用C语言实现利用TCP协议编写一个简单的文件服务器和客户端。 支持功能： 1. list：列表远端目录下的文件和目录信息（目录用方括号括起以示区分） 2. pwd: 查看远端当前目录 3. lpwd: 查看本地当前目录 4. cd xxxxx: 切换远端当前目录为xxxxx 5. lcd xxxxx: 切换本地当前目录为xxxxx 6. down xxxxx: 下载远端当前目录下的文件xxxxx到客户端的当前目录下 7. exit：断开连接并退出。

以下是基于C语言实现的简单文件服务器和客户端的代码，支持上述功能：

服务器端代码：

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

#define PORT 9090
#define BUFFER_SIZE 1024
#define MAX_COMMAND_LENGTH 100
#define MAX_PATH_LENGTH 200

void list_files(int client_socket, char *current_directory);
void change_directory(int client_socket, char *current_directory, char *new_directory);
void download_file(int client_socket, char *current_directory, char *filename);

int main()
{
    int server_socket, client_socket;
    struct sockaddr_in server_address, client_address;
    socklen_t client_address_size;
    char buffer[BUFFER_SIZE];
    char current_directory[MAX_PATH_LENGTH];
    char command[MAX_COMMAND_LENGTH];

    // create server socket
    server_socket = socket(AF_INET, SOCK_STREAM, 0);
    if(server_socket == -1)
    {
        perror("Failed to create server socket");
        exit(EXIT_FAILURE);
    }

    // set server address
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = INADDR_ANY;
    server_address.sin_port = htons(PORT);

    // bind server socket to address
    if(bind(server_socket, (struct sockaddr *)&server_address, sizeof(server_address)) == -1)
    {
        perror("Failed to bind server socket");
        exit(EXIT_FAILURE);
    }

    // listen for client connections
    if(listen(server_socket, 5) == -1)
    {
        perror("Failed to listen for connections");
        exit(EXIT_FAILURE);
    }

    printf("Server started and listening on port %d...\n", PORT);

    // main server loop
    while(1)
    {
        client_address_size = sizeof(client_address);

        // accept client connection
        client_socket = accept(server_socket, (struct sockaddr *)&client_address, &client_address_size);
        if(client_socket == -1)
        {
            perror("Failed to accept client connection");
            continue;
        }

        printf("Client connected from %s:%d\n", inet_ntoa(client_address.sin_addr), ntohs(client_address.sin_port));

        // send welcome message to client
        strcpy(buffer, "Welcome to the file server!\n");
        send(client_socket, buffer, strlen(buffer), 0);

        // set initial current directory to server's working directory
        getcwd(current_directory, MAX_PATH_LENGTH);

        // main client loop
        while(1)
        {
            // receive command from client
            memset(buffer, 0, BUFFER_SIZE);
            recv(client_socket, buffer, BUFFER_SIZE, 0);
            sscanf(buffer, "%s", command);

            // handle command
            if(strcmp(command, "list") == 0)
            {
                list_files(client_socket, current_directory);
            }
            else if(strcmp(command, "pwd") == 0)
            {
                sprintf(buffer, "%s\n", current_directory);
                send(client_socket, buffer, strlen(buffer), 0);
            }
            else if(strcmp(command, "cd") == 0)
            {
                char new_directory[MAX_PATH_LENGTH];
                sscanf(buffer, "%*s %s", new_directory);
                change_directory(client_socket, current_directory, new_directory);
            }
            else if(strcmp(command, "down") == 0)
            {
                char filename[MAX_PATH_LENGTH];
                sscanf(buffer, "%*s %s", filename);
                download_file(client_socket, current_directory, filename);
            }
            else if(strcmp(command, "exit") == 0)
            {
                printf("Client disconnected from %s:%d\n", inet_ntoa(client_address.sin_addr), ntohs(client_address.sin_port));
                close(client_socket);
                break;
            }
            else
            {
                strcpy(buffer, "Invalid command\n");
                send(client_socket, buffer, strlen(buffer), 0);
            }
        }
    }

    return 0;
}

void list_files(int client_socket, char *current_directory)
{
    DIR *dir;
    struct dirent *entry;
    char buffer[BUFFER_SIZE];

    dir = opendir(current_directory);
    if(dir == NULL)
    {
        perror("Failed to open directory");
        return;
    }

    while((entry = readdir(dir)) != NULL)
    {
        if(entry->d_type == DT_DIR)
        {
            sprintf(buffer, "[%s]\n", entry->d_name);
        }
        else
        {
            sprintf(buffer, "%s\n", entry->d_name);
        }

        send(client_socket, buffer, strlen(buffer), 0);
    }

    closedir(dir);
}

void change_directory(int client_socket, char *current_directory, char *new_directory)
{
    char buffer[BUFFER_SIZE];

    if(chdir(new_directory) == -1)
    {
        perror("Failed to change directory");
        strcpy(buffer, "Failed to change directory\n");
        send(client_socket, buffer, strlen(buffer), 0);
    }
    else
    {
        getcwd(current_directory, MAX_PATH_LENGTH);
        sprintf(buffer, "Changed directory to %s\n", current_directory);
        send(client_socket, buffer, strlen(buffer), 0);
    }
}

void download_file(int client_socket, char *current_directory, char *filename)
{
    char buffer[BUFFER_SIZE];
    char filepath[MAX_PATH_LENGTH];
    int file_descriptor, bytes_read;

    // create file path
    sprintf(filepath, "%s/%s", current_directory, filename);

    // open file for reading
    file_descriptor = open(filepath, O_RDONLY);
    if(file_descriptor == -1)
    {
        perror("Failed to open file for reading");
        strcpy(buffer, "Failed to open file for reading\n");
        send(client_socket, buffer, strlen(buffer), 0);
        return;
    }

    // send file contents to client
    while((bytes_read = read(file_descriptor, buffer, BUFFER_SIZE)) > 0)
    {
        send(client_socket, buffer, bytes_read, 0);
    }

    close(file_descriptor);
}

客户端代码：

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

#define SERVER_IP "127.0.0.1"
#define PORT 9090
#define BUFFER_SIZE 1024
#define MAX_COMMAND_LENGTH 100
#define MAX_PATH_LENGTH 200

int main()
{
    int client_socket;
    struct sockaddr_in server_address;
    char buffer[BUFFER_SIZE];
    char command[MAX_COMMAND_LENGTH];
    char current_directory[MAX_PATH_LENGTH];

    // create client socket
    client_socket = socket(AF_INET, SOCK_STREAM, 0);
    if(client_socket == -1)
    {
        perror("Failed to create client socket");
        exit(EXIT_FAILURE);
    }

    // set server address
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = inet_addr(SERVER_IP);
    server_address.sin_port = htons(PORT);

    // connect to server
    if(connect(client_socket, (struct sockaddr *)&server_address, sizeof(server_address)) == -1)
    {
        perror("Failed to connect to server");
        exit(EXIT_FAILURE);
    }

    // receive welcome message from server
    memset(buffer, 0, BUFFER_SIZE);
    recv(client_socket, buffer, BUFFER_SIZE, 0);
    printf("%s", buffer);

    // set initial current directory to client's working directory
    getcwd(current_directory, MAX_PATH_LENGTH);

    // main client loop
    while(1)
    {
        // get user command
        printf("> ");
        fgets(command, MAX_COMMAND_LENGTH, stdin);
        command[strlen(command)-1] = '\0'; // remove newline character

        // send command to server
        send(client_socket, command, strlen(command), 0);

        // handle command
        if(strcmp(command, "list") == 0)
        {
            // receive file list from server
            memset(buffer, 0, BUFFER_SIZE);
            while(recv(client_socket, buffer, BUFFER_SIZE, 0) > 0)
            {
                printf("%s", buffer);
                memset(buffer, 0, BUFFER_SIZE);
            }
        }
        else if(strcmp(command, "pwd") == 0)
        {
            // receive current directory from server
            memset(buffer, 0, BUFFER_SIZE);
            recv(client_socket, buffer, BUFFER_SIZE, 0);
            printf("%s", buffer);
        }
        else if(strcmp(command, "cd") == 0)
        {
            // receive change directory result from server
            memset(buffer, 0, BUFFER_SIZE);
            recv(client_socket, buffer, BUFFER_SIZE, 0);
            printf("%s", buffer);
        }
        else if(strcmp(command, "down") == 0)
        {
            char filename[MAX_PATH_LENGTH];
            FILE *file;
            int bytes_received;

            // get filename from user
            printf("Filename: ");
            fgets(filename, MAX_PATH_LENGTH, stdin);
            filename[strlen(filename)-1] = '\0'; // remove newline character

            // open file for writing
            file = fopen(filename, "w");
            if(file == NULL)
            {
                perror("Failed to open file for writing");
                continue;
            }

            // receive file contents from server
            while((bytes_received = recv(client_socket, buffer, BUFFER_SIZE, 0)) > 0)
            {
                fwrite(buffer, 1, bytes_received, file);
            }

            fclose(file);
            printf("File downloaded successfully\n");
        }
        else if(strcmp(command, "exit") == 0)
        {
            close(client_socket);
            printf("Disconnected from server\n");
            break;
        }
        else
        {
            printf("Invalid command\n");
        }
    }

    return 0;
}