listen()函数中backlog参数分析

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=;  

    if(argc!=){
        puts("usage# ./aworker listenqueuelen");
        exit();
    }
    queuelen=atoi(argv[]);  

    listenfd = socket(AF_INET,SOCK_STREAM,);  

    bzero(&servaddr,sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons();  

    bind(listenfd,(struct sockaddr*)&servaddr,sizeof(servaddr));  

    listen(listenfd,queuelen);
    sleep(); //将这个注释，会出现另一种情况哟~~
    while()
    {
        connfd = accept(listenfd,NULL,);
        if(connfd == -)
        {
            perror("accept error");
            continue;
        }
        puts("new connection...");
    }
    return ;
}  

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();  

    for(int i=;i<;i++)
    {
        cpid = fork();
        if(cpid == )
        {
            sockfd = socket(AF_INET,SOCK_STREAM,);
            rc = connect(sockfd,(struct sockaddr*)&servaddr,sizeof(servaddr));
            if(rc == -)
            {
                perror("connect error");
                exit();
            }
            printf("pid#%d connected...\n",getpid());
            sleep();
            close(sockfd);
            exit();
        }
    }     

    while()
    {
        cpid = wait(NULL);
        if(cpid==-){
            perror("end of wait");
            break;
        }
        printf("pid#%d exit...\n",cpid);
    }
    return ;
}  

实验结果

服务器端显示：

root@cloud2:~/slp/NetWrokProgram/server# ./aworker
new connection...
new connection...
new connection...
new connection...
new connection... 

客户端显示：

root@cloud2:~/slp/NetWrokProgram/client# ./a.out
pid# connected...
pid# connected...
pid# connected...
pid# exit...
pid# exit...
pid# exit...
pid# connected...
pid# connected...
pid# exit...
pid# 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# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# 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(); //将这个注释，会出现另一种情况哟~~
while()
{
    connfd = accept(listenfd,NULL,);
    ....  

同样的运行服务端结果如下：

root@cloud2:~/slp/NetWrokProgram/server# ./aworker
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

root@cloud2:~/slp/NetWrokProgram/client# ./a.out
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# connected...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# exit...
pid# 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。

转自：http://blog.csdn.net/ordeder/article/details/21551567