How to detect the multicast group address?

I have a problem with UDP Multicast. I managed to send UDP Multicast telegrams and receive them. But when I receive the telegram I need to find out to which multicast group address it was sent. So far I have:

Code:

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recvfrom( hSocket,
        pcData,
        nMaxDataLen,
        0,
        (SOCKADDR*) &from,
        &len);

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When I read from.sin_addr.S_un.S_addr; then I get the original IP Adresse from the sender PC but I also would like to know to which group address it was sent. Is there a way to get this infoamtion?

1) it is "datagram" not "telegram" :)
2) As far as I know, the receiver can't know what the sender used to do the multicast, so if this is critical information, you need to make this part of your protocol.

I can't make it part of my protocol because it's a 3rd party software from where I receive the datagram. I have read through the internet and sometime I found in some forums the function WSARecvMsg where some people say that I could get these information from if I would use this:

Code:

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int nEnable = 1;
setsockopt(hTmpSocket, IPPROTO_IP, IP_PKTINFO, (char FAR*)&nEnable, sizeof(nEnable));

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But my compiler (VS2010) says the function WSARecvMsg is unknown even though I included Mswsock.h

Quote:

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Originally Posted by grka

...
But my compiler (VS2010) says the function WSARecvMsg is unknown even though I included Mswsock.h

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From MSDN article WSARecvMsg:

Quote:

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Note The function pointer for the WSARecvMsg function must be obtained at run time by making a call to the WSAIoctl function with the SIO_GET_EXTENSION_FUNCTION_POINTER opcode specified. The input buffer passed to the WSAIoctl function must contain WSAID_WSARECVMSG, a globally unique identifier (GUID) whose value identifies the WSARecvMsg extension function. On success, the output returned by the WSAIoctl function contains a pointer to the WSARecvMsg function. The WSAID_WSARECVMSG GUID is defined in the Mswsock.h header file.

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