目录
- 00. 目录
- 01. event_base_loop函数
- 02. event_base_dispatch函数
- 03. event_base_loopexit函数
- 04. event_base_loopbreak函数
- 05. event_base_got_exit函数
- 06. event_base_got_break函数
- 07. event_base_dump_events函数
- 08. event_base_dump_events函数
- 09. 废弃的事件循环函数
- 10.参考
00. 目录
声明: 该博客来源于传智播客C++学院相关培训参考手册
01. event_base_loop函数
一旦有了一个已经注册了某些事件的event_base(关于如何创建和注册事件请看笔记四),就需要让libevent等待事件并且通知事件的发生。
event_base_loop函数
/**Wait for events to become active, and run their callbacks.This is a more flexible version of event_base_dispatch().By default, this loop will run the event base until either there are no morepending or active events, or until something calls event_base_loopbreak() orevent_base_loopexit(). You can override this behavior with the 'flags'argument.@param eb the event_base structure returned by event_base_new() orevent_base_new_with_config()@param flags any combination of EVLOOP_ONCE | EVLOOP_NONBLOCK@return 0 if successful, -1 if an error occurred, or 1 if we exited becauseno events were pending or active.@see event_base_loopexit(), event_base_dispatch(), EVLOOP_ONCE,EVLOOP_NONBLOCK*/
int event_base_loop(struct event_base *base, int flag); //while(1) { .... }
功能:等待事件被触发, 然后执行对应的回调函数
参数:base: event_base_new的返回值flag: 标志
返回值:成功: 0成功 1表示没有事件触发失败: -1
循环相关标志:
/** @name Loop flagsThese flags control the behavior of event_base_loop().*/
/**@{*/
/** Block until we have an active event, then exit once all active events* have had their callbacks run. */
#define EVLOOP_ONCE 0x01
/** Do not block: see which events are ready now, run the callbacks* of the highest-priority ones, then exit. */
#define EVLOOP_NONBLOCK 0x02
/**@}*/
默认情况下,event_base_loop()函数运行event_base直到其中没有已经注册的事件为止。执行循环的时候,函数重复地检查是否有任何已经注册的事件被触发(比如说,读事件的文件描述符已经就绪,可以读取了;或者超时事件的超时时间即将到达)。如果有事件被触发,函数标记被触发的事件为“激活的”,并且执行这些事件。
在flags参数中设置一个或者多个标志就可以改变event_base_loop()的行为。如果设置了EVLOOP_ONCE,循环将等待某些事件成为激活的,执行激活的事件直到没有更多的事件可以执行,然会返回。如果设置了EVLOOP_NONBLOCK,循环不会等待事件被触发:循环将仅仅检测是否有事件已经就绪,可以立即触发,如果有,则执行事件的回调。
完成工作后,如果正常退出,event_base_loop()返回0;如果因为后端中的某些未处理错误而退出,则返回-1。
为帮助大家理解,这里给出event_base_loop()的算法概要:
while (any events are registered with the loop) {if (EVLOOP_NONBLOCK was set, or any events are already active)If any registered events have triggered, mark them active.elseWait until at least one event has triggered, and mark it active.for (p = 0; p < n_priorities; ++p {if (any event with priority of p is active) {Run all active events with priority of p.break; /* Do not run any events of a less important priority */}}if (EVLOOP_ONCE was set or EVLOOP_NONBLOCK was set)break;
}
02. event_base_dispatch函数
event_base_dispatch()等同于没有设置标志的event_base_loop()。所以,event_base_dispatch()将一直运行,直到没有已经注册的事件了,或者调用了event_base_loopbreak()或者event_base_loopexit()为止。
/**Event dispatching loopThis loop will run the event base until either there are no more pending oractive, or until something calls event_base_loopbreak() orevent_base_loopexit().@param base the event_base structure returned by event_base_new() orevent_base_new_with_config()@return 0 if successful, -1 if an error occurred, or 1 if we exited becauseno events were pending or active.@see event_base_loop()*/
int event_base_dispatch(struct event_base *base);
等价于没有设置标志的 event_base_loop函数
event_base_dispatch()将一直运行,直到没有已经注册的事件了,或者调用了event_base_loopbreak()或者event_base_loopexit()为止。
这些函数定义在<event2/event.h>中,从libevent 1.0版就存在了。
03. event_base_loopexit函数
/**Exit the event loop after the specified timeThe next event_base_loop() iteration after the given timer expires willcomplete normally (handling all queued events) then exit withoutblocking for events again.Subsequent invocations of event_base_loop() will proceed normally.@param eb the event_base structure returned by event_init()@param tv the amount of time after which the loop should terminate,or NULL to exit after running all currently active events.@return 0 if successful, or -1 if an error occurred@see event_base_loopbreak()*/
int event_base_loopexit(struct event_base *base, const struct timeval *tv);
功能:让event_base在给定时间之后停止循环。
参数:base event_base_new的返回值tv 表示延时的时间,如果为NULL 立即停止循环,没有延时返回值:成功: 0成功 失败: -1
注意:
如果event_base当前正在执行任何激活事件的回调,则回调会继续运行,直到运行完所有激活事件的回调之才退出。
04. event_base_loopbreak函数
/**Abort the active event_base_loop() immediately.event_base_loop() will abort the loop after the next event is completed;event_base_loopbreak() is typically invoked from this event's callback.This behavior is analogous to the "break;" statement.Subsequent invocations of event_loop() will proceed normally.@param eb the event_base structure returned by event_init()@return 0 if successful, or -1 if an error occurred@see event_base_loopexit()*/
int event_base_loopbreak(struct event_base *base);
功能:让event_base立即停止循环。
参数:base event_base_new的返回值
返回值:成功: 0成功 失败: -1
这些函数声明在<event2/event.h>中。event_break_loopexit()函数首次在libevent 1.0c版本中实现;event_break_loopbreak()首次在libevent 1.4.3版本中实现。
注意:
event_base_loopbreak()让event_base立即退出循环。它与event_base_loopexit(base,NULL)的不同在于,如果event_base当前正在执行激活事件的回调,它将在执行完当前正在处理的事件后立即退出。
event_base_loopexit(base,NULL)和event_base_loopbreak(base)在事件循环没有运行时的行为不同:前者安排下一次事件循环在下一轮回调完成后立即停止(就好像带EVLOOP_ONCE标志调用一样);后者却仅仅停止当前正在运行的循环,如果事件循环没有运行,则没有任何效果。
官方参考示例一: 立即退出循环
#include <event2/event.h>/* Here's a callback function that calls loopbreak */
void cb(int sock, short what, void *arg)
{struct event_base *base = arg;event_base_loopbreak(base);
}void main_loop(struct event_base *base, evutil_socket_t watchdog_fd)
{struct event *watchdog_event;/* Construct a new event to trigger whenever there are any bytes toread from a watchdog socket. When that happens, we'll call thecb function, which will make the loop exit immediately withoutrunning any other active events at all.*/watchdog_event = event_new(base, watchdog_fd, EV_READ, cb, base);event_add(watchdog_event, NULL);event_base_dispatch(base);
}
官方参考示例二: 执行事件循环10秒,然后退出
#include <event2/event.h>void run_base_with_ticks(struct event_base *base)
{struct timeval ten_sec;ten_sec.tv_sec = 10;ten_sec.tv_usec = 0;/* Now we run the event_base for a series of 10-second intervals, printing"Tick" after each. For a much better way to implement a 10-secondtimer, see the section below about persistent timer events. */while (1) {/* This schedules an exit ten seconds from now. */event_base_loopexit(base, &ten_sec);event_base_dispatch(base);puts("Tick");}
}
测试代码: 每隔3秒钟输出一个字符串
#include <stdio.h>
#include <event.h>int main(void)
{struct event_base *base = NULL;struct timeval tmo = {3, 0}; base = event_base_new();if (NULL == base){ printf("event_base_new failded...\n");return 1;}while(1){ //设置三秒钟退出事件循环event_base_loopexit(base, &tmo); //进入事件循环event_base_dispatch(base);printf("hello itcast\n");}event_base_free(base);return 0;
}
05. event_base_got_exit函数
有时候需要知道对event_base_dispatch()或者event_base_loop()的调用是正常退出的,还是因为调用event_base_loopexit()或者event_base_break()而退出的。可以调用下述函数来确定是否调用了loopexit或者break函数。
/**Checks if the event loop was told to exit by event_loopexit().This function will return true for an event_base at every point afterevent_loopexit() is called, until the event loop is next entered.@param eb the event_base structure returned by event_init()@return true if event_base_loopexit() was called on this event base,or 0 otherwise@see event_base_loopexit()@see event_base_got_break()*/
int event_base_got_exit(struct event_base *base);
功能:判断循环是否因为调用event_base_loopexit()或者event_base_break()而退出的时候返回true,否则返回false。下次启动事件循环的时候,这些值会被重设。
参数:base event_base_new的返回值
返回值:true 循环是因为调用对应的函数而退出0 其它情况
参考示例:
#include <stdio.h>
#include <event.h>int main(void)
{struct event_base *base = NULL;struct timeval tmo = {3, 0}; base = event_base_new();if (NULL == base){ printf("event_base_new failded...\n");return 1;} while(1){ event_base_loopexit(base, &tmo);event_base_dispatch(base);printf("hello itcast\n");//如果事件循环因为event_base_loopexit而退出 返回trueif (event_base_got_exit(base)){printf("event_base_got_exit return true\n");}} event_base_free(base);return 0;
}
06. event_base_got_break函数
有时候需要知道对event_base_dispatch()或者event_base_loop()的调用是正常退出的,还是因为调用event_base_loopexit()或者event_base_break()而退出的。可以调用下述函数来确定是否调用了loopexit或者break函数。
/**Checks if the event loop was told to abort immediately by event_loopbreak().This function will return true for an event_base at every point afterevent_loopbreak() is called, until the event loop is next entered.@param eb the event_base structure returned by event_init()@return true if event_base_loopbreak() was called on this event base,or 0 otherwise@see event_base_loopbreak()@see event_base_got_exit()*/
int event_base_got_break(struct event_base *base);
功能:判断循环是否因为调用event_base_loopexit()或者event_base_break()而退出的时候返回true,否则返回false。下次启动事件循环的时候,这些值会被重设。
参数:base event_base_new的返回值
返回值:true 循环是因为调用对应的函数而退出0 其它情况
07. event_base_dump_events函数
有时候需要在事件回调中获取当前时间的近似视图,但不想调用gettimeofday()(可能是因为OS将gettimeofday()作为系统调用实现,而你试图避免系统调用的开销)。
在回调中,可以请求libevent开始本轮回调时的当前时间视图。
void event_base_dump_events(struct event_base *, FILE *);/** Sets 'tv' to the current time (as returned by gettimeofday()),looking at the cached value in 'base' if possible, and callinggettimeofday() or clock_gettime() as appropriate if there is nocached time.Generally, this value will only be cached while actuallyprocessing event callbacks, and may be very inaccuate if yourcallbacks take a long time to execute.Returns 0 on success, negative on failure.*/
int event_base_gettimeofday_cached(struct event_base *base,struct timeval *tv);
如果event_base当前正在执行回调,event_base_gettimeofday_cached()函数设置tv_out参数的值为缓存的时间。否则,函数调用evutil_gettimeofday()获取真正的当前时间。成功时函数返回0,失败时返回负数。返回值:成功 0失败 负数
注意:
注意,因为libevent在开始执行回调的时候时间值会被缓存,所以这个值至少是有一点不精确。如果回调执行很长时间,这个值将非常不精确。
这个函数是libevent 2.0.4-alpha新引入的。
08. event_base_dump_events函数
为帮助调试程序(或者调试libevent),有时候可能需要已经加入到event_base的所有事件及其状态的完整列表。调用event_base_dump_events()可以将这个列表输出到指定的文件中。
这个列表是人可读的,未来版本的libevent将会改变其格式。
void event_base_dump_events(struct event_base *base, FILE *file);
功能:转储event_base的状态到文件中。
参数:base event_base_new的返回值file FILE类型指针返回值:无
这个函数在libevent 2.0.1-alpha版本中引入。
09. 废弃的事件循环函数
前面已经讨论过,老版本的libevent 具有“当前”event_base的概念。
本文讨论的某些事件循环函数具有操作当前event_base的变体。除了没有base参数外,这些函数跟当前新版本函数的行为相同。
2.0版本之前的event_base是不支持锁的,所以这些函数并不是完全线程安全的:不允许在执行事件循环的线程之外的其他线程中调用*_loopbreak()或者_loopexit()*函数。
10.参考
相关书籍: http://www.wangafu.net/~nickm/libevent-book/Ref2_eventbase.html
官方参考网站: https://www.monkey.org/~provos/libevent/doxygen-2.0.1/index.html