多路IO转接（多路IO复用）
内核监听多个文件描述符的属性（读写缓冲区）变化，如果变化，将事件告知给应用层。
常用的3种复用技术：select，poll，epoll
#include <stdio.h>
#include <iostream>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <cstring>
#include <arpa/inet.h>
#include <unistd.h>
#include <netinet/in.h>
#define PORT 8001
using namespace std;
int main(int argc, char *argv[])
{int lfd = socket(AF_INET, SOCK_STREAM, 0);if (lfd < 0){cout << "chen" << endl;return 1;}cout << "socket success" << endl;struct sockaddr_in local;local.sin_family = AF_INET;local.sin_port = htons(PORT);local.sin_addr.s_addr = 0;int ans = bind(lfd, (struct sockaddr *)&local, sizeof(local));if (ans < 0){cout << "biao" << endl;return 1;}cout << "bind success" << endl;ans = listen(lfd, 128);if (ans < 0){cout << "ren" << endl;return 1;}cout << "listen success" << endl;int maxfd = lfd; // 最大的文件描述符fd_set oldset, rset;FD_ZERO(&oldset);FD_ZERO(&rset);// 将lfd添加oldset集合中FD_SET(lfd, &oldset);while (1){rset = oldset; // 将oldset赋值给需要监听的rset集合int n = select(maxfd + 1, &rset, NULL, NULL, NULL);if (n < 0){perror("错误!");break;}else if (n == 0){continue; // 没有变化 继续监听}else // 监听到了文件描述符的变化{// lfd变化 代表有新的连接到来if (FD_ISSET(lfd, &rset)){struct sockaddr_in cliaddr;socklen_t len = sizeof(cliaddr);char ip[16] = "";// 提取新的连接int cfd = accept(lfd, (struct sockaddr *)&cliaddr, &len);printf("new client ip = %s port = %d \n", inet_ntop(AF_INET, &(cliaddr.sin_addr.s_addr), ip, 16), ntohs(cliaddr.sin_port));// 将cfd添加至oldset集合中,以下次监听FD_SET(cfd, &oldset);if (cfd > maxfd)maxfd = cfd;// 如果只有lfd变化 continueif (--n == 0)continue;}// cfd 遍历lfd之后的文件描述符是否在rset集合中,如果在则cfd变化for (int i = lfd + 1; i <= maxfd; i++){if (FD_ISSET(i, &rset)){char buffer[1500];int ret = read(i, buffer, sizeof(buffer));if (ret < 0){perror("read 错误!");close(i);FD_CLR(i, &oldset);}else if (ret == 0){printf("client close\n");close(i);FD_CLR(i, &oldset);}else{buffer[ret] = 0;printf("读到的内容:%s", buffer);write(i, buffer, ret);}}}}}return 0;
}
