数中处理的。

还是以epoll为例，在epoll_dispatch函数有以下一段代码：

    if (tv != NULL) {
        timeout = evutil_tv_to_msec_(tv);
        if (timeout < 0 || timeout > MAX_EPOLL_TIMEOUT_MSEC) {
            /* Linux kernels can wait forever if the timeout is
             * too big; see comment on MAX_EPOLL_TIMEOUT_MSEC. */
            timeout = MAX_EPOLL_TIMEOUT_MSEC;
        }
    }

    epoll_apply_changes(base);
    event_changelist_remove_all_(&base->changelist, base);

    EVBASE_RELEASE_LOCK(base, th_base_lock);

    //epoll_wait的最后一个参数即为超时时间
    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);
    }

从上面代码可以看出，是通过epoll_wait的超时机制来实现定时器的，这样我们就可以知道，其实定时器就是利用了select和epoll_wait等这些系统函数的超时机制，才实现的定时器。

总的来讲，定时器就是在事件主循环中，等待网络调用超时，当超时以后，将任务写入队列，然后处理队列，调用回调函数，这样就实现了定时器。

2. 定时器代码实现

看libevent源代码中例子：

#include <sys/types.h>
#include <event2/event-config.h>
#include <sys/stat.h>
#include <time.h>
#ifdef EVENT__HAVE_SYS_TIME_H
#include <sys/time.h>
#endif
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#include <event2/event.h>
#include <event2/event_struct.h>
#include <event2/util.h>

struct timeval lasttime;

int event_is_persistent;

static void
timeout_cb(evutil_socket_t fd, short event, void *arg)
{
    struct timeval newtime, difference;
    struct event *timeout = (struct event*)arg;
    double elapsed;

    evutil_gettimeofday(&newtime, NULL);
    evutil_timersub(&newtime, &lasttime, &difference);
    elapsed = difference.tv_sec +
        (difference.tv_usec / 1.0e6);

    printf("timeout_cb called at %d: %.3f seconds elapsed.\n",
        (int)newtime.tv_sec, elapsed);
    lasttime = newtime;

    if (! event_is_persistent) {
        struct timeval tv;
        evutil_timerclear(&tv);
        tv.tv_sec = 2;
        event_add(timeout, &tv);
    }
}

int
main(int argc, char **argv)
{
    struct event timeout;
    struct timeval tv;
    struct event_base *base;
    int flags;

    if (argc == 2 && !strcmp(argv[1], "-p")) {
        event_is_persistent = 1;
        flags = EV_PERSIST;
    } else {
        event_is_persistent = 0;
        flags = 0;
    }

    /* Initalize the event library */
    base = event_base_new();

    /* Initalize one event */
    event_assign(&timeout, base, -1, flags, timeout_cb, (void*) &timeout);

    evutil_timerclear(&tv);
    tv.tv_sec = 3;
    event_add(&timeout, &tv);

    evutil_gettimeofday(&lasttime, NULL);

    event_base_dispatch(base);

    return (0);
}

实现三秒调用一次回调函数，执行结果如下：

timeout_cb called at 1535528104: 3.001 seconds elapsed.
timeout_cb called at 1535528107: 3.000 seconds elapsed.
timeout_cb called at 1535528110: 3.001 seconds elapsed.
timeout_cb called at 1535528113: 2.999 seconds elapsed.
timeout_cb called at 1535528116: 3.000 seconds elapsed.
timeout_cb called at 1535528119: 3.002 seconds elapsed.
timeout_cb called at 1535528122: 2.999 seconds elapsed.
timeout_cb called at 1535528125: 3.001 seconds elapsed.
timeout_cb called at 1535528128: 3.000 seconds elapsed.
timeout_cb called at 1535528131: 3.000 seconds elapsed.
timeout_cb called at 1535528134: 2.999 seconds elapsed.
timeout_cb called at 1535528137: 3.000 seconds elapsed.
timeout_cb called at 1535528140: 3.000 seconds elapsed.
timeout_cb called at 1535528143: 3.000 seconds elapsed.
timeout_cb called at 1535528146: 3.000 seconds elapsed.
timeout_cb called at 1535528149: 3.002 seconds elapsed.