使用select方式实现一个C/S架构的应用； 要求：服务器同时支持多个客户端连接，一个客户端发送消息，服务器回复 一个XML消息（内容不限）；

以下是使用select方式实现C/S架构应用的代码示例：

服务器端代码：

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

#define MAX_CLIENTS 5
#define BUFFER_SIZE 1024

int main(int argc, char *argv[])
{
    int server_fd, client_fds[MAX_CLIENTS], max_fd, activity, i, valread;
    struct sockaddr_in server_addr, client_addr;
    char buffer[BUFFER_SIZE] = {0};
    fd_set read_fds;

    // 创建服务器socket
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0)
    {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // 设置服务器socket的地址和端口号
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(atoi(argv[1]));

    // 绑定服务器socket
    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0)
    {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    // 监听服务器socket
    if (listen(server_fd, MAX_CLIENTS) < 0)
    {
        perror("listen failed");
        exit(EXIT_FAILURE);
    }

    printf("Server is listening on port %d.\n", atoi(argv[1]));

    // 初始化客户端socket数组
    for (i = 0; i < MAX_CLIENTS; i++)
    {
        client_fds[i] = 0;
    }

    while (1)
    {
        // 清空文件描述符集
        FD_ZERO(&read_fds);

        // 将服务器socket加入文件描述符集
        FD_SET(server_fd, &read_fds);
        max_fd = server_fd;

        // 将所有已连接的客户端socket加入文件描述符集
        for (i = 0; i < MAX_CLIENTS; i++)
        {
            int fd = client_fds[i];

            if (fd > 0)
            {
                FD_SET(fd, &read_fds);
                if (fd > max_fd)
                {
                    max_fd = fd;
                }
            }
        }

        // 监听文件描述符集上的事件
        activity = select(max_fd + 1, &read_fds, NULL, NULL, NULL);

        if ((activity < 0) && (errno != EINTR))
        {
            printf("select error");
        }

        // 处理服务器socket上的连接请求
        if (FD_ISSET(server_fd, &read_fds))
        {
            int new_socket, addr_len = sizeof(client_addr);

            if ((new_socket = accept(server_fd, (struct sockaddr *)&client_addr, (socklen_t *)&addr_len)) < 0)
            {
                perror("accept failed");
                exit(EXIT_FAILURE);
            }

            printf("New client connected: %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

            // 将新连接的客户端socket加入客户端socket数组
            for (i = 0; i < MAX_CLIENTS; i++)
            {
                if (client_fds[i] == 0)
                {
                    client_fds[i] = new_socket;
                    break;
                }
            }
        }

        // 处理客户端socket上的消息
        for (i = 0; i < MAX_CLIENTS; i++)
        {
            int fd = client_fds[i];

            if (FD_ISSET(fd, &read_fds))
            {
                if ((valread = read(fd, buffer, BUFFER_SIZE)) == 0)
                {
                    // 客户端断开连接，将其socket从客户端socket数组中移除
                    close(fd);
                    client_fds[i] = 0;
                    printf("Client disconnected.\n");
                }
                else
                {
                    // 处理客户端发送的消息
                    printf("Received message from client: %s\n", buffer);

                    // 回复XML消息给客户端
                    char *xml_message = "<message>XML message</message>";
                    send(fd, xml_message, strlen(xml_message), 0);
                }
            }
        }
    }

    return 0;
}

客户端代码：

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

#define BUFFER_SIZE 1024

int main(int argc, char *argv[])
{
    int client_fd, valread;
    struct sockaddr_in server_addr;
    char buffer[BUFFER_SIZE] = {0};

    // 创建客户端socket
    if ((client_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0)
    {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // 设置服务器socket的地址和端口号
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr(argv[1]);
    server_addr.sin_port = htons(atoi(argv[2]));

    // 连接服务器socket
    if (connect(client_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0)
    {
        perror("connect failed");
        exit(EXIT_FAILURE);
    }

    printf("Connected to server: %s:%d\n", argv[1], atoi(argv[2]));

    // 发送消息给服务器
    char *message = "Hello server!";
    send(client_fd, message, strlen(message), 0);
    printf("Sent message to server: %s\n", message);

    // 接收服务器回复的消息
    valread = read(client_fd, buffer, BUFFER_SIZE);
    printf("Received message from server: %s\n", buffer);

    close(client_fd);

    return 0;
}

服务器端使用了select函数来同时处理多个客户端连接，客户端连接时会发送Hello server!消息给服务器，服务器会回复一个XML消息。