select()函数（I/O多路复用）-并发服务器的实现

注意select函数调用后对文件描述符集合的影响

服务器端

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#include <sys/select.h>
#include <sys/time.h>

//使用select函数实现TCP并发服务器

#define N 128

#define errlog(errmsg) do{perror(errmsg); exit(1);}while(0)

int main(int argc, const char *argv[])
{
    int sockfd;
    struct sockaddr_in serveraddr, clientaddr;
    int acceptfd;
    socklen_t addrlen = sizeof(struct sockaddr_in);
    fd_set readfds;
    int maxfd;

    fd_set tempfds;

    int i = 0;

    char buf[N] = {};

    //创建套接字
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        errlog("fail to socket");
    }

    //填充网络信息结构体
    //inet_addr 将点分十进制转化成网络字节
    //htons表示将主机字节序转化成网络字节序
    //atoi 将字符串转化成整型数据
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_addr.s_addr = inet_addr(argv[1]);
    serveraddr.sin_port = htons(atoi(argv[2]));

    //将套接字与IP地址和端口号绑定
    if(bind(sockfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) < 0)
    {
        errlog("fail to bind");
    }

    //将套接字设置为被动监听状态
    if(listen(sockfd, 10) < 0)
    {
        errlog("fail to listen");
    }

    //第一步：清空集合
    FD_ZERO(&readfds);

    //第二步：将文件描述符添加到集合当中
    //注意：当select函数调用成功后，他会清除没有准备就绪的文件描述符，所以需要每次重复添加
    FD_SET(sockfd, &readfds);

    maxfd = sockfd;


    while(1)
    {
        tempfds = readfds;

        //第三步：调用select函数将添加进去的文件描述符准备就绪
        if(select(maxfd + 1, &tempfds, NULL, NULL, NULL) < 0)   //此处若不用第三方tempfds代替的话，select执行后会将集合中没有准备就绪的
                                                            //文件描述符删除，若删除的话会将sockfd清除，那样的话
        {
            errlog("fail to select");
        }

        //使用FD_ISSET判断文件描述符
        for(i = 0; i < maxfd + 1; i++)
        {
            if(FD_ISSET(i, &tempfds) == 1)      //
            {
                if(i == sockfd)         
                {
                    //接收客户端的连接请求
                    if((acceptfd = accept(sockfd, (struct sockaddr *)&clientaddr, &addrlen)) < 0)
                    {
                        errlog("fail to accept");
                    } 

                    printf("%s ---> %d\n", inet_ntoa(clientaddr.sin_addr), ntohs(clientaddr.sin_port));

                    FD_SET(acceptfd, &readfds);

                    maxfd = maxfd > acceptfd ? maxfd : acceptfd;

                }
                else
                {

                    if(recv(i, buf, N, 0) < 0)
                    {
                        errlog("fail to recv");
                    }

                    if(strncmp(buf, "quit", 4) == 0)
                    {
                        printf("%s is quited...\n", inet_ntoa(clientaddr.sin_addr));
                        break;
                    }
                    else
                    {
                        printf("from client >>> %s\n", buf);

                        strcat(buf, " from server...");

                        send(i, buf, N, 0);

                    }

                }   
            }
        }
    }

    close(sockfd);
    close(acceptfd);
    return 0;
}

客户端

`#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <string.h>

#define N 128

#define errlog(errmsg) do{perror(errmsg); exit(1);}while(0)

int main(int argc, const char *argv[])
{
    int sockfd;
    struct sockaddr_in serveraddr;

    socklen_t addrlen = sizeof(struct sockaddr_in);

    char buf[N] = {};

    //创建套接字
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        errlog("fail to socket");
    }

    //填充网络信息结构体
    //inet_addr 将点分十进制转化成网络字节
    //htons表示将主机字节序转化成网络字节序
    //atoi 将字符串转化成整型数据
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_addr.s_addr = inet_addr(argv[1]);
    serveraddr.sin_port = htons(atoi(argv[2]));

    //向服务器发送连接请求
    if(connect(sockfd, (struct sockaddr *)&serveraddr, addrlen) < 0)
    {
        errlog("fail to accept");
    }

    //与服务器进行通信
    while(1)
    {
        fgets(buf, N, stdin);
        buf[strlen(buf) - 1] = '\0';

        if(send(sockfd, buf, N, 0) < 0)
        {
            errlog("fail to send");
        }

        if(strncmp(buf, "quit", 4) == 0)
        {
            break;
        }

        if(recv(sockfd, buf, N, 0) < 0)
        {
            errlog("fail to recv");
        }
        printf("%s\n", buf);

    }

    close(sockfd);
    return 0;
}