libevent笔记5:水位watermarks

bufferevent中提供了对读写回调的触发条件及最大缓存长度的设置,即低高水位:

  • 低水位:是读写回调函数的最低触发数据长度,当输入/输出缓存区中的数据长度小于低水位时,读/写回调函数不会被触发;
  • 高水位:是缓存区的最大接收长度,当输入/输出缓存区中的数据长度大于高水位时,不会继续向缓存区中增加数据。

水位设置函数bufferevent_setwatermark

void bufferevent_setwatermark (struct bufferevent *bufev, short events, size_t lowmark, size_t highmark)
该函数能够为一个给定的bufferevent设置指定事件的低高水位。若events为EV_READ则为设置读回调函数的水位;events为EV_WRITE则为设置写回调函数的水位,

Demo

服务端:在创建了与客户端连接的bufferevent后,设置低水位为6,高水位为10。

//server.c
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

// 读缓冲区回调
void read_cb(struct bufferevent *bev, void *arg)
{
    char buf[1024] = {0};   
    bufferevent_read(bev, buf, sizeof(buf));
    //这里不能直接输出字符串
    for(int i = 0; i < 20; ++i)
    {
       printf("%c", buf[i]);
    }
    printf("\n");
}
 
void cb_listener(struct evconnlistener *listener, evutil_socket_t fd, 
        struct sockaddr *addr, int len, void *ptr){
   
   struct sockaddr_in *addr_in = (struct socketaddr*)addr;
   printf("connect new client from: %s\n", inet_ntoa(addr_in->sin_addr));
   struct event_base* base = (struct event_base*)ptr;
   // 通信操作
   // 添加新事件
   struct bufferevent *bev;
   bev = bufferevent_socket_new(base, fd, BEV_OPT_CLOSE_ON_FREE);
 
   // 给bufferevent缓冲区设置回调
   bufferevent_setcb(bev, read_cb, NULL, NULL, NULL);
   bufferevent_enable(bev, EV_READ);
   bufferevent_enable(bev, EV_WRITE);

   //设置读写的高低水位
   bufferevent_setwatermark(bev, EV_READ|EV_WRITE, 6, 10);  
}

int main(int argc, const char* argv[])
{
    // init server 
    struct sockaddr_in serv;
    memset(&serv, 0, sizeof(serv));
    serv.sin_family = AF_INET;
    serv.sin_port = htons(9995);
    serv.sin_addr.s_addr = htonl(INADDR_ANY);
 
    struct event_base* base;
    base = event_base_new();
    // 创建套接字
    // 绑定
    // 接收连接请求
    struct evconnlistener* listener;
    listener = evconnlistener_new_bind(base, cb_listener, base, 
                                  LEV_OPT_CLOSE_ON_FREE | LEV_OPT_REUSEABLE, 
                                  36, (struct sockaddr*)&serv, sizeof(serv));
 
    event_base_dispatch(base);
    evconnlistener_free(listener);
    event_base_free(base);
    return 0;
}

客户端:

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
 
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);
}

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);

    event_add(ev, NULL); 
    event_base_dispatch(base);
    event_base_free(base);
    return 0;
}

客户端发送数据:

sunminming@sunminming:~/libevent/watermask$ ./client 
服务器已连接
a
qwertyuio

服务器接收的数据:

sunminming@sunminming:~/libevent/watermask$ ./server 
connect new client from: 127.0.0.1
a
qwertyu

我们从客户端一共发送了两次信息,第一次发送3个字符:'a','\n','\0';第二次发送11个字符'q','w','e','r','t','y','u','i','o','\n','\0')。
因此,读回调函数输出的前10个字符应该为:'a','\n','\0','q','w','e','r','t','y','u';之后还输出了10个buf数组中原本就存在的'\0';最后输出换行符。

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