使用socket编程(一) 中的主程序，存在以下两个问题：

    1）. 有时候重启会出错，提示“bind error: Address already in use”。解决此问题用到setsockopt(）函数；

       2）.  同时只能处理单用户请求；

下面进一步完善”socket编程(一)“的主程序，使用它能同时处理多个客户端请求。

主要技巧是使用fork()产生子进程，让子进程去处理客户端请求。

#include "../apue.h"

int main(void){         int listenfd;         listenfd=socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);         if(listenfd < 0)                 err_exit("listen error");         struct sockaddr_in servaddr;         memset(&servaddr, 0, sizeof(servaddr));         servaddr.sin_family = AF_INET;         servaddr.sin_port = htons(5188);         servaddr.sin_addr.s_addr = htonl(INADDR_ANY);         //servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");         //inet_aton("127.0.0.1", $servaddr.sin_addr);         int on=1;         if(setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on))<0)                 err_exit("setsockopt error");         if(bind(listenfd, (struct sockaddr*)&servaddr, sizeof(servaddr))<0)                 err_exit("bind error");         if(listen(listenfd, SOMAXCONN)<0)                 err_exit("listen error");         struct sockaddr_in peeraddr;         socklen_t peerlen = sizeof(peeraddr);         int conn;         pid_t pid;         while (1){                 if((conn = accept(listenfd, (struct sockaddr*)&peeraddr, &peerlen))<0)                         err_exit("accept error");                 printf("ip=%s,port=%d\n", inet_ntoa(peeraddr.sin_addr), peeraddr.sin_port);                 pid = fork();                 if(pid==-1)                         err_exit("fork error");                 if(pid==0){                         close(listenfd);                         while(1){                         char buf[1024];                         int ret;                     memset(buf, 0, sizeof(buf));                     ret = read(conn, buf, sizeof(buf));                                 if(ret==0){                                         printf("client close\n");                                         break;                                 }                                 else if(ret==-1)                                         err_exit("ret error");                     fputs(buf, stdout);                     write(conn, buf, ret);                                 }                                 exit(EXIT_SUCCESS);                 }else{                         close(conn);                 }         }         return 0; }