请用C++winsock2写一个TCP/IP文件传输（指定ip和端口）的客户端和服务器的完整实例

C++的Winsock2库主要用于Windows平台下的网络编程，包括TCP/IP协议。以下是一个简单的TCP/IP文件传输的客户端和服务器的例子，假设我们只处理文本文件的上传和下载：

**服务器端（server.cpp）：**

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>
#include <fstream>
#pragma comment(lib, "Ws2_32.lib")

const char* serverIP = "127.0.0.1";
int serverPort = 9000;

// 创建套接字、绑定地址、监听等
SOCKET serverSocket;
void initServer() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "Error initializing Winsock." << std::endl;
        return;
    }

    serverSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (serverSocket == INVALID_SOCKET) {
        std::cerr << "Error creating socket." << std::endl;
        WSACleanup();
        return;
    }

    sockaddr_in serverAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = inet_addr(serverIP);
    serverAddr.sin_port = htons(serverPort);

    if (bind(serverSocket, (SOCKADDR*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) {
        std::cerr << "Error binding to address." << std::endl;
        closesocket(serverSocket);
        WSACleanup();
        return;
    }

    listen(serverSocket, SOMAXCONN); // 监听连接

    std::cout << "Server listening on " << serverIP << ":" << serverPort << std::endl;
}

// 处理客户端请求
void handleClient(SOCKET clientSocket) {
    char buffer[1024];
    int bytesRead;

    while ((bytesRead = recv(clientSocket, buffer, sizeof(buffer), 0)) > 0) {
        std::string message(buffer, bytesRead);
        if (message == "upload") {  // 接收上传指令
            std::ifstream file("file.txt", std::ios::binary);
            if (file.is_open()) {
                send(clientSocket, "Upload started.", strlen("Upload started."), 0);
                std::copy(file.rdbuf(), file.rdbuf() + file.gcount(), buffer);
                send(clientSocket, buffer, file.gcount(), 0);
                file.close();
            } else {
                send(clientSocket, "File not found.", strlen("File not found."), 0);
            }
        } else if (message == "download") {  // 接收下载指令
            send(clientSocket, "Downloading...", strlen("Downloading..."), 0);
            std::ofstream outputFile("file.txt", std::ios::binary | std::ios::out);
            if (!outputFile) {
                send(clientSocket, "Failed to create output file.", strlen("Failed to create output file."), 0);
            } else {
                sendfile(clientSocket, stdout, NULL, -1, NULL, NULL);
                outputFile.close();
                send(clientSocket, "Download completed.", strlen("Download completed."), 0);
            }
        } else {
            send(clientSocket, "Unknown command.", strlen("Unknown command."), 0);
        }
    }

    if (bytesRead == 0 || bytesRead == -1) {  // 错误或关闭
        shutdown(clientSocket, SD_SEND);
        closesocket(clientSocket);
    }
}

int main() {
    initServer();

    while (true) {
        SOCKET clientSocket = accept(serverSocket, NULL, NULL);
        if (clientSocket == INVALID_SOCKET) {
            break; // 处理错误
        }
        std::cout << "Accepted connection from " << inet_ntoa(((sockaddr_in*)getpeername(clientSocket))->sin_addr) << "." << std::endl;
        handleClient(clientSocket);
    }

    closesocket(serverSocket);
    WSACleanup();
    return 0;
}

**客户端（client.cpp）：**

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

const char* serverIP = "127.0.0.1";
int serverPort = 9000;

void connectToServer() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "Error initializing Winsock." << std::endl;
        return;
    }

    SOCKET clientSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (clientSocket == INVALID_SOCKET) {
        std::cerr << "Error creating socket." << std::endl;
        WSACleanup();
        return;
    }

    sockaddr_in serverAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = inet_addr(serverIP);
    serverAddr.sin_port = htons(serverPort);

    if (connect(clientSocket, (SOCKADDR*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) {
        std::cerr << "Error connecting to server." << std::endl;
        closesocket(clientSocket);
        WSACleanup();
        return;
    }

    std::string command;
    std::getline(std::cin, command);

    switch (command[0]) {
    case 'u': // 上传
        std::ifstream file("file.txt", std::ios::binary);
        if (file.is_open()) {
            send(clientSocket, "Uploading...", strlen("Uploading..."), 0);
            std::copy(file.rdbuf(), file.rdbuf() + file.gcount(), buffer);
            send(clientSocket, buffer, file.gcount(), 0);
            file.close();
            std::cout << "Uploaded successfully.\n";
        } else {
            std::cerr << "File not found.\n";
        }
        break;
    case 'd': // 下载
        std::ofstream outputFile("file.txt", std::ios::binary | std::ios::out);
        if (!outputFile) {
            std::cerr << "Failed to create output file.\n";
        } else {
            send(clientSocket, "Downloading...\n", strlen("Downloading...\n"), 0);
            sendfile(clientSocket, outputFile.getHandle(), NULL, -1, NULL, NULL);
            outputFile.close();
            std::cout << "Downloaded successfully.\n";
        }
        break;
    default:
        std::cerr << "Invalid command.\n";
        break;
    }

    closesocket(clientSocket);
    WSACleanup();
}

int main() {
    connectToServer();
    return 0;
}

注意：这是一个简化的示例，实际应用需要处理更多的边界情况，并添加异常处理和错误消息。同时，`sendfile`函数用于高效的文件传输。