selectIO多路复用并发服务器，是通过轮询检测文件描述符来实现并发

将内核要检测文件描述符放入集合中，调用select函数，通知内核区检测文件描述符集合中的文件描述符是否准备就绪，即对应的空间中是否有数据

对准备就绪的文件描述符指向对应的IO操作，让再循环通过select监测

#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>#define ERR_MSG(msg) do{\fprintf(stderr, "line:%d ", __LINE__);\perror(msg);\
}while(0)#define IP "192.168.114.134"   //本机IP ifconfig
#define PORT 8899           //1024-49151int main(int argc, const char *argv[])
{//创建流式套接字int sfd = socket(PF_INET, SOCK_STREAM, 0);if(sfd < 0){ERR_MSG("socket");return -1;}printf("sfd = %d\n", sfd);//允许端口快速重用int reuse = 1;if(setsockopt(sfd, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse)) < 0){ERR_MSG("setsockopt");return -1;}printf("允许端口快速重用成功\n");//填充地址信息结构体，//真实的地址信息结构体根据地址族指定 AF_INET:man 7 ipstruct sockaddr_in sin;sin.sin_family      = AF_INET;          //必须填AF_INET;sin.sin_port        = htons(PORT) ;     //端口号，1024~49151sin.sin_addr.s_addr = inet_addr(IP);    //本机IP， ifconfig//将IP和端口绑定到套接字上;if(bind(sfd, (struct sockaddr*)&sin, sizeof(sin)) < 0){ERR_MSG("bind");return -1;}printf("bind success __%d__\n", __LINE__);//将套接字设置为被动监听状态，监听是否有客户端连接成功;if(listen(sfd, 128) < 0){ERR_MSG("listen");return -1;}printf("listen success __%d__\n", __LINE__);//创建select队列结构体fd_set readfds , tempfds;FD_ZERO(&readfds);FD_SET(sfd,&readfds);FD_SET(0,&readfds);int maxfd = sfd;int sel = 0;struct sockaddr_in cin;     //存储连接成功的客户端地址信息socklen_t addrlen = sizeof(cin);char buf[1024] = "";ssize_t res = 0;   //读取到的个数//用来存储对应用户的地址信息结构体struct sockaddr_in client_arr[1020];//循环监听文件描述符是否启动while (1){tempfds = readfds;sel = select(maxfd+1,&tempfds,NULL,NULL,NULL);if(sel<0){ERR_MSG("select");return -1;}else if (sel == 0){printf("超时\n");return -1;}for (int i = 0;i<=maxfd;i++){if (FD_ISSET(i,&tempfds)){if (i == 0){int send_fd = -1;  //要发给那个用户对应的文件描述符int res_sc = 0;printf("终端有数据写入\n");	//不将缓存区中的内容取出//select会一直监听到该文件描述准备就绪res_sc = scanf("%d %s",&send_fd,buf);if (res_sc < 2){fprintf(stderr,"请输入正确的格式（int string ）\n");continue;}if (send_fd < sfd || send_fd > 1024 || FD_ISSET(send_fd,&readfds)){fprintf(stderr,"文件描述符错误\n");continue;}if(send(send_fd,buf,sizeof(buf), 0) < 0){ERR_MSG("send");return -1;}}else if (sfd == i){//阻塞函数，从已完成连接的队列头中获取一个客户端信息，生成一个新的文件描述符，//该文件描述符才是与客户端通信的文件描述符！！！int newfd = accept(sfd, (struct sockaddr*)&cin, &addrlen);if(newfd < 0){ERR_MSG("accept");return -1;}printf("[%s:%d] newfd=%d 连接成功__%d__\n", \inet_ntoa(cin.sin_addr), ntohs(cin.sin_port), newfd, __LINE__);client_arr[newfd-4] = cin;FD_SET(newfd,&readfds);maxfd = maxfd>newfd?maxfd:newfd;}else{bzero(buf, sizeof(buf));//接收res = recv(i, buf, sizeof(buf), 0);if(res < 0){ERR_MSG("recv");return -1;}else if(0 == res){printf("[%s:%d] newfd=%d 客户端下线__%d__\n", \inet_ntoa(client_arr[i-4].sin_addr), ntohs(client_arr[i-4].sin_port), i, __LINE__);close(i);FD_CLR(i,&readfds);maxfd = maxfd==i?maxfd-1:maxfd;continue;}printf("[%s:%d] newfd=%d : %s __%d__\n", \inet_ntoa(client_arr[i-4].sin_addr), ntohs(client_arr[i-4].sin_port), i, buf, __LINE__);strcpy(buf,"收到");if(send(i, buf, strlen(buf), 0) < 0){ERR_MSG("send");return -1;}printf("发送成功\n");}}}}/*while(1){bzero(buf, sizeof(buf));bzero(send_b, sizeof(send_b));//接收res = recv(newfd, buf, sizeof(buf), 0);if(res < 0){ERR_MSG("recv");return -1;}else if(0 == res){printf("[%s:%d] newfd=%d 客户端下线__%d__\n", \inet_ntoa(cin.sin_addr), ntohs(cin.sin_port), newfd, __LINE__);break;}printf("[%s:%d] newfd=%d : %s __%d__\n", \inet_ntoa(cin.sin_addr), ntohs(cin.sin_port), newfd, buf, __LINE__);//发送 将数据拼接字符串后发送回去fprintf(stdout,"回答的内容(不要超过1024字节)>>");fscanf(stdin,"%s",send_b);if(send(newfd, send_b, sizeof(send_b), 0) < 0){ERR_MSG("send");return -1;}printf("发送成功\n");}
*/close(sfd);return 0;
}