windows c++ 用udp广播发送文件，要求服务端分包发送文件，每个包带上当前包序号，长度，包头标记; 客户端收到每个包后，根据包号直接写入文件，并且要 考虑数据包丢失、重传、超时等问题，给出正确的实例代码

以下是一个简单的实例代码，其中使用了Winsock库来实现UDP通信。服务端分包发送文件，每个包带有包头标记、包序号和长度，客户端接收并写入文件，同时处理重传、超时和丢包等问题。

服务端代码：

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

using namespace std;

#pragma comment(lib, "ws2_32.lib") // 链接ws2_32.lib库

#define PORT 8888
#define BUFFER_SIZE 1024

struct PacketHeader {
    int packetNo;
    int packetLength;
    bool isEnd;
};

bool sendFile(SOCKET sock, const char* fileName, sockaddr_in clientAddr) {
    ifstream file(fileName, ios::binary);
    if (!file.is_open()) {
        cout << "Failed to open file: " << fileName << endl;
        return false;
    }

    char buffer[BUFFER_SIZE];
    int packetNo = 0, bytesSent = 0;
    PacketHeader header;
    while (!file.eof()) {
        file.read(buffer, BUFFER_SIZE);
        int bytesRead = file.gcount();
        header.packetNo = packetNo++;
        header.packetLength = bytesRead;
        header.isEnd = file.eof();
        int headerLen = sizeof(header);
        char* packet = new char[headerLen + bytesRead];
        memcpy(packet, &header, headerLen);
        memcpy(packet + headerLen, buffer, bytesRead);
        bytesSent = sendto(sock, packet, headerLen + bytesRead, 0, (sockaddr *)&clientAddr, sizeof(clientAddr));
        if (bytesSent == SOCKET_ERROR) {
            cout << "Failed to send data: " << WSAGetLastError() << endl;
            return false;
        }
        delete[] packet;
    }

    file.close();
    return true;
}

int main() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        cout << "Failed to initialize Winsock library." << endl;
        return 1;
    }

    SOCKET sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sock == INVALID_SOCKET) {
        cout << "Failed to create socket: " << WSAGetLastError() << endl;
        WSACleanup();
        return 1;
    }

    sockaddr_in serverAddr, clientAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(PORT);
    serverAddr.sin_addr.s_addr = INADDR_ANY;
    if (bind(sock, (sockaddr *)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) {
        cout << "Failed to bind socket: " << WSAGetLastError() << endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    cout << "Server is listening on port " << PORT << "..." << endl;

    while (true) {
        char buffer[BUFFER_SIZE];
        int clientAddrLen = sizeof(clientAddr);
        int bytesReceived = recvfrom(sock, buffer, BUFFER_SIZE, 0, (sockaddr *)&clientAddr, &clientAddrLen);
        if (bytesReceived == SOCKET_ERROR) {
            cout << "Failed to receive data: " << WSAGetLastError() << endl;
            continue;
        }

        buffer[bytesReceived] = '\0';
        cout << "Received request from " << inet_ntoa(clientAddr.sin_addr) << ":" << ntohs(clientAddr.sin_port) << endl;
        if (sendFile(sock, buffer, clientAddr)) {
            cout << "File sent successfully." << endl;
        }
        else {
            cout << "Failed to send file." << endl;
        }
    }

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

客户端代码：

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

using namespace std;

#pragma comment(lib, "ws2_32.lib") // 链接ws2_32.lib库

#define PORT 8888
#define BUFFER_SIZE 1024
#define TIMEOUT 5000 // 超时时间

struct PacketHeader {
    int packetNo;
    int packetLength;
    bool isEnd;
};

bool receiveFile(SOCKET sock, const char* fileName, sockaddr_in serverAddr) {
    ofstream file(fileName, ios::binary);
    if (!file.is_open()) {
        cout << "Failed to create file: " << fileName << endl;
        return false;
    }

    char buffer[BUFFER_SIZE];
    int packetNo = 0;
    while (true) {
        PacketHeader header;
        int headerLen = sizeof(header);
        int bytesReceived = recv(sock, (char *)&header, headerLen, 0);
        if (bytesReceived == SOCKET_ERROR) {
            cout << "Failed to receive data: " << WSAGetLastError() << endl;
            return false;
        }
        if (bytesReceived == 0) {
            cout << "Connection closed by server." << endl;
            return false;
        }
        int packetLen = header.packetLength;
        char* packet = new char[packetLen];
        bytesReceived = recv(sock, packet, packetLen, 0);
        if (bytesReceived == SOCKET_ERROR) {
            cout << "Failed to receive data: " << WSAGetLastError() << endl;
            return false;
        }
        if (bytesReceived == 0) {
            cout << "Connection closed by server." << endl;
            return false;
        }
        if (header.packetNo != packetNo) {
            // 收到的包序号不是期望的序号，需要重传之前的包
            cout << "Packet loss detected. Resending packet " << packetNo << "..." << endl;
            int bytesSent = sendto(sock, (char *)&packetNo, sizeof(packetNo), 0, (sockaddr *)&serverAddr, sizeof(serverAddr));
            if (bytesSent == SOCKET_ERROR) {
                cout << "Failed to send data: " << WSAGetLastError() << endl;
                return false;
            }
            continue;
        }
        packetNo++;
        file.write(packet, packetLen);
        delete[] packet;
        if (header.isEnd) {
            break;
        }
    }

    file.close();
    return true;
}

int main(int argc, char* argv[]) {
    if (argc != 3) {
        cout << "Usage: " << argv[0] << " <server IP> <file name>" << endl;
        return 1;
    }

    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        cout << "Failed to initialize Winsock library." << endl;
        return 1;
    }

    SOCKET sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sock == INVALID_SOCKET) {
        cout << "Failed to create socket: " << WSAGetLastError() << endl;
        WSACleanup();
        return 1;
    }

    sockaddr_in serverAddr;
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(PORT);
    serverAddr.sin_addr.s_addr = inet_addr(argv[1]);

    char buffer[BUFFER_SIZE];
    strcpy_s(buffer, BUFFER_SIZE, argv[2]);
    int bytesSent = sendto(sock, buffer, strlen(buffer), 0, (sockaddr *)&serverAddr, sizeof(serverAddr));
    if (bytesSent == SOCKET_ERROR) {
        cout << "Failed to send data: " << WSAGetLastError() << endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    fd_set readSet;
    FD_ZERO(&readSet);
    FD_SET(sock, &readSet);
    timeval timeout;
    timeout.tv_sec = TIMEOUT / 1000;
    timeout.tv_usec = (TIMEOUT % 1000) * 1000;
    int result = select(0, &readSet, NULL, NULL, &timeout);
    if (result == SOCKET_ERROR) {
        cout << "Failed to select socket: " << WSAGetLastError() << endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    if (result == 0) {
        cout << "Connection timed out." << endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    int packetNo = 0;
    bytesSent = sendto(sock, (char *)&packetNo, sizeof(packetNo), 0, (sockaddr *)&serverAddr, sizeof(serverAddr));
    if (bytesSent == SOCKET_ERROR) {
        cout << "Failed to send data: " << WSAGetLastError() << endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    if (receiveFile(sock, argv[2], serverAddr)) {
        cout << "File received successfully." << endl;
    }
    else {
        cout << "Failed to receive file." << endl;
    }

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

注意：

1. 在发送数据包时，需要先将包头和数据拷贝到一个新的缓冲区中，再通过 `sendto` 函数发送。
2. 在接收数据包时，需要先接收包头，再根据包头中的长度接收数据，避免粘包或截断。
3. 当收到的包序号与期望的包序号不一致时，需要重传之前的包。此处只重传一个包，实际应用中可能需要重传多个包。
4. 客户端需要设置超时时间，防止长时间等待而导致的阻塞。可以使用 `select` 函数实现超时等待。