libevent 初探

来源：互联网发布：广联达水利水电软件编辑：程序博客网时间：2024/05/22 03:14

Luo Weifeng 2011-8-27

The libevent API provides a mechanism to execute a callback function when a specific event occurs on a file descriptor or after a timeout has been reached. Furthermore, libevent also support callbacks due to signals or regular timeouts.

摘录自官方：http://monkey.org/~provos/libevent/

libevent是什么？其实就是封装了诸如 poll select等的事件分发函数的一个事件分发处理库。应用逻辑可以对指定时间指定事件处理程序，支持的事件有： timeout， signal 和用户事件。开发过linux 上网络应用程序的银儿应该很容易理解这个东西。说白了就是一个大循环不停的检测各种注册了的事件，如果某个事件发生了，就触发该事件的注册处理函数。

libevent的使用耶比较广泛，从memcache等到开源的一些虚拟机通信，云计算领域等。

下面看一个自带的demo：

/*  This exmple program provides a trivial server program that listens for TCP  connections on port 9995.  When they arrive, it writes a short message to  each client connection, and closes each connection once it is flushed.  Where possible, it exits cleanly in response to a SIGINT (ctrl-c).*/#include <string.h>#include <errno.h>#include <stdio.h>#include <signal.h>#ifndef WIN32#include <netinet/in.h># ifdef _XOPEN_SOURCE_EXTENDED#  include <arpa/inet.h># endif#include <sys/socket.h>#endif#include <event2/bufferevent.h>#include <event2/buffer.h>#include <event2/listener.h>#include <event2/util.h>#include <event2/event.h>static const char MESSAGE[] = "Hello, World!\n";static const int PORT = 9995;static void listener_cb(struct evconnlistener *, evutil_socket_t,    struct sockaddr *, int socklen, void *);static void conn_writecb(struct bufferevent *, void *);static void conn_eventcb(struct bufferevent *, short, void *);static void signal_cb(evutil_socket_t, short, void *);intmain(int argc, char **argv){struct event_base *base;struct evconnlistener *listener;    struct event *signal_event;struct sockaddr_in sin;#ifdef WIN32WSADATA wsa_data;WSAStartup(0x0201, &wsa_data);#endif        // eventbase 对象是所有event的管理者，维护了整个event系统的状态，队列，event堆等base = event_base_new();if (!base) {fprintf(stderr, "Could not initialize libevent!\n");return 1;}        // 标准 tcpsocketmemset(&sin, 0, sizeof(sin));sin.sin_family = AF_INET;sin.sin_port = htons(PORT);    //  这里将网络事件注册到libevent中，由listener_cb 处理事件listener = evconnlistener_new_bind(base, listener_cb, (void *)base,    LEV_OPT_REUSEABLE|LEV_OPT_CLOSE_ON_FREE, -1,    (struct sockaddr*)&sin,    sizeof(sin));if (!listener) {fprintf(stderr, "Could not create a listener!\n");return 1;}    // 这里创建的一个信号处理机制， 当出现SIGINT信号时使用signal_cb进行处理signal_event = evsignal_new(base, SIGINT, signal_cb, (void *)base);        // 将刚刚创建的信号处理注册到libevent处理中if (!signal_event || event_add(signal_event, NULL)<0) {fprintf(stderr, "Could not create/add a signal event!\n");return 1;}    //这里应该是主要的分发大循环    event_base_dispatch(base);evconnlistener_free(listener);event_free(signal_event);event_base_free(base);printf("done\n");return 0;}// 这应该是事件处理函数了static voidlistener_cb(struct evconnlistener *listener, evutil_socket_t fd,    struct sockaddr *sa, int socklen, void *user_data){struct event_base *base = user_data;struct bufferevent *bev;bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);if (!bev) {fprintf(stderr, "Error constructing bufferevent!");event_base_loopbreak(base);return;}bufferevent_setcb(bev, NULL, conn_writecb, conn_eventcb, NULL);bufferevent_enable(bev, EV_WRITE);bufferevent_disable(bev, EV_READ);bufferevent_write(bev, MESSAGE, strlen(MESSAGE));}static voidconn_writecb(struct bufferevent *bev, void *user_data){struct evbuffer *output = bufferevent_get_output(bev);if (evbuffer_get_length(output) == 0) {printf("flushed answer\n");bufferevent_free(bev);}}static voidconn_eventcb(struct bufferevent *bev, short events, void *user_data){if (events & BEV_EVENT_EOF) {printf("Connection closed.\n");} else if (events & BEV_EVENT_ERROR) {printf("Got an error on the connection: %s\n",    strerror(errno));/*XXX win32*/}/* None of the other events can happen here, since we haven't enabled * timeouts */bufferevent_free(bev);}static voidsignal_cb(evutil_socket_t sig, short events, void *user_data){struct event_base *base = user_data;struct timeval delay = { 2, 0 };printf("Caught an interrupt signal; exiting cleanly in two seconds.\n");event_base_loopexit(base, &delay);}


注意到这个例子非常的简单， 这里注册了两个类型的事件，一种是网络连接时间，当访问本地 localhost:9995 的时候，产生了一个connect事件，然后，由conn_event_cb 回调
函数进行处理，返回MESSAGE 这个字符串，在浏览器中能看见， signal_cb处理 SIGNAL_INT  ,通俗的理解为Ctrol+C 。	
					
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