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) {
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
#include
#include
#include
#ifdef EVENT__HAVE_SYS_TIME_H
#include
#endif
#include
#include
#include
#include
#include
#include
#include
#include
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;
}
base = event_base_new();
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.
