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【C++网络编程】第5篇：UDP与广播通信

article 2025/3/31 2:21:39

一、UDP协议核心特性

1. UDP vs TCP

特性	UDP	TCP
连接方式	无连接	面向连接（三次握手）
可靠性	不保证数据到达或顺序	可靠传输（超时重传、顺序控制）
传输效率	低延迟，高吞吐	相对较低（因握手和确认机制）
适用场景	实时音视频、广播、在线游戏	文件传输、Web请求、数据库操作

2. UDP数据包结构

**首部（8字节）**：

| 源端口（2） | 目标端口（2） |
| 数据包长度（2） | 校验和（2） |

数据载荷：最大长度受限于IPv4的MTU（通常1500字节）。

3. 不同方式介绍

单播（Unicast）：1 对 1（普通 UDP/TCP 通信）。
广播（Broadcast）：1 对同一子网内所有设备。
组播（Multicast）：1 对一组指定的设备（跨子网）。

二、UDP单播通信实现

1. UDP服务器与客户端流程

服务器：socket() → bind() → recvfrom() → sendto()
客户端：socket() → sendto() → recvfrom()

2. 完整代码示例

UDP服务器（接收并回复）

#include <iostream>
#include <winsock2.h>
#include <ws2tcpip.h>  
#pragma comment(lib, "ws2_32.lib")

int main() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "WSAStartup failed!" << std::endl;
        return 1;
    }

    SOCKET sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    sockaddr_in serverAddr{};
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serverAddr.sin_port = htons(8080);

    bind(sock, (sockaddr*)&serverAddr, sizeof(serverAddr));
    std::cout << "UDP Server listening on port 8080..." << std::endl;

    char buffer[1024];
    sockaddr_in clientAddr{};
    int clientAddrLen = sizeof(clientAddr);

    while (true) {
        int bytesReceived = recvfrom(sock, buffer, sizeof(buffer), 0,
            (sockaddr*)&clientAddr, &clientAddrLen);
        if (bytesReceived == SOCKET_ERROR) {
            std::cerr << "recvfrom failed: " << WSAGetLastError() << std::endl;
            continue;
        }

        char clientIp[INET_ADDRSTRLEN] = { 0 };
        if (InetNtopA(
            AF_INET,                            // IPv4
            &clientAddr.sin_addr,               // 输入地址结构体
            clientIp,                           // 输出缓冲区
            sizeof(clientIp)                    // 缓冲区大小
        ) == NULL) {
            std::cerr << "IP conversion failed: " << WSAGetLastError() << std::endl;
            strcpy_s(clientIp, "unknown");      // 错误时显示 "unknown"
        }

        std::cout << "Received " << bytesReceived << " bytes from "
            << clientIp << ":"                  // 使用转换后的 IP 字符串
            << ntohs(clientAddr.sin_port) << std::endl;

        // 原样返回数据
        sendto(sock, buffer, bytesReceived, 0,
            (sockaddr*)&clientAddr, clientAddrLen);
    }

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

UDP客户端（发送消息）

#include <iostream>
#include <winsock2.h>
#include <ws2tcpip.h> 
#pragma comment(lib, "ws2_32.lib")

int main() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "WSAStartup failed!" << std::endl;
        return 1;
    }

    SOCKET sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

    sockaddr_in serverAddr{};
    serverAddr.sin_family = AF_INET;
    inet_pton(AF_INET, "127.0.0.1", &(serverAddr.sin_addr.s_addr));
    serverAddr.sin_port = htons(8080);

    const char* message = "Hello UDP Server!";
    sendto(sock, message, strlen(message), 0,
        (sockaddr*)&serverAddr, sizeof(serverAddr));

    char buffer[1024];
    int bytesReceived = recvfrom(sock, buffer, sizeof(buffer), 0, nullptr, nullptr);
    if (bytesReceived > 0) {
        std::cout << "Server echoed: " << std::string(buffer, bytesReceived) << std::endl;
    }

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

测试结果
在这里插入图片描述

三、UDP广播通信

1. 广播地址与设置

广播地址：255.255.255.255（全局广播）或子网广播地址（如192.168.1.255）。
套接字选项：启用SO_BROADCAST。

int enable = 1;
setsockopt(sock, SOL_SOCKET, SO_BROADCAST, (char*)&enable, sizeof(enable));

2. 广播发送端代码

#include <iostream>
#include <winsock2.h>
#include <ws2tcpip.h>  
#pragma comment(lib, "ws2_32.lib")

int main() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "WSAStartup failed!" << std::endl;
        return 1;
    }

    SOCKET sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

    // 启用广播
    int enable = 1;
    setsockopt(sock, SOL_SOCKET, SO_BROADCAST, (char*)&enable, sizeof(enable));

    sockaddr_in broadcastAddr{};
    broadcastAddr.sin_family = AF_INET;
    inet_pton(AF_INET, "127.0.0.1", &(broadcastAddr.sin_addr.s_addr));
    broadcastAddr.sin_port = htons(8080);

    const char* message = "Broadcast Message!";
    sendto(sock, message, strlen(message), 0,
        (sockaddr*)&broadcastAddr, sizeof(broadcastAddr));

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

3. 服务器端修改部分

        std::cout << "Received " << bytesReceived << " bytes from "
            << clientIp << ":"                  // 使用转换后的 IP 字符串
            << ntohs(clientAddr.sin_port) << std::endl;
        std::cout << "Datas:" << std::string(buffer, bytesReceived) << std::endl;

4. 测试结果

在这里插入图片描述

四、UDP组播（Multicast）

1. 组播地址范围

IPv4：224.0.0.0 ~ 239.255.255.255（如239.255.0.1）。

2. 接收端

#include <iostream>
#include <winsock2.h>
#include <ws2tcpip.h>
#pragma comment(lib, "ws2_32.lib")

int main() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "WSAStartup failed!" << std::endl;
        return 1;
    }

    // 创建 UDP 套接字
    SOCKET sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sock == INVALID_SOCKET) {
        std::cerr << "socket() failed: " << WSAGetLastError() << std::endl;
        WSACleanup();
        return 1;
    }

    // 设置端口复用（允许其他进程绑定相同端口）
    int reuse = 1;
    if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR,
        (char*)&reuse, sizeof(reuse)) == SOCKET_ERROR) {
        std::cerr << "setsockopt(SO_REUSEADDR) failed: " << WSAGetLastError() << std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 绑定到本地端口
    sockaddr_in localAddr{};
    localAddr.sin_family = AF_INET;
    localAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    localAddr.sin_port = htons(8080);
    if (bind(sock, (sockaddr*)&localAddr, sizeof(localAddr)) == SOCKET_ERROR) {
        std::cerr << "bind() failed: " << WSAGetLastError() << std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 加入组播组
    ip_mreq mreq{};
    if (inet_pton(AF_INET, "239.255.255.250", &(mreq.imr_multiaddr.s_addr)) != 1) {
        std::cerr << "inet_pton() failed for multicast address" << std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    mreq.imr_interface.s_addr = htonl(INADDR_ANY);  // 使用默认网络接口

    if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP,
        (char*)&mreq, sizeof(mreq)) == SOCKET_ERROR) {
        std::cerr << "setsockopt(IP_ADD_MEMBERSHIP) failed: " << WSAGetLastError() << std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    std::cout << "Listening for multicast on 239.255.255.250:8080..." << std::endl;

    char buffer[1024];
    sockaddr_in senderAddr{};
    int senderAddrLen = sizeof(senderAddr);

    while (true) {
        // 接收数据
        int bytesReceived = recvfrom(sock, buffer, sizeof(buffer) - 1, 0,
            (sockaddr*)&senderAddr, &senderAddrLen);
        if (bytesReceived == SOCKET_ERROR) {
            std::cerr << "recvfrom() failed: " << WSAGetLastError() << std::endl;
            continue;
        }

        // 显示来源信息
        char senderIp[INET_ADDRSTRLEN] = { 0 };
        inet_ntop(AF_INET, &senderAddr.sin_addr, senderIp, INET_ADDRSTRLEN);
        std::cout << "Received " << bytesReceived << " bytes from "
            << senderIp << ":" << ntohs(senderAddr.sin_port) << std::endl;

        // 安全处理数据（防止缓冲区溢出）
        buffer[bytesReceived] = '\0';  // 添加字符串终止符
        std::cout << "Data: " << buffer << std::endl;
    }

    // 退出时清理（虽然 while(true) 会阻止执行到这里）
    closesocket(sock);
    WSACleanup();
    return 0;
}

3. 发送端

#include <iostream>
#include <winsock2.h>
#include <ws2tcpip.h>
#pragma comment(lib, "ws2_32.lib")

int main() {
    // 初始化 Winsock
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "WSAStartup failed: " << WSAGetLastError() << std::endl;
        return 1;
    }

    // 创建 UDP 套接字
    SOCKET sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sock == INVALID_SOCKET) {
        std::cerr << "socket() failed: " << WSAGetLastError() << std::endl;
        WSACleanup();
        return 1;
    }

    // 设置组播 TTL（控制传输范围）
    int ttl = 32;
    if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL,
        (char*)&ttl, sizeof(ttl)) == SOCKET_ERROR) {
        std::cerr << "setsockopt(IP_MULTICAST_TTL) failed: " << WSAGetLastError() << std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 构建组播目标地址
    sockaddr_in multicastAddr{};
    multicastAddr.sin_family = AF_INET;
    multicastAddr.sin_port = htons(8080); // 目标端口

    // 转换组播地址
    if (inet_pton(AF_INET, "239.255.255.250", &multicastAddr.sin_addr.s_addr) != 1) {
        std::cerr << "inet_pton() failed for multicast address" << std::endl;
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 发送数据
    const char* msg = "Hello Multicast!";
    int msgLen = static_cast<int>(strlen(msg));
    int bytesSent = sendto(sock, msg, msgLen, 0,
        (sockaddr*)&multicastAddr, sizeof(multicastAddr));

    if (bytesSent == SOCKET_ERROR) {
        std::cerr << "sendto() failed: " << WSAGetLastError() << std::endl;
    }
    else if (bytesSent != msgLen) {
        std::cerr << "sendto() partial send: " << bytesSent << "/" << msgLen << std::endl;
    }
    else {
        std::cout << "Successfully sent " << bytesSent
            << " bytes to multicast group 239.255.255.250:8080" << std::endl;
    }

    // 清理资源
    closesocket(sock);
    WSACleanup();
    return 0;
}