安装

 libevent安装很简单，首先可以从github上clone源码/或直接下载压缩。具体命令如下：

sh autogen.sh
./configure
make
make install
make verify

简介

       libevent 是通过注册事件和回调函数，当事件发生后，则执行对应的回调函数。libevent 支持三种事件:网络IO,定时器和信号；定时器的数据结构是最小堆(min heap),网络IO和信号使用数据结构为双向列表;

样例分析

       hello-world.c  该文件见sample目录，本例子提供了一个简单的例子，监听9995端口，当连接到达后返回一个短消息。源码如下：

/*
  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 *);

int
main(int argc, char **argv)
{
        struct event_base *base;
        struct evconnlistener *listener;
        struct event *signal_event;

        struct sockaddr_in sin;
#ifdef WIN32
        WSADATA wsa_data;
        WSAStartup(0x0201, &wsa_data);
#endif
        base = event_base_new();
        if (!base) {
                fprintf(stderr, "Could not initialize libevent!\n");
                return 1;
        }
        memset(&sin, 0, sizeof(sin));
        sin.sin_family = AF_INET;
        sin.sin_port = htons(PORT);

        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;
        }

        signal_event = evsignal_new(base, SIGINT, signal_cb, (void *)base);

        if (!signal_event || event_add(signal_event, NULL)<0) {
                fprintf(stderr, "Could not create/add a signal event!\n");
                return 1;
        }

        event_base_dispatch(base);

        evconnlistener_free(listener);
        event_free(signal_event);
        event_base_free(base);

        printf("done\n");
        return 0;
}

static void
listener_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 void
conn_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 void
conn_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 void
signal_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);
}

event_base_new/event_base_free

创建event_base，在使用libevent函数之前需要分配一个或者多个event_base结构体,使用结束后使用event_base_free方法释放申请的内存；

evconnlistener_new_bind

绑定客户端连接请求到达后的回调方法，其中listener_cb为回调方法；

evconnlistener_new_bind(base, listener_cb, (void *)base,
            LEV_OPT_REUSEABLE|LEV_OPT_CLOSE_ON_FREE, -1,
            (struct sockaddr*)&sin,
            sizeof(sin));

listener_cb

客户端连接请求到达后的回调方法

     struct event_base *base = user_data;
     struct bufferevent *bev; //封装了socket 封装了两个event 对应的 write read 
     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); //绑定回调方法； conn_writecb,conn_eventcb
     bufferevent_enable(bev, EV_WRITE); //设置写方法有效
     bufferevent_disable(bev, EV_READ);//设置读无效  
     bufferevent_write(bev, MESSAGE, strlen(MESSAGE));

bufferevent_setcb 

设置读写对应的回调函数

void bufferevent_setcb(struct bufferevent *bufev,
        bufferevent_data_cb readcb, //读事件
        bufferevent_data_cb writecb,//写事件 
        bufferevent_event_cb eventcb, //连接事件
        void *cbarg);

evsignal_new

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