本文共 3218 字，大约阅读时间需要 10 分钟。

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 
   
    #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 voidtimeout_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); }}intmain(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.