用select实现服务器并发：

linux@linux:~/study/1001$ cat server.c 
#include <myhead.h>
 
#define ERR_MSG(msg) do{\
	printf("%d\n",__LINE__);\
	perror(msg);\
}while(0)
 
#define PORT 8880
 
#define IP "192.168.31.38"
 
int main(int argc, const char *argv[])
{
	//创建流式套接字 socket
	int sfd = socket(AF_INET,SOCK_STREAM,0);
	if(sfd == -1){
		ERR_MSG("socket");
		return -1;
	}
	printf("socket create success! sfd = %d\n",sfd);
 
	//允许端口快速的被复用
	int reuse = 1;                                                            
	if(setsockopt(sfd, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse)) < 0)
	{
		ERR_MSG("setsockopt");
		return -1;
	}
	printf("允许端口快速的被复用成功\n");
 
	//绑定服务器的地址信息 --->必须绑定 bind
	struct sockaddr_in sin;
	sin.sin_family = AF_INET;
	sin.sin_port = htons(PORT);
	sin.sin_addr.s_addr = inet_addr(IP);
	if(bind(sfd,(struct sockaddr*)&sin,sizeof(sin)) == -1){
		ERR_MSG("bind");
		return -1;
	}
	printf("bind success!\n");
	//将套接字设置为被动监听状态 listen
	if(listen(sfd,128) == -1){
		ERR_MSG("listen");
		return -1;
	}
	printf("listen success!\n");
	fd_set readfds,tempfds;
	//清空集合
	FD_ZERO(&readfds);
 
	//添加文件描述符
	FD_SET(0,&readfds);
	FD_SET(sfd,&readfds);
 
	struct sockaddr_in cin;
	socklen_t addrlen = sizeof(cin);
	int newfd = -1;
	char buf[32];
	ssize_t res = 0;
	int s_res = -1;
	int maxfd = sfd;
	int sndfd = -1;
	struct sockaddr_in save[1024];
	while(1){
		tempfds = readfds;
		maxfd = maxfd>newfd?maxfd:newfd;
		s_res = select(maxfd+1,&tempfds,NULL,NULL,NULL);
		if(s_res == -1){
			ERR_MSG("select");
			return -1;
		}else if(s_res == 0){
			printf("time out\n");
			break;
		}
		for(int i = 0;i <= maxfd;i++){
			if(FD_ISSET(i,&tempfds) == 0)
				continue;
			if(0 == i){
				printf("触发键盘输入事件\n");
				bzero(buf,sizeof(buf));
				int ret = scanf("%d %s",&sndfd,buf);
				if(ret != 2){
					printf("您的输入格式有误：sndfd buf\n");
					return -1;
				}
				if(sndfd <= 2 || FD_ISSET(sndfd,&readfds) == 0){
					printf("您输入的文件描述符不合法 snfd = %d\n",sndfd);
					return -1;
				}
				//发送
				if(send(sndfd,buf,sizeof(buf),0) == -1){
					ERR_MSG("send");
					return -1;
				}
				printf("send success!\n");
 
			}else if(sfd == i){
				printf("触发客户端连接事件\n");
				//获取一个已经完成的客户端信息，生成一个新的文件描述符 accept
				newfd = accept(sfd,(struct sockaddr*)&cin,&addrlen); 
				if(newfd == -1){
					ERR_MSG("accept");
					return -1;
				}
				printf("[%s:%d]客户端链接成功! newfd = %d\n",inet_ntoa(cin.sin_addr),ntohs(cin.sin_port),newfd);
				save[newfd] = cin;
				FD_SET(newfd,&readfds);
 
			}else{
				printf("触发接收客户端数据事件\n");
				//接收
				bzero(buf,sizeof(buf));
				res = recv(i,buf,sizeof(buf),0);
				if(res == -1){
					ERR_MSG("recv");
					return -1;
				}else if(res == 0){
					printf("[%s:%d]客户端已下线! newfd = %d\n",inet_ntoa(save[i].sin_addr),ntohs(save[i].sin_port),i);
					close(i);
					FD_CLR(i,&readfds);
					while(FD_ISSET(maxfd,&readfds) == 0 && maxfd-->=0);
					continue;
				}
				printf("[%s:%d] newfd = %d buf = %s \n",inet_ntoa(save[i].sin_addr),ntohs(save[i].sin_port),i,buf);
 
				//发送
				if(send(i,buf,sizeof(buf),0) == -1){
					ERR_MSG("send");
					return -1;
				}
				printf("send success!\n");
			}
		}
	}
	//关闭文件描述符
	if(close(sfd)<0){
		ERR_MSG("close");
		return -1;
	}
	if(close(newfd) < 0){
		ERR_MSG("close");
		return -1;
	}
	return 0;
}

运行结果：