Linux下的單進程多用戶TCP服務器，採用select方法實現。

/*************************************************
* File name   : server.c
* Description : 單進程併發服務器
* Author      : [email protected]
* Version     : V1.0
* Date        :
* Compiler    : arm-linux-gcc-4.4.3
* Target      : mini2440(Linux-2.6.32)
* History     :
*   <author>  <time>   <version >   <desc>
*************************************************/
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/time.h>
#include <stdlib.h>

#define PORT 1234               //服務器端口
#define BACKLOG 5               //listen隊列中等待的連接數
#define MAXDATASIZE 1024        //緩衝區大小

typedef struct _CLIENT
{
    int fd;                     //客戶端socket描述符
    char name[20];              //客戶端名稱
    struct sockaddr_in addr;    //客戶端地址信息結構體
    char data[MAXDATASIZE];     //客戶端私有數據指針
} CLIENT;

void process_client(CLIENT * client, char *recvbuf, int len);   //客戶請求處理函數

/*************************************************
* Function    : main()
* Description :
* Calls       : process_client()
* Called By   :
* Input       :
* Output      :
* Return      :
*************************************************/
void main(int argc ,char **argv)
{
    int i, maxi, maxfd, sockfd;
    int nready;
    ssize_t n;
    fd_set rset, allset;        //select所需的文件描述符集合
    int listenfd, connectfd;    //socket文件描述符
    struct sockaddr_in server;  //服務器地址信息結構體

    CLIENT client[FD_SETSIZE];  //FD_SETSIZE爲select函數支持的最大描述符個數
    char recvbuf[MAXDATASIZE];  //緩衝區
    int sin_size;               //地址信息結構體大小

    if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {                           //調用socket創建用於監聽客戶端的socket
        perror("Creating socket failed.");
        exit(1);
    }

    int opt = SO_REUSEADDR;
    setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));  //設置socket屬性

    bzero(&server, sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(PORT);
    server.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(listenfd, (struct sockaddr *)&server, sizeof(struct sockaddr)) == -1)
    {                           //調用bind綁定地址
        perror("Bind error.");
        exit(1);
    }

    if (listen(listenfd, BACKLOG) == -1)
    {                           //調用listen開始監聽
        perror("listen() error\n");
        exit(1);
    }

    //初始化select
    maxfd = listenfd;
    maxi = -1;
    for (i = 0; i < FD_SETSIZE; i++)
    {
        client[i].fd = -1;
    }
    FD_ZERO(&allset);           //清空
    FD_SET(listenfd, &allset);  //將監聽socket加入select檢測的描述符集合

    while (1)
    {
        struct sockaddr_in addr;
        rset = allset;
        nready = select(maxfd + 1, &rset, NULL, NULL, NULL);    //調用select
        printf("Select() break and the return num is %d. \n", nready);

        if (FD_ISSET(listenfd, &rset))
        {                       //檢測是否有新客戶端請求
            printf("Accept a connection.\n");
            //調用accept，返回服務器與客戶端連接的socket描述符
            sin_size = sizeof(struct sockaddr_in);
            if ((connectfd =
                 accept(listenfd, (struct sockaddr *)&addr, (socklen_t *) & sin_size)) == -1)
            {
                perror("Accept() error\n");
                continue;
            }

            //將新客戶端的加入數組
            for (i = 0; i < FD_SETSIZE; i++)
            {
                if (client[i].fd < 0)
                {
                    char buffer[20];
                    client[i].fd = connectfd;   //保存客戶端描述符
                    memset(buffer, '0', sizeof(buffer));
                    sprintf(buffer, "Client[%.2d]", i);
                    memcpy(client[i].name, buffer, strlen(buffer));
                    client[i].addr = addr;
                    memset(buffer, '0', sizeof(buffer));
                    sprintf(buffer, "Only For Test!");
                    memcpy(client[i].data, buffer, strlen(buffer));
                    printf("You got a connection from %s:%d.\n", inet_ntoa(client[i].addr.sin_addr),ntohs(client[i].addr.sin_port));
                    printf("Add a new connection:%s\n",client[i].name);
                    break;
                }
            }

            if (i == FD_SETSIZE)
                printf("Too many clients\n");
            FD_SET(connectfd, &allset); //將新socket連接放入select監聽集合
            if (connectfd > maxfd)
                maxfd = connectfd;  //確認maxfd是最大描述符
            if (i > maxi)       //數組最大元素值
                maxi = i;
            if (--nready <= 0)
                continue;       //如果沒有新客戶端連接，繼續循環
        }

        for (i = 0; i <= maxi; i++)
        {
            if ((sockfd = client[i].fd) < 0)    //如果客戶端描述符小於0，則沒有客戶端連接，檢測下一個
                continue;
            // 有客戶連接，檢測是否有數據
            if (FD_ISSET(sockfd, &rset))
            {
                printf("Receive from connect fd[%d].\n", i);
                if ((n = recv(sockfd, recvbuf, MAXDATASIZE, 0)) == 0)
                {               //從客戶端socket讀數據，等於0表示網絡中斷
                    close(sockfd);  //關閉socket連接
                    printf("%s closed. User's data: %s\n", client[i].name, client[i].data);
                    FD_CLR(sockfd, &allset);    //從監聽集合中刪除此socket連接
                    client[i].fd = -1;  //數組元素設初始值，表示沒客戶端連接
                }
                else
                    process_client(&client[i], recvbuf, n); //接收到客戶數據，開始處理
                if (--nready <= 0)
                    break;      //如果沒有新客戶端有數據，跳出for循環回到while循環
            }
        }
    }
    close(listenfd);            //關閉服務器監聽socket
}

/*************************************************
* Function    : process_client()
* Description : 處理客戶端連接函數
* Calls       :
* Called By   : main()
* Input       :
* Output      :
* Return      :
*************************************************/
void process_client(CLIENT * client, char *recvbuf, int len)
{
    char sendbuf[MAXDATASIZE];
    int i;

    printf("Received client( %s ) message: %s\n", client->name, recvbuf);

    for (i = 0; i < len - 1; i++)
    {
        sendbuf[i] = recvbuf[len - i - 2];
    }

    sendbuf[len - 1] = '\0';

    send(client->fd, sendbuf, strlen(sendbuf), 0);
}

測試結果：