libevent笔记4：Filter_bufferevent过滤器

Filter_bufferevent是一种基于bufferevent的过滤器，其本身也是一个bufferevent。能够对底层bufferevent输入缓存区中的数据进行操作（加/解密等）后再读取，同样也能在一定的操作后再将数据写入底层bufferevent的输出缓存区。需要注意的是，在创建Filter_bufferevent后，底层bufferevent的读写回调函数就不会再生效了，而缓存区的回调函数依旧有效。

Filter_bufferevent相关函数

struct bufferevent *bufferevent_filter_new (struct bufferevent underlying, bufferevent_filter_cb input_filter, bufferevent_filter_cb output_filter, int options, void(free_context)(void *), void *ctx)：创建一个过滤器，参数列表如下：

• struct bufferevent *underlying：需要过滤的底层bufferevent；
• bufferevent_filter_cb input_filter/bufferevent_filter_cb output_filter：对底层bufferevent的输入/输出缓存器进行操作的过滤器函数，这两个过滤器函数会在其源缓存区中有数据时触发；
• int option：关于bufferevent的设置；
• void(*free_context)(void *)：释放时调用的函数，返回值及参数列表均为空；
• void *ctx：传递给过滤器函数的参数；

typedef enum bufferevent_filter_result(* bufferevent_filter_cb)(struct evbuffer *src, struct evbuffer *dst, ev_ssize_t dst_limit, enum bufferevent_flush_mode mode, void *ctx)：需要自定义，并传送给bufferevent_filter_new()的过滤器函数，参数列表如下：

• struct evbuffer *src：需要处理的数据的源缓存区，即从这个缓存中取需要过滤的数据；
• struct evbuffer *dst：需要处理的数据的目的缓存区，即将处理后的数据放入这个缓存中；
• ev_ssize_t dst_limit：目标缓存区的长度，过滤器可以忽略这个参数；
• enum bufferevent_flush_mode mode：过滤模式，来告诉过滤器数据是因为什么进过滤器的。
• void *ctx：上一个函数中给定的参数；
• typedef enum bufferevent_filter_result：过滤函数的返回值，包括：BEV_OK(正常)；BEV_NEED_MORE(需要继续读数据)；BEV_ERROR(发生错误)；

过滤器的源/目缓存区

过滤器函数的参数列表中包含了过滤器的源/目的缓存区。而对于输入/输出过滤器，源/目缓存区是不同，而这对理解过滤器至关重要。可以使用一段简单的程序来确定过滤器的参数与底层bufferevent及Filter_bufferevent的关系，客户端正常连接服务器并发生数据，服务器创建一个Filter_bufferevent并两个bufferevent的一共4个缓存区，过滤器则输出源/目的缓存区的地址。

• 客户端：

#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#include <event2/event.h>
#include <event2/bufferevent.h>
 
void read_cb(struct bufferevent *bev, void *arg)
{
    char buf[1024] = {0}; 
    bufferevent_read(bev, buf, sizeof(buf));
    printf(buf);
}
 
void write_cb(struct bufferevent *bev, void *arg)
{
   printf("我是写缓冲区的回调函数...您已发送
"); 
}
 
void event_cb(struct bufferevent *bev, short events, void *arg)
{
    if (events & BEV_EVENT_EOF)
    {
        printf("connection closed
");  
    }
    else if(events & BEV_EVENT_ERROR)   
    {
        printf("some other error
");
    }
    else if(events & BEV_EVENT_CONNECTED)
    {
        printf("服务器已连接
");
        return;
    }
    
    bufferevent_free(bev);
    printf("free bufferevent...
");
}
 
void send_cb(evutil_socket_t fd, short what, void *arg)
{
    char buf[1024] = {0}; 
    struct bufferevent* bev = (struct bufferevent*)arg;
    read(fd, buf, sizeof(buf));
    bufferevent_write(bev, buf, strlen(buf)-1);
}
 
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.
");
    event_base_loopexit(base, &delay);
}

int main(int argc, const char* argv[])
{
    struct event_base* base;
    base = event_base_new();
 
 
    struct bufferevent* bev;
    bev = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);
 
    // 连接服务器
    struct sockaddr_in serv;
    memset(&serv, 0, sizeof(serv));
    serv.sin_family = AF_INET;
    serv.sin_port = htons(9995);
    evutil_inet_pton(AF_INET, "127.0.0.1", &serv.sin_addr.s_addr);
    bufferevent_socket_connect(bev, (struct sockaddr*)&serv, sizeof(serv));
 
    // 设置回调
    bufferevent_setcb(bev, read_cb, NULL, event_cb, NULL);
    bufferevent_enable(bev, EV_READ | EV_PERSIST);
    // 创建一个事件
    struct event* ev = event_new(base, STDIN_FILENO, 
                                 EV_READ|EV_PERSIST, send_cb, bev);

    //for Ctrl+C
    struct event *signal_event;
    signal_event = evsignal_new(base, SIGINT, signal_cb, (void *)base);
    event_add(signal_event, NULL);
    event_add(ev, NULL);  
    event_base_dispatch(base);
    event_base_free(base);
    return 0;
}

• 服务端：

#include <event2/bufferevent.h>
#include <event2/event.h>
#include <event2/buffer.h>
#include <arpa/inet.h>// interner address
#include <unistd.h>//os
#include <stdio.h>
#include <malloc.h>
#include <string.h>
#include <event2/listener.h>
#include <sys/types.h>
#include <errno.h>

enum bufferevent_filter_result input_cb(struct evbuffer *src, struct evbuffer *dst, 
		ev_ssize_t dst_limit, enum bufferevent_flush_mode mode, void *ctx)
{
   //输出缓存区的地址
   printf("the src and dst in fun input_cb: %ld, %ld
", src, dst);
   //清空源缓存区，避免被一直调用
   evbuffer_drain(src, 1024);
   return BEV_OK;   
}

enum bufferevent_filter_result output_cb(struct evbuffer *src, struct evbuffer *dst,
                ev_ssize_t dst_limit, enum bufferevent_flush_mode mode, void *ctx)
{
   //输出缓存区的地址
   printf("the src and dst in fun output_cb: %ld, %ld
", src, dst);
   //清空源缓存区，避免被一直调用
   evbuffer_drain(src, 1024);
   return BEV_OK;
}


void read_cb(struct bufferevent *bev, void *arg)
{
   char buf[1024] = {0};
   bufferevent_read(bev, buf, 1024);
   printf("%s
", buf);
}

void write_cb(struct bufferevnet *bev, void *arg)
{
   printf("write_cb
"); 
}

void listener_cb(struct evconnlistener *listener, evutil_socket_t fd,
		 struct sockaddr *addr, int len, void *ptr)
{
   struct sockaddr_in *caddr = (struct sockaddr_in *)addr;
   struct event_base *base = (struct event_base *)ptr;
   
   //init bufferevent
   struct bufferevent *bev;
   bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);
   
   bufferevent_setcb(bev, NULL, NULL, NULL, NULL);
   bufferevent_enable(bev, EV_WRITE | EV_READ);
   //bufferevent_write(bev, "Hello client!", strlen("Hello client!")+1);
   
   struct bufferevent *filter_bev = bufferevent_filter_new(bev, input_cb, output_cb, BEV_OPT_CLOSE_ON_FREE, NULL, NULL);
   //设置Filter_bufferevent的读写函数
   bufferevent_setcb(filter_bev, read_cb, write_cb, NULL, NULL);
   bufferevent_enable(filter_bev, EV_READ|EV_WRITE);
   //输出底层bufferevent的缓存区地址
   printf("the input evbuffer and output evbuffer of underlying bufferevent: %ld, %ld
", bufferevent_get_input(bev), bufferevent_get_output(bev));
   //输出Filter_bufferevent的缓存区地址
   printf("the input evbuffer and output evbuffer of filter bufferevent: %ld, %ld
", bufferevent_get_input(filter_bev), bufferevent_get_output(filter_bev));
   //先Filter_bufferevent写
   bufferevent_write(filter_bev, "abc", sizeof("abc"));
}


int main(int argc, const char *argv[])
{
   //init server
   struct sockaddr_in servaddr;
   memset(&servaddr, 0, sizeof(servaddr));
   servaddr.sin_family = AF_INET;
   servaddr.sin_port = htons(9995);
   servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

   //init event_base
   struct event_base *base;
   base = event_base_new();

   //init linstener
   struct evconnlistener *listener;
   listener = evconnlistener_new_bind(base, listener_cb, base, 
		   LEV_OPT_CLOSE_ON_FREE|LEV_OPT_REUSEABLE_PORT, 36, 
		   (struct socketaddr *)&servaddr, sizeof(servaddr));
   event_base_dispatch(base);

   evconnlistener_free(listener);
   event_base_free(base);
}

服务端运行结果：

sunminming@sunminming:~/libevent/filter$ ./addrdemo 
the input evbuffer and output evbuffer of underlying bufferevent: 94493149330208, 94493149330352
the input evbuffer and output evbuffer of filter bufferevent: 94493149331184, 94493149331328
the src and dst in fun output_cb: 94493149331328, 94493149330352
the src and dst in fun input_cb: 94493149330208, 94493149331184

可以看出来：

• 输入过滤器中的源缓存区与底层buferevent的输入缓存区地址一致，目的缓存区与Filter_bufferevent的输入缓存区地址一致。
• 输出过滤器中的源缓存区与Filter_bufferevent的输出缓存区地址一致，目的缓存区与底层bufferevent的输出缓存区地址一致。
  因此，4个缓存区与2个过滤器函数的关系可以画成下图：
  

Demo

之后就是过滤器正常使用的Demo：输入过滤器会将底层bufferevent的接受到的数据加1处理，而输出过滤器则会将输出的数据复制一份加在原数据的末尾。

• 服务端：

#include <event2/bufferevent.h>
#include <event2/event.h>
#include <event2/buffer.h>
#include <arpa/inet.h>// interner address
#include <unistd.h>//os
#include <stdio.h>
#include <malloc.h>
#include <string.h>
#include <event2/listener.h>
#include <sys/types.h>
#include <errno.h>

enum bufferevent_filter_result input_cb(struct evbuffer *src, struct evbuffer *dst, 
		ev_ssize_t dst_limit, enum bufferevent_flush_mode mode, void *ctx)
{
   //处理输入的数据：从源缓存区读，并加1后放入目的缓存区
   char buf[1024];
   memset(buf, '', sizeof(buf));
   evbuffer_remove(src, buf, sizeof(buf));
   int len = strlen(buf);
   printf("%d", len);
   for(int i = 0; i < len; ++i)
   {
      ++buf[i];
   }
   buf[2*len] = '
';
   evbuffer_add(dst, buf, len);
   return BEV_OK;   
}

enum bufferevent_filter_result output_cb(struct evbuffer *src, struct evbuffer *dst,
                ev_ssize_t dst_limit, enum bufferevent_flush_mode mode, void *ctx)
{
   //处理输出的数据：从源缓存区读，复制后放入目的缓存区
   char buf[1024] = {0};
   memset(buf, '', sizeof(buf));
   evbuffer_remove(src, buf, sizeof(buf));
   int len = strlen(buf);
   for(int i = len; i < 2*len; ++i)
   {
      buf[i] = buf[i - len];
   }
   evbuffer_add(dst, buf, 2*len);
   return BEV_OK;
}


void read_cb(struct bufferevent *bev, void *arg)
{
   char buf[1024] = {0};
   bufferevent_read(bev, buf, 1024);
   printf("%s
", buf);
}

void listener_cb(struct evconnlistener *listener, evutil_socket_t fd,
		 struct sockaddr *addr, int len, void *ptr)
{
   struct sockaddr_in *caddr = (struct sockaddr_in *)addr;
   struct event_base *base = (struct event_base *)ptr;
   
   //init bufferevent
   struct bufferevent *bev;
   bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);
   
   bufferevent_setcb(bev, NULL, NULL, NULL, NULL);
   bufferevent_enable(bev, EV_WRITE | EV_READ);
   
   struct bufferevent *filter_bev = bufferevent_filter_new(bev, input_cb, output_cb, BEV_OPT_CLOSE_ON_FREE, NULL, NULL);
   bufferevent_setcb(filter_bev, read_cb, NULL, NULL, NULL);
   bufferevent_enable(filter_bev, EV_READ|EV_WRITE);

   bufferevent_write(filter_bev, "abc", sizeof("abc"));
}


int main(int argc, const char *argv[])
{
   //init server
   struct sockaddr_in servaddr;
   memset(&servaddr, 0, sizeof(servaddr));
   servaddr.sin_family = AF_INET;
   servaddr.sin_port = htons(9995);
   servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

   //init event_base
   struct event_base *base;
   base = event_base_new();

   //init linstener
   struct evconnlistener *listener;
   listener = evconnlistener_new_bind(base, listener_cb, base, 
		   LEV_OPT_CLOSE_ON_FREE|LEV_OPT_REUSEABLE_PORT, 36, 
		   (struct socketaddr *)&servaddr, sizeof(servaddr));
   event_base_dispatch(base);

   evconnlistener_free(listener);
   event_base_free(base);
}

客户端与上一节相同。流程见注释。