定时器是一种特殊类型的时间间隔测量的时钟，定时器发出提示代表时间间隔已过期。下面我们就用网络库Libevent实现简单的定时器功能：每5秒执行一次回调函数，打印当前时间。
#include <stdio.h>
#include <time.h>
#include <event2/event.h>
#include <event2/event_struct.h>
void timeout_cb(evutil_socket_t fd, short event, void *arg)
{
time_t t = time(NULL);
printf("%s", ctime(&t));
}
int main(void)
{
struct event timeout;
struct event_base *base;
struct timeval tv;
base = event_base_new();
event_assign(&timeout, base, -1, EV_PERSIST, timeout_cb, NULL);
evutil_timerclear(&tv);
tv.tv_sec = 5;
tv.tv_usec = 0;
event_add(&timeout, &tv);
event_base_dispatch(base);
return 0;
}
[ycxie@fedora Workspace]$ gcc -o libevent_timer -levent -Wall libevent_timer.c [ycxie@fedora Workspace]$ ./libevent_timer Tue Jul 17 12:32:14 2018 Tue Jul 17 12:32:19 2018 Tue Jul 17 12:32:24 2018 Tue Jul 17 12:32:29 2018 Tue Jul 17 12:32:34 2018
为使用方便，libevent提供了一些以evtimer_开头的宏，用于替代event_*调用来操作纯超时事件。使用这些宏能改进代码的清晰性。
#include <stdio.h>
#include <time.h>
#include <event2/event.h>
#include <event2/event_struct.h>
void timeout_cb(evutil_socket_t fd, short event, void *arg)
{
time_t t = time(NULL);
printf("%s", ctime(&t));
struct timeval tv;
evutil_timerclear(&tv);
tv.tv_sec = 5;
tv.tv_usec = 0;
struct event *timeout = arg;
evtimer_add(timeout, &tv);
}
int main(void)
{
struct event timeout;
struct event_base *base;
struct timeval tv;
base = event_base_new();
evtimer_assign(&timeout, base, timeout_cb, &timeout);
evutil_timerclear(&tv);
tv.tv_sec = 5;
tv.tv_usec = 0;
evtimer_add(&timeout, &tv);
event_base_dispatch(base);
return 0;
}
[ycxie@fedora Workspace]$ gcc -o libevent_timer -levent -Wall libevent_timer.c [ycxie@fedora Workspace]$ ./libevent_timer Tue Jul 17 13:01:57 2018 Tue Jul 17 13:02:02 2018 Tue Jul 17 13:02:07 2018 Tue Jul 17 13:02:12 2018 Tue Jul 17 13:02:17 2018