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：
TCP中有如下两个队列：
- 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。
- 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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