#include <sys/select.h>
/* According to earlier standards */
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int nfds, fd_set *readfds, fd_set *writefds,fd_set *exceptfds, struct timeval *timeout);nfds:         监控的文件描述符集里最大文件描述符加1，因为此参数会告诉内核检测前多少个文件描述符的状态readfds：    监控有读数据到达文件描述符集合，传入传出参数writefds：    监控写数据到达文件描述符集合，传入传出参数exceptfds：    监控异常发生达文件描述符集合,如带外数据到达异常，传入传出参数timeout：    定时阻塞监控时间，3种情况1.NULL，永远等下去2.设置timeval，等待固定时间3.设置timeval里时间均为0，检查描述字后立即返回，轮询struct timeval {long tv_sec; /* seconds */long tv_usec; /* microseconds */};void FD_CLR(int fd, fd_set *set);     //把文件描述符集合里fd清0int FD_ISSET(int fd, fd_set *set);     //测试文件描述符集合里fd是否置1void FD_SET(int fd, fd_set *set);     //把文件描述符集合里fd位置1void FD_ZERO(fd_set *set);             //把文件描述符集合里所有位清0

/* server.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include "wrap.h"#define MAXLINE 80
#define SERV_PORT 6666int main(int argc, char *argv[])
{int i, maxi, maxfd, listenfd, connfd, sockfd;int nready, client[FD_SETSIZE];     /* FD_SETSIZE 默认为 1024 */ssize_t n;fd_set rset, allset;char buf[MAXLINE];char str[INET_ADDRSTRLEN];             /* #define INET_ADDRSTRLEN 16 */socklen_t cliaddr_len;struct sockaddr_in cliaddr, servaddr;listenfd = Socket(AF_INET, SOCK_STREAM, 0);bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(SERV_PORT);Bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr));Listen(listenfd, 20);         /* 默认最大128 */maxfd = listenfd;             /* 初始化 */
maxi = -1;                    /* client[]的下标 */for (i = 0; i < FD_SETSIZE; i++)client[i] = -1;         /* 用-1初始化client[] */FD_ZERO(&allset);
FD_SET(listenfd, &allset); /* 构造select监控文件描述符集 */for ( ; ; ) {rset = allset;             /* 每次循环时都从新设置select监控信号集 */nready = select(maxfd+1, &rset, NULL, NULL, NULL);if (nready < 0)perr_exit("select error");if (FD_ISSET(listenfd, &rset)) { /* new client connection */cliaddr_len = sizeof(cliaddr);connfd = Accept(listenfd, (struct sockaddr *)&cliaddr, &cliaddr_len);printf("received from %s at PORT %d\n",inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),ntohs(cliaddr.sin_port));for (i = 0; i < FD_SETSIZE; i++) {if (client[i] < 0) {client[i] = connfd; /* 保存accept返回的文件描述符到client[]里 */break;}}/* 达到select能监控的文件个数上限 1024 */if (i == FD_SETSIZE) {fputs("too many clients\n", stderr);exit(1);}FD_SET(connfd, &allset);     /* 添加一个新的文件描述符到监控信号集里 */if (connfd > maxfd)maxfd = connfd;         /* select第一个参数需要 */if (i > maxi)maxi = i;                 /* 更新client[]最大下标值 */if (--nready == 0)continue;                 /* 如果没有更多的就绪文件描述符继续回到上面select阻塞监听,负责处理未处理完的就绪文件描述符 */}for (i = 0; i <= maxi; i++) {     /* 检测哪个clients 有数据就绪 */if ( (sockfd = client[i]) < 0)continue;if (FD_ISSET(sockfd, &rset)) {if ( (n = Read(sockfd, buf, MAXLINE)) == 0) {Close(sockfd);        /* 当client关闭链接时，服务器端也关闭对应链接 */FD_CLR(sockfd, &allset); /* 解除select监控此文件描述符 */client[i] = -1;} else {int j;for (j = 0; j < n; j++)buf[j] = toupper(buf[j]);Write(sockfd, buf, n);}if (--nready == 0)break;}}}close(listenfd);return 0;
}

/* client.c */
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>
#include "wrap.h"#define MAXLINE 80
#define SERV_PORT 6666int main(int argc, char *argv[])
{struct sockaddr_in servaddr;char buf[MAXLINE];int sockfd, n;sockfd = Socket(AF_INET, SOCK_STREAM, 0);bzero(&servaddr, sizeof(servaddr));servaddr.sin_family = AF_INET;inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);servaddr.sin_port = htons(SERV_PORT);Connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));while (fgets(buf, MAXLINE, stdin) != NULL) {Write(sockfd, buf, strlen(buf));n = Read(sockfd, buf, MAXLINE);if (n == 0)printf("the other side has been closed.\n");elseWrite(STDOUT_FILENO, buf, n);}Close(sockfd);return 0;
}