用c语言编写一个客户端和服务器端程序，客户端连接到服务器端后，给服务器发出读取一个文档的指令“read a.txt”，服务器读取a.txt，如果该文档存在，则返回结果并显示，如果该文件不存在，则返回“The file is not exist”

以下是用C语言编写的一个简单的客户端和服务器端程序，实现了客户端连接到服务器端后，给服务器发出读取一个文档的指令“read a.txt”，服务器读取a.txt，如果该文档存在，则返回结果并显示，如果该文件不存在，则返回“The file is not exist”：

客户端代码：client.c

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

#define SERVER_IP "127.0.0.1"
#define SERVER_PORT 8888

int main(int argc, char const *argv[])
{
    int client_socket;
    struct sockaddr_in server_address;
    char message[1024] = {0};
    char buffer[1024] = {0};

    // 创建socket
    client_socket = socket(AF_INET, SOCK_STREAM, 0);
    if (client_socket == -1)
    {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    // 设置服务器地址信息
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(SERVER_PORT);
    if (inet_pton(AF_INET, SERVER_IP, &server_address.sin_addr) <= 0)
    {
        perror("invalid address");
        exit(EXIT_FAILURE);
    }

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

    // 发送指令
    strcpy(message, "read a.txt");
    if (send(client_socket, message, strlen(message), 0) < 0)
    {
        perror("send failed");
        exit(EXIT_FAILURE);
    }

    // 接收结果
    if (recv(client_socket, buffer, sizeof(buffer), 0) < 0)
    {
        perror("receive failed");
        exit(EXIT_FAILURE);
    }

    // 显示结果
    printf("%s\n", buffer);

    // 关闭socket
    close(client_socket);

    return 0;
}

服务器端代码：server.c

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

#define SERVER_PORT 8888

int main(int argc, char const *argv[])
{
    int server_socket, client_socket;
    struct sockaddr_in server_address, client_address;
    char buffer[1024] = {0};
    FILE *fp;

    // 创建socket
    server_socket = socket(AF_INET, SOCK_STREAM, 0);
    if (server_socket == -1)
    {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    // 设置服务器地址信息
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = INADDR_ANY;
    server_address.sin_port = htons(SERVER_PORT);

    // 绑定socket
    if (bind(server_socket, (struct sockaddr *)&server_address, sizeof(server_address)) < 0)
    {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    // 监听socket
    if (listen(server_socket, 3) < 0)
    {
        perror("listen failed");
        exit(EXIT_FAILURE);
    }

    // 接收连接
    printf("Waiting for client connection...\n");
    if ((client_socket = accept(server_socket, (struct sockaddr *)&client_address, (socklen_t *)&client_address)) < 0)
    {
        perror("accept failed");
        exit(EXIT_FAILURE);
    }
    printf("Client connected successfully!\n");

    // 接收指令
    if (recv(client_socket, buffer, sizeof(buffer), 0) < 0)
    {
        perror("receive failed");
        exit(EXIT_FAILURE);
    }

    // 处理指令
    if (strncmp(buffer, "read ", 5) == 0)
    {
        fp = fopen(buffer + 5, "r");
        if (fp != NULL)
        {
            // 读取文件内容并发送
            while (fgets(buffer, sizeof(buffer), fp) != NULL)
            {
                if (send(client_socket, buffer, strlen(buffer), 0) < 0)
                {
                    perror("send failed");
                    exit(EXIT_FAILURE);
                }
            }
            fclose(fp);
        }
        else
        {
            // 文件不存在
            strcpy(buffer, "The file is not exist");
            if (send(client_socket, buffer, strlen(buffer), 0) < 0)
            {
                perror("send failed");
                exit(EXIT_FAILURE);
            }
        }
    }

    // 关闭socket
    close(client_socket);
    close(server_socket);

    return 0;
}

编译和运行：

1. 分别保存上面的客户端代码和服务器端代码到 client.c 和 server.c 文件中；
2. 在终端中执行下面的命令来分别编译客户端和服务器端程序：

gcc -o client client.c
gcc -o server server.c

3. 在终端中先运行服务器端程序：

./server

4. 在终端中再运行客户端程序：

./client

5. 如果 a.txt 文件存在，则会显示该文件的内容；如果 a.txt 文件不存在，则会显示 “The file is not exist”。