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”。

好的，我可以为您提供一个基于C语言的简单实现。以下是服务器端和客户端的代码：

服务器端：

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

#define MAXLINE 1024
#define PORT_NUMBER 8080

int main() {
    int server_fd, new_socket, valread;
    struct sockaddr_in address;
    int opt = 1;
    int addrlen = sizeof(address);
    char buffer[MAXLINE] = {0};
    char *hello = "Hello from server";
    char *file = "a.txt";

    // Creating socket file descriptor
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // Forcefully attaching socket to the port 8080
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(PORT_NUMBER);

    // Forcefully attaching socket to the port 8080
    if (bind(server_fd, (struct sockaddr *) &address, sizeof(address)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }
    if (listen(server_fd, 3) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }
    if ((new_socket = accept(server_fd, (struct sockaddr *) &address, (socklen_t *) &addrlen)) < 0) {
        perror("accept");
        exit(EXIT_FAILURE);
    }

    valread = read(new_socket, buffer, MAXLINE);
    if (strcmp(buffer, "read a.txt") == 0) {
        int fd = open(file, O_RDONLY);
        if (fd == -1) {
            send(new_socket, "The file is not exist", strlen("The file is not exist"), 0);
        } else {
            char file_content[MAXLINE];
            memset(file_content, 0, MAXLINE);
            read(fd, file_content, MAXLINE);
            send(new_socket, file_content, strlen(file_content), 0);
        }
    } else {
        send(new_socket, hello, strlen(hello), 0);
    }
    return 0;
}

客户端：

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

#define MAXLINE 1024
#define PORT_NUMBER 8080

int main() {
    int sock = 0, valread;
    struct sockaddr_in serv_addr;
    char *hello = "read a.txt";
    char buffer[MAXLINE] = {0};

    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        printf("\n Socket creation error \n");
        return -1;
    }

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(PORT_NUMBER);

    // Convert IPv4 and IPv6 addresses from text to binary form
    if (inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr) <= 0) {
        printf("\nInvalid address/ Address not supported \n");
        return -1;
    }

    if (connect(sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        printf("\nConnection Failed \n");
        return -1;
    }

    send(sock, hello, strlen(hello), 0);
    valread = read(sock, buffer, MAXLINE);
    printf("%s\n", buffer);
    close(sock);
    return 0;
}

注意：这只是一个简单的示例，实际应用中需要考虑更多的错误处理和安全问题。