多播（自发自收）

#include <winsock2.h>
#include <stdio.h>
#define BUFSIZE 1024
#define MAXADDRSTR 16
#define LOOPCOUNT 100
/* 检查系统中是否安装了合适版本的Winsock DLL。*/
int CheckWinsockVersion(void)
{
    WORD wVersionRequested;
    WSADATA wsaData;
   
    int err;
    // 异步I/O和多址广播只有在Winsock2.0以上版本才支持
    wVersionRequested = MAKEWORD(2, 2);
    err = WSAStartup(wVersionRequested, &wsaData);
    if (err == 0)
    {
        if ((LOBYTE(wsaData.wVersion) == 2) && (HIBYTE(wsaData.wVersion) == 2))
        {
            // 确认Winsock DLL 支持2.0
            return 0;
        }
        WSACleanup();
        err = WSAVERNOTSUPPORTED; // 不支持，失败返回
    }
   
    /* Tell the user that we couldn't find a usable WinsockDLL. */
    printf("Winsock DLL does not support requested API version./n");
    return err;
}
int main()
{
    int nRet, i;
    int nIP_TTL = 1; // 在本子网中传播
    BOOL bFlag;
    DWORD dFlag, cbRet;
    int iLen = MAXADDRSTR;
    char strDestMulti[MAXADDRSTR] = "224.1.1.1";
    SOCKADDR_IN stSrcAddr, stDestAddr;
    SOCKET hSock, hNewSock;
    u_short nDestPort = 6666;
    WSABUF stWSABuf;
    char achInBuf[BUFSIZE];
    char achOutBuf[] = "Message number:        ";
   
    nRet = CheckWinsockVersion(); // 检测Winsock版本号
    if (nRet)
    {
        printf("WSAStartup failed: %d/r/n", nRet);
        exit(1);
    }
   
    // 将字符串地址转化成套接字地址
    nRet = WSAStringToAddress(strDestMulti, // address string
                            AF_INET,        // address family
                            NULL,            // protocol info structure
                            (LPSOCKADDR)&stDestAddr, // socket address string
                            &iLen);            // length of socket structure
    if (nRet)
    {
        printf("WSAStringToAddress(%s)failed,Err:%d/n", strDestMulti, WSAGetLastError());
        exit(1);
    }
   
    // 创建一个多播套接字
    hSock = WSASocket(AF_INET, SOCK_DGRAM, IPPROTO_UDP,
        (LPWSAPROTOCOL_INFO)NULL, 0, WSA_FLAG_OVERLAPPED | WSA_FLAG_MULTIPOINT_C_LEAF | WSA_FLAG_MULTIPOINT_D_LEAF);
    if (hSock == INVALID_SOCKET)
    {
        printf("WSASock() failed, Err:%d/n", WSAGetLastError());
        exit(1);
    }
   
    bFlag = TRUE; // 设置套接口为可重用端口地址
    nRet = setsockopt(hSock,    // socket
            SOL_SOCKET,            // socket level
            SO_REUSEADDR,        // socket option
            (char*)&bFlag,        // option value
            sizeof(bFlag));        // size of value
    if (nRet == SOCKET_ERROR)
    {
        printf("setsockopt() SO_REUSEADDR failed,Err:%d/n", WSAGetLastError());
    }
   
    // 将套接口绑定到用户指定端口及默认的接口
    stSrcAddr.sin_family = PF_INET;
    stSrcAddr.sin_port = htons(nDestPort);
    stSrcAddr.sin_addr.s_addr = INADDR_ANY;
    nRet = bind(hSock, (struct sockaddr FAR*)&stSrcAddr, sizeof(struct sockaddr));
   
    if (nRet == SOCKET_ERROR)
    {
        printf("bind failed, Err:%d/n", WSAGetLastError());
        exit(1);
    }
   
    // 设置多播数据报传送范围(TTL)
    nRet = WSAIoctl(hSock,
            SIO_MULTICAST_SCOPE,
            &nIP_TTL,
            sizeof(nIP_TTL),
            NULL,
            0,
            &cbRet,
            NULL,
            NULL);
    if (nRet)
    {
        printf("WSAIoctl(SIO_MULTICAST_SCOPE) failed, Err:%d/n", WSAGetLastError());
        exit(0);
    }
    // 允许内部回送(LOOPBACK)。Windows 95不支持该选项
    bFlag = TRUE;
    nRet = WSAIoctl(hSock,
            SIO_MULTIPOINT_LOOPBACK,
            &bFlag,
            sizeof(bFlag),
            NULL,
            0,
            &cbRet,
            NULL,
            NULL);
    if (nRet)
    {
        printf("WSAIoctl(SIO_MULTIPOINT_LOOPBACK)failed, Err: %d/n", WSAGetLastError());
        exit(0);
    }
   
    stDestAddr.sin_family = AF_INET;
    nRet = WSAHtons(hSock,
            nDestPort,
            &(stDestAddr.sin_port));
    if (nRet == SOCKET_ERROR)
    {
        printf("WSAHtons() failed, Err: %d/n", WSAGetLastError());
        exit(0);
    }
   
    // 加入到指定多址广播组，指定为即为发送者又做接收者
    // 在IP Multicast中，返回的套接字描述符和输入的套接字描述符相同
    hNewSock = WSAJoinLeaf(hSock,
        (PSOCKADDR)&stDestAddr,
        sizeof(stDestAddr),
        NULL,
        NULL,
        NULL,
        NULL,
        JL_BOTH);
    if (hNewSock == INVALID_SOCKET)
    {
        printf("WSAJoinLeaf() failed, Err: %d/n", WSAGetLastError());
        exit(0);
    }
   
    // 在循环中发送/接收数据。
    for (i = 0; i < LOOPCOUNT; i++)
    {
        static iCounter = 1;
        stWSABuf.buf = achOutBuf;
        stWSABuf.len = sizeof(achOutBuf);
       
        cbRet = 0;
        itoa(iCounter++, &achOutBuf[16], 10);
        nRet = WSASendTo(hSock,
                &stWSABuf,
                1,
                &cbRet,
                0,
                (struct sockaddr*)&stDestAddr,
                sizeof(struct sockaddr),
                NULL,
                NULL);
        if (nRet == SOCKET_ERROR)
        {
            printf("WSASendTo() failed. Err: %d/n", WSAGetLastError());
            exit(0);
        }
        stWSABuf.buf = achInBuf;
        stWSABuf.len = BUFSIZE;
        cbRet = 0;
        iLen = sizeof(stSrcAddr);
        dFlag = 0;
       
        nRet = WSARecvFrom(hSock,
                &stWSABuf,
                1,
                &cbRet,
                &dFlag,
                (struct sockaddr*)&stSrcAddr,
                &iLen,
                NULL,
                NULL);
        if (nRet == SOCKET_ERROR)
        {
            printf("WSARecvFrom() failed, Err:%d/n", WSAGetLastError());
            exit(0);
        }
        else
        {
            u_short nPort = 0;
            char achAddr[MAXADDRSTR + 3] = {0};
            iLen = MAXADDRSTR + 3;
            nRet = WSAAddressToString(
                    (struct sockaddr*)&stSrcAddr,
                    sizeof(stSrcAddr),
                    NULL,
                    achAddr,
                    (unsigned long*)&iLen);
            if (nRet == SOCKET_ERROR)
            {
                printf("WSAAddressToString() failed, Err:%d/n", WSAGetLastError());
                exit(0);
            }
            nRet = WSANtohs(hSock,
                    stSrcAddr.sin_family,
                    &nPort);
            if (nRet == SOCKET_ERROR)
            {
                printf("WSANtohs() failed, Err:%d/n", WSAGetLastError());
                exit(0);
            }
           
            printf("received %d bytes from %s, port %d:%s/r/n",
                cbRet, achAddr[0] ? achAddr : "??", nPort, achInBuf);
        }
    }
   
    closesocket(hNewSock);
    closesocket(hSock);
    WSACleanup();
    return 0;
}