socket通信之listen函数

妖狐艹你老母 2022-12-22 00:47 179阅读 0赞

# socket通信之listen函数 #

listen函数原型如下：

#include <sys/types.h>
    #include <sys/socket.h>
    
    int listen(int sockfd, int backlog);

第一个参数sockfd为创建socket返回的文件描述符。

第二个参数backlog为建立好连接处于ESTABLISHED状态的队列的长度。

backlog的最大值128（linux原文描述如下）：

If  the  backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is silently truncated to that value; the default value in this file is 128.  In kernels before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.

下面附上TCP三次握手的执行过程图：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMjI4MTI4NDk_size_16_color_FFFFFF_t_70_pic_center]

TCP中有如下两个队列：

1.  SYN队列（半连接队列）：当服务器端收到客户端的SYN报文时，会响应SYN/ACK报文，然后连接就会进入SYN RECEIVED状态，处于SYN RECEIVED状态的连接被添加到SYN队列，并且当它们的状态改变为ESTABLISHED时，即当接收到3次握手中的ACK分组时，将它们移动到accept队列。SYN队列的大小由内核参数/proc/sys/net/ipv4/tcp\_max\_syn\_backlog设置。
2.  accept队列（完全连接队列）：accept队列存放的是已经完成TCP三次握手的连接，而accept系统调用只是简单地从accept队列中取出连接而已，并不是调用accept函数才会完成TCP三次握手，accept队列的大小可以通过listen函数的第二个参数控制。

## 实验 ##

下面我们用实验验证下backlog这个参数的含义：

服务器端代码如下：

#include <sys/types.h>
    #include <sys/socket.h>
    #include <arpa/inet.h>
    #include <string.h>
    #include <stdio.h>
    
    int main() { 
        
        // create socket
        int listenfd = socket(AF_INET, SOCK_STREAM, 0);
        if(-1 == listenfd) { 
            printf("create socket error");
            return -1;
        }
        
        // bind port 
        struct sockaddr_in bindaddr;
        bindaddr.sin_family = AF_INET;
        bindaddr.sin_addr.s_addr = htonl(INADDR_ANY);
        bindaddr.sin_port = htons(3000);
        if(-1 == bind(listenfd, (struct sockaddr *)&bindaddr, sizeof(bindaddr))) { 
            printf("bind error");
            return -1;
        }
        
        // start listen
        if (listen(listenfd, 2) == -1) { 
            printf("listem error");
            return -1;
        }
        
        while(1){ }
        return 0;
    }

客户端代码如下：

#include <sys/types.h>
    #include <sys/socket.h>
    #include <arpa/inet.h>
    #include <string.h>
    #include <stdio.h>
    #include <pthread.h>
    
    #define PORT 3000
    #define thread_num 10
    
    struct sockaddr_in serv_addr;
    
    void *func() { 
        int conn_fd;
        conn_fd = socket(AF_INET, SOCK_STREAM, 0);
        printf("conn_fd : %d\n", conn_fd);
    
        if( connect(conn_fd, (struct sockaddr *)&serv_addr, sizeof(struct sockaddr_in)) == -1) { 
            printf("connect error\n");
        }
    
        while(1) { }
    }
    
    int main(int argc,char *argv[]) { 
        serv_addr.sin_family = AF_INET;
        serv_addr.sin_port = htons(PORT);
    	serv_addr.sin_addr.s_addr = inet_addr("10.0.4.129");;
    
        pthread_t pid[thread_num];
        int i;
        for(i = 0 ; i < thread_num; ++i) { 
            pthread_create(&pid[i], NULL, &func, NULL);
        }
    
        for(i = 0 ; i < thread_num; ++i) { 
            pthread_join(pid[i], NULL);
        }
    
        return 0;
    }

由于建立连接和关闭连接的速度非常快，肉眼无法观察到，所以需要将变化的内容存到文本中，然后通过观察文本中的内容进行分析：

# watch -n 1 "netstat -anotp|grep 3000 >> a.txt"

启动服务器端，一开始没有连接进来：

tcp        0      0 0.0.0.0:3000            0.0.0.0:*               LISTEN      65667/./server.out   off (0.00/0/0)

然后启动客户端，用10个线程去连接服务器，因此服务器上有10条连接，其中只有三个连接是ESTABLISHED（backlog+1）：

tcp        3      0 0.0.0.0:3000            0.0.0.0:*               LISTEN      65667/./server.out   keepalive (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57492        SYN_RECV    -                    on (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57484        SYN_RECV    -                    on (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57488        SYN_RECV    -                    on (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57478        SYN_RECV    -                    on (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57482        SYN_RECV    -                    on (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57480        SYN_RECV    -                    on (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57486        SYN_RECV    -                    on (0.87/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57476        ESTABLISHED -                    off (0.00/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57474        ESTABLISHED -                    off (0.00/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57490        ESTABLISHED -                    off (0.00/0/0)

过了一会，连接超时会自动关闭：

tcp        3      0 0.0.0.0:3000            0.0.0.0:*               LISTEN      65667/./server.out   off (0.00/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57476        ESTABLISHED -                    off (0.00/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57474        ESTABLISHED -                    off (0.00/0/0)
    tcp        0      0 10.0.4.129:3000         10.0.4.120:57490        ESTABLISHED -                    off (0.00/0/0)

## 探究Netty中SO\_BACKLOG与listen函数中backlog的关系 ##

package com.morris.netty.basic;
    
    import io.netty.bootstrap.ServerBootstrap;
    import io.netty.buffer.ByteBuf;
    import io.netty.buffer.Unpooled;
    import io.netty.channel.*;
    import io.netty.channel.nio.NioEventLoopGroup;
    import io.netty.channel.socket.SocketChannel;
    import io.netty.channel.socket.nio.NioServerSocketChannel;
    
    public class NettyServer { 
    
        public static final int PORT = 8899;
    
        public static void main(String[] args) throws InterruptedException { 
    
            EventLoopGroup bossGroup = new NioEventLoopGroup(); // 负责接收客户端连接
            EventLoopGroup workerGroup = new NioEventLoopGroup(3); // 负责读取和发送数据
    
            try { 
                ServerBootstrap b = new ServerBootstrap();
                b.group(bossGroup, workerGroup)
                        .channel(NioServerSocketChannel.class)
                        .option(ChannelOption.SO_BACKLOG, 8)
                        .childHandler(new ChannelInitializer<SocketChannel>() { 
                            @Override
                            public void initChannel(SocketChannel ch) { 
                                ch.pipeline().addLast(new SimpleChannelInboundHandler() { 
                                    @Override
                                    protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception { 
                                        ByteBuf receiveByteBuf = (ByteBuf) msg;
                                        byte[] bytes = new byte[receiveByteBuf.readableBytes()];
                                        receiveByteBuf.readBytes(bytes);
    
                                        System.out.println("receive from client: " + new String(bytes));
    
                                        ByteBuf sendByteBuf = Unpooled.copiedBuffer("hello client".getBytes());
                                        ctx.writeAndFlush(sendByteBuf);
                                        ctx.close();
                                    }
                                });
                            }
                        });
    
                // 启动 server.
                ChannelFuture f = b.bind(PORT).sync();
    
                System.out.println("server is start on port: " + PORT);
    
                // 等待socket关闭
                f.channel().closeFuture().sync();
            } finally { 
                workerGroup.shutdownGracefully();
                bossGroup.shutdownGracefully();
            }
        }
    
    }

使用strace命令根据程序所产生的系统调用：

# javac -cp ".:/root/java/netty/libs/dom4j-1.6.1.jar:/root/java/netty/libs/hamcrest-core-1.3.jar:/root/java/netty/libs/javassist-3.18.1-GA.jar:/root/java/netty/libs/jboss-marshalling-2.0.6.Final.jar:/root/java/netty/libs/jboss-marshalling-serial-2.0.6.Final.jar:/root/java/netty/libs/jdom-1.1.3.jar:/root/java/netty/libs/jibx-extras-1.3.1.jar:/root/java/netty/libs/jibx-run-1.3.1.jar:/root/java/netty/libs/joda-time-2.9.5.jar:/root/java/netty/libs/json-simple-1.1.1.jar:/root/java/netty/libs/junit-4.13.jar:/root/java/netty/libs/log4j-1.2.17.jar:/root/java/netty/libs/lombok-1.18.2.jar:/root/java/netty/libs/msgpack-0.6.12.jar:/root/java/netty/libs/netty-all-4.1.51.Final.jar:/root/java/netty/libs/protobuf-java-3.6.1.jar:/root/java/netty/libs/slf4j-api-1.7.30.jar:/root/java/netty/libs/slf4j-log4j12-1.7.30.jar:/root/java/netty/libs/xml-apis-1.0.b2.jar:/root/java/netty/libs/xpp3-1.1.3.4.O.jar" NettyServer.java 
    # strace -ff -o out java -cp ".:/root/java/netty/libs/dom4j-1.6.1.jar:/root/java/netty/libs/hamcrest-core-1.3.jar:/root/java/netty/libs/javassist-3.18.1-GA.jar:/root/java/netty/libs/jboss-marshalling-2.0.6.Final.jar:/root/java/netty/libs/jboss-marshalling-serial-2.0.6.Final.jar:/root/java/netty/libs/jdom-1.1.3.jar:/root/java/netty/libs/jibx-extras-1.3.1.jar:/root/java/netty/libs/jibx-run-1.3.1.jar:/root/java/netty/libs/joda-time-2.9.5.jar:/root/java/netty/libs/json-simple-1.1.1.jar:/root/java/netty/libs/junit-4.13.jar:/root/java/netty/libs/log4j-1.2.17.jar:/root/java/netty/libs/lombok-1.18.2.jar:/root/java/netty/libs/msgpack-0.6.12.jar:/root/java/netty/libs/netty-all-4.1.51.Final.jar:/root/java/netty/libs/protobuf-java-3.6.1.jar:/root/java/netty/libs/slf4j-api-1.7.30.jar:/root/java/netty/libs/slf4j-log4j12-1.7.30.jar:/root/java/netty/libs/xml-apis-1.0.b2.jar:/root/java/netty/libs/xpp3-1.1.3.4.O.jar" NettyServer

在out.69240文件中有下面这么两行：

bind(45, { sa_family=AF_INET6, sin6_port=htons(8899), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, 28) = 0
    listen(45, 8)

可见在Netty中设置的ChannelOption.SO\_BACKLOG其实就是listen的第二个参数backlog。

## 修改内核参数 ##

永久修改：直接修改配置文件`/etc/sysctl.conf`。

临时修改：使用`sysctl`命令，系统重启或者网络重启（systemctl retart network）了，设置就会失效。

sysctl的使用：

*  \-a：显示所有的系统参数。
 *  \-w：临时修改某个指定参数的值，如`net.ipv4.tcp_syncookies = 1`。

例如修改TCP接收缓冲区的大小：

在netty中是这么设置的：

.option(ChannelOption.SO_SNDBUF, 1024)

实际上底层的系统调用为：

setsockopt(45, SOL_SOCKET, SO_RCVBUF, [1024], 4) = 0

当然也可以全局配置整个操作系统中所有TCP的接收缓冲区的大小：

# sysctl -r net.ipv4.tcp_rmem=1024 4096 65535

查看操作系统默认的TCP的接收缓冲区的大小：

# cat /proc/sys/net/ipv4/tcp_rmem
    4096	87380	6291456

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