Libevent源码学习---bufferevent

╰+哭是因爲堅強的太久メ 2022-06-17 10:57 282阅读 0赞

代码也可以参考我的[gayhub][]

libevent的bufferevent在event的基础上自己维护了一个buffer，并提供了一系列的API，这样读写起来更加方便．

先看看bufferevent结构体

struct bufferevent {  
            struct event_base *ev_base;  
            const struct bufferevent_ops *be_ops;  
            struct event ev_read;  
            struct event ev_write;  
            struct evbuffer *input;  
            struct evbuffer *output;  
            //……  
            bufferevent_data_cb readcb;  
            bufferevent_data_cb writecb;  
            bufferevent_event_cb errorcb;  
            //……  
    }

我们注意到有三个回调函数，分别对应读，写，错误，这种构造有点类似`select`．  
除此之外，这个结构体中还内置了读写缓冲：`struct evbuffer *input`和`struct evbuffer *output`  
当有数据被读入的时候，`readcb`被调用，当`output`被输出完成的时候，`writecb`被调用，当网络I/O出现错误，如链接中断超时等时，`errorcb`被调用

bufferevent提供了一些API，先来学习如何使用bufferevent：

## **1.设置socket为非阻塞的** ##

evutil_make_socket_nonblocking(fd);

该函数实际上是对`fcntl`的简单封装，先`get`当前的flag或上`NON_BLOCKING`

## **2.使用`bufferevent_socket_new`创建一个`struct bufferevent*bev`，关联该`sockfd`，托管给`event_base`：** ##

//函数原型
    struct bufferevent * bufferevent_socket_new(struct event_base*base,evutil_socket_t fd,int options);
    
    //使用方法
    struct bufferevent *bev;
    bev=bufferevent_socket_new(base,fd,BEV_OPT_CLOSE_ON_FREE);

再看看源码，这里面做了哪些事情：

struct bufferevent *
    bufferevent_socket_new(struct event_base *base, evutil_socket_t fd,
        int options)
    {
        struct bufferevent_private *bufev_p;
        struct bufferevent *bufev;
    
    
        if ((bufev_p = mm_calloc(1, sizeof(struct bufferevent_private)))== NULL)
            return NULL;
    
            //bufferevent_init_common中调用evebuffer_new()初始化input和output
        if (bufferevent_init_common(bufev_p, base, &bufferevent_ops_socket,
                        options) < 0) {
            mm_free(bufev_p);
            return NULL;
        }
        bufev = &bufev_p->bev;
        evbuffer_set_flags(bufev->output, EVBUFFER_FLAG_DRAINS_TO_FD);
    
        //event_assign初始化了ev_read和ev_write事件
        event_assign(&bufev->ev_read, bufev->ev_base, fd,
            EV_READ|EV_PERSIST, bufferevent_readcb, bufev);
        event_assign(&bufev->ev_write, bufev->ev_base, fd,
            EV_WRITE|EV_PERSIST, bufferevent_writecb, bufev);
    
            //给output添加了一个callback
        evbuffer_add_cb(bufev->output, bufferevent_socket_outbuf_cb, bufev);
    
        evbuffer_freeze(bufev->input, 0);
        evbuffer_freeze(bufev->output, 1);
    
        return bufev;
    }

## **3.设置读写对应的回调函数** ##

//函数原型
    void bufferevent_setcb(struct bufferevent *bufev,
                bufferevent_data_cb readcb,
                bufferevent_data_cb writecb,
                bufferevent_event_cb eventcb,
                void *cbarg);

## **4.起用读写事件，其实是调用了`event_add`将相应的事件添加到Reactor中。正如文档所说，相应事件不设置为true，bufferevent是不会读写数据的** ##

//函数原型
    int bufferevent_enable(struct bufferevent*bufev, short event);
    
    //eg.
    bufferevent_enable(bev,EV_READ|EV_WRITE)

简单看一下该函数的实现

int
    bufferevent_enable(struct bufferevent *bufev, short event)
    {
        if (event & EV_READ) {
            if (bufferevent_add(&bufev->ev_read, bufev->timeout_read) == -1)
                return (-1);
        }
        if (event & EV_WRITE) {
            if (bufferevent_add(&bufev->ev_write, bufev->timeout_write) == -1)
                return (-1);
        }
    
        bufev->enabled |= event;
        return (0);
    }
    
    
    //static表明了其是一个内部函数
    static int
    bufferevent_add(struct event *ev, int timeout)
    {
        struct timeval tv, *ptv = NULL;
    
        if (timeout) {
            evutil_timerclear(&tv);
            tv.tv_sec = timeout;
            ptv = &tv;
        }
    
        return (event_add(ev, ptv));//此处调用event_add将event添加到Reactor中
    }

## **使用bufferevent简单的httpclient** ##

参考上述步骤：  
我们使用bufferevent可以实现一个简单的`httpclient`

`gethostbyname`获取`baidu`的主机地址，向80端口发起http请求，将百度的h5页面write到标准输出

#include <stdio.h>
    #include <stdlib.h>
    #include <errno.h>
    #include <assert.h>
    #include <unistd.h>
    #include <signal.h>
    
    #include <event2/event.h>
    #include <event2/bufferevent.h>
    #include <sys/socket.h>
    #include <string.h>
    
    #include <netinet/in.h>
    #include <netdb.h>
    
    void eventcb(struct bufferevent*bev,short events,void *ptr)
    {
        if(events&BEV_EVENT_CONNECTED)
        {
        printf("ok connected\r\n");
        }
        else if(events&(BEV_EVENT_ERROR|BEV_EVENT_EOF))
        {
        struct event_base *base=ptr;
        printf("error\r\n");
        printf("closing \r\n");
        bufferevent_free(bev);
        event_base_loopexit(base,NULL);
        }
    }
    
    void readcb(struct bufferevent*bev,void *arg )
    {
        char buf[1024];
        printf("readcb\r\n");
        int n;
        struct evbuffer *input;
        input = bufferevent_get_input(bev);
        while((n=evbuffer_remove(input,buf,sizeof(buf)))>0)
        {
    //  fwrite(buf,1,n,stdout);
        write(1,buf,n);
        }
    
    }
    
    int main(int argc,char **argv)
    {
    
        struct event_base* base;
        struct bufferevent* bev;
        struct sockaddr_in sin;
        const char hostname[]="www.baidu.com";
        const char query[]=
        "GET / HTTP/1.0\r\n"
        "Host: www.baidu.com\r\n"
        "\r\n";
    
    
        struct hostent * h;
        h=gethostbyname(hostname);
        if(!h)
        {
        fprintf(stderr,"Couldn't lookup %s: %s",hostname,hstrerror(h_errno));
        return 1;
        }
        if(h->h_addrtype!=AF_INET)
        {
        fprintf(stderr,"No ipv6 support sorry.");
        return 1;
        }  
        //#define h_addr h_addr_list[0]
    
        memset(&sin,0,sizeof(sin));
        sin.sin_family=AF_INET;
        sin.sin_port=htons(80);
        sin.sin_addr=*(struct in_addr*)h->h_addr;
    
        base = event_base_new();
        assert(base != NULL);
    
        bev=bufferevent_socket_new(base,-1,BEV_OPT_CLOSE_ON_FREE);
        bufferevent_setcb(bev,readcb,NULL,eventcb,base);
        bufferevent_enable(bev,EV_READ|EV_WRITE);
        evbuffer_add(bufferevent_get_output(bev),query,strlen(query));
    
        if(bufferevent_socket_connect(bev,
            (struct sockaddr*)&sin,sizeof(sin))<0)
        {
        bufferevent_free(bev);
        return -1;
        }
        event_base_dispatch(base);
        return 0;
    }

## **参考** ##

1.[http://blog.csdn.net/yusiguyuan/article/details/17455963][http_blog.csdn.net_yusiguyuan_article_details_17455963]  
2.libevent深入浅出

[gayhub]: https://github.com/NearXdu/LibEvent
[http_blog.csdn.net_yusiguyuan_article_details_17455963]: http://blog.csdn.net/yusiguyuan/article/details/17455963