listen()函数中backlog参数分析
背景知识
Unix网络编程描述如下:
总结
0. accept()函数不参与三次握手,而只负责从已建立连接队列中取出一个连接和sockfd进行绑定;
1. backlog参数决定了未完成队列和已完成队列中连接数目之和的最大值(从内核角度看,是否这个和就是等于sock->recv_queue ?);
2. accept()函数调用,会从已连接队列中取出一个“连接”(可以是一个描述连接的数据结构,listensocket->sock->recv_queue[sk_buff] ? ),未完成队列和已完成队列中连接数目 之和将减少1;即accept将监听套接字对应的sock的接收队列中的已建立连接的sk_buff取下(从该sk_buff中可以获得对端主机的发送过来的tcp/ip数据包)
3. 监听套接字的已完成队列中的元素个数大于0,那么该套接字是可读的。
4. 当程序调用accept的时候(设置阻塞参数),那么判定该套接字是否可读,不可读则进入睡眠,直至已完成队列中的元素个数大于0(监听套接字可读)而唤起监听进程。
实例分析1
将服务器端的listen函数backlog设置为2,用20个客户端与服务器建立连接,查看连接的建立情况。
服务器代码:
- #include <stdio.h>
- #include<unistd.h>
- #include<sys/types.h> /* basic system data types */
- #include<sys/socket.h> /* basic socket definitions */
- #include<netinet/in.h> /* sockaddr_in{} and other Internet defns */
- #include<arpa/inet.h> /* inet(3) functions */
- #include<sys/epoll.h> /* epoll function */
- #include<fcntl.h>
- #include<stdlib.h>
- #include<errno.h>
- #include<stdio.h>
- #include<string.h>
- int main(int argc,char*argv[])
- {
- int listenfd,connfd;
- struct sockaddr_in cliaddr,servaddr;
- int queuelen=5;
- if(argc!=2){
- puts("usage# ./aworker listenqueuelen");
- exit(0);
- }
- queuelen=atoi(argv[1]);
- listenfd = socket(AF_INET,SOCK_STREAM,0);
- bzero(&servaddr,sizeof(servaddr));
- servaddr.sin_family = AF_INET;
- servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
- servaddr.sin_port = htons(2989);
- bind(listenfd,(struct sockaddr*)&servaddr,sizeof(servaddr));
- listen(listenfd,queuelen);
- sleep(60); //将这个注释,会出现另一种情况哟~~
- while(1)
- {
- connfd = accept(listenfd,NULL,0);
- if(connfd == -1)
- {
- perror("accept error");
- continue;
- }
- puts("new connection...");
- }
- return 0;
- }
client代码
- #include "client.h"
- //void cli_hander(int sockfd,)
- int main()
- {
- int sockfd;
- int rc;
- int cpid;
- struct sockaddr_in servaddr;
- bzero(&servaddr,sizeof(servaddr));
- servaddr.sin_family = AF_INET;
- inet_pton(AF_INET,"127.0.0.1",&servaddr.sin_addr);
- servaddr.sin_port = htons(2989);
- for(int i=0;i<20;i++)
- {
- cpid = fork();
- if(cpid == 0)
- {
- sockfd = socket(AF_INET,SOCK_STREAM,0);
- rc = connect(sockfd,(struct sockaddr*)&servaddr,sizeof(servaddr));
- if(rc == -1)
- {
- perror("connect error");
- exit(0);
- }
- printf("pid#%d connected...\n",getpid());
- sleep(3);
- close(sockfd);
- exit(0);
- }
- }
- while(1)
- {
- cpid = wait(NULL);
- if(cpid==-1){
- perror("end of wait");
- break;
- }
- printf("pid#%d exit...\n",cpid);
- }
- return 0;
- }
实验结果:
服务器端显示:
- [email protected]:~/slp/NetWrokProgram/server# ./aworker 2
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
客户端显示:
- [email protected]:~/slp/NetWrokProgram/client# ./a.out
- pid#16697 connected...
- pid#16699 connected...
- pid#16698 connected...
- pid#16697 exit...
- pid#16699 exit...
- pid#16698 exit...
- pid#16700 connected...
- pid#16701 connected...
- pid#16700 exit...
- pid#16701 exit...
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- connect error: Connection timed out
- pid#16702 exit...
- pid#16703 exit...
- pid#16704 exit...
- pid#16705 exit...
- pid#16706 exit...
- pid#16707 exit...
- pid#16708 exit...
- pid#16709 exit...
- pid#16710 exit...
- pid#16711 exit...
- pid#16712 exit...
- pid#16713 exit...
- pid#16714 exit...
- pid#16715 exit...
- pid#16716 exit...
- end of wait: No child processes
结果分析:
同时建立连接的客户端进程共有20个,可是只有5个完成了连接的建立,其他15个没有成功。有趣的是,建立的5个链接中有3个是马上建立的,2个是过了一段时间后后来才建立的。
实例分析2
将server端的代码中的sleep(60)注释,即服务端listen即开始进入while循环中的accept阻塞:
- ...
- listen(listenfd,queuelen);
- sleep(60); //将这个注释,会出现另一种情况哟~~
- while(1)
- {
- connfd = accept(listenfd,NULL,0);
- ....
同样的运行,结果如下:
- [email protected]:~/slp/NetWrokProgram/server# ./aworker 2
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
- new connection...
客户端:
- [email protected]:~/slp/NetWrokProgram/client# ./a.out
- pid#16736 connected...
- pid#16737 connected...
- pid#16738 connected...
- pid#16739 connected...
- pid#16740 connected...
- pid#16741 connected...
- pid#16742 connected...
- pid#16743 connected...
- pid#16744 connected...
- pid#16745 connected...
- pid#16746 connected...
- pid#16747 connected...
- pid#16748 connected...
- pid#16749 connected...
- pid#16750 connected...
- pid#16751 connected...
- pid#16752 connected...
- pid#16753 connected...
- pid#16755 connected...
- pid#16754 connected...
- pid#16736 exit...
- pid#16737 exit...
- pid#16738 exit...
- pid#16739 exit...
- pid#16740 exit...
- pid#16741 exit...
- pid#16742 exit...
- pid#16743 exit...
- pid#16744 exit...
- pid#16745 exit...
- pid#16746 exit...
- pid#16747 exit...
- pid#16748 exit...
- pid#16749 exit...
- pid#16750 exit...
- pid#16751 exit...
- pid#16752 exit...
- pid#16753 exit...
- pid#16755 exit...
- pid#16754 exit...
- end of wait: No child processes
结果分析:
由于每个连接在建立之后,已完成队列中的连接马上就被accept给读取了,所以已完成和未完成队列中的连接数之和根本不可能超过backlog限定的个数。
原文链接:
http://blog.****.net/ordeder/article/details/21551567