How to pass user-defined data to a worker thread using IOCP?

Question

Hey... I created a small test server using I/O completion ports and winsock. I can successfully connect and associate a socket handle with the completion port. But I don´t know how to pass user-defined data-structures into the wroker thread...

What I´ve tried so far was passing a user-structure as (ULONG_PTR)&structure as the Completion Key in the association-call of CreateIoCompletionPort() But that did not work.

Now I tried defining my own OVERLAPPED-structure and using CONTAINING_RECORD() as described here http://msdn.microsoft.com/en-us/magazine/cc302334.aspx and http://msdn.microsoft.com/en-us/magazine/bb985148.aspx. But that does not work, too. (I get freaky values for the contents of pHelper)

So my Question is: How can I pass data to the worker thread using WSARecv(), GetQueuedCompletionStatus() and the Completion packet or the OVERLAPPED-strucutre?

EDIT: How can I successfully transmit "per-connection-data"?... It seems like I got the art of doing it (like explained in the two links above) wrong.

Here goes my code: (Yes, its ugly and its only TEST-code)

struct helper
    {
        SOCKET m_sock;
        unsigned int m_key;
        OVERLAPPED over;
    };


///////

SOCKET newSock = INVALID_SOCKET;
    WSABUF wsabuffer;
    char cbuf[250];
    wsabuffer.buf = cbuf;
    wsabuffer.len = 250;
    DWORD flags, bytesrecvd;


    while(true)
    {
        newSock = accept(AcceptorSock, NULL, NULL);
        if(newSock == INVALID_SOCKET)
            ErrorAbort("could not accept a connection");

        //associate socket with the CP
        if(CreateIoCompletionPort((HANDLE)newSock, hCompletionPort, 3,0) != hCompletionPort)
            ErrorAbort("Wrong port associated with the connection");
        else
            cout << "New Connection made and associated\n";

        helper* pHelper = new helper;
        pHelper->m_key = 3;
        pHelper->m_sock = newSock;
        memset(&(pHelper->over), 0, sizeof(OVERLAPPED));
        flags = 0;
        bytesrecvd = 0;

        if(WSARecv(newSock, &wsabuffer, 1, NULL, &flags, (OVERLAPPED*)pHelper, NULL) != 0)
        {
            if(WSAGetLastError() != WSA_IO_PENDING)
                ErrorAbort("WSARecv didnt work");
        }
    }

    //Cleanup
    CloseHandle(hCompletionPort);
    cin.get();
    return 0;
}

DWORD WINAPI ThreadProc(HANDLE h)
{
    DWORD dwNumberOfBytes = 0;
    OVERLAPPED* pOver = nullptr;
    helper* pHelper = nullptr;
    WSABUF RecvBuf;
    char cBuffer[250];
    RecvBuf.buf = cBuffer;
    RecvBuf.len = 250;
    DWORD dwRecvBytes = 0;
    DWORD dwFlags = 0;
    ULONG_PTR Key = 0;

    GetQueuedCompletionStatus(h, &dwNumberOfBytes, &Key, &pOver, INFINITE);

    //Extract helper
    pHelper = (helper*)CONTAINING_RECORD(pOver, helper, over);


    cout << "Received Overlapped item" << endl;
    if(WSARecv(pHelper->m_sock, &RecvBuf, 1, &dwRecvBytes, &dwFlags, pOver, NULL) != 0)
        cout << "Could not receive data\n";
    else
        cout << "Data Received: " << RecvBuf.buf << endl;

    ExitThread(0);
}

Answer 1

You can send special-purpose data of your own to the completion port via PostQueuedCompletionStatus.

The I/O completion packet will satisfy an outstanding call to the GetQueuedCompletionStatus function. This function returns with the three values passed as the second, third, and fourth parameters of the call to PostQueuedCompletionStatus. The system does not use or validate these values. In particular, the lpOverlapped parameter need not point to an OVERLAPPED structure.