GetQueuedCompletionStatus initial WSARecv with 0 lpNumberOfBytesRecvd issue

Question

I am reading about IOCP through this article

http://www.winsocketdotnetworkprogramming.com/winsock2programming/winsock2advancediomethod5i.html

This first WSARecv function he used with 0 lpNumberOfBytesRecvd to initiate the completion ports

if (WSARecv(Accept, &(PerIoData->DataBuf), 1, &RecvBytes, &Flags, &(PerIoData->Overlapped), NULL) == SOCKET_ERROR)

            {

                        if (WSAGetLastError() != ERROR_IO_PENDING)

                        {

                                    printf("WSARecv() failed with error %d\n", WSAGetLastError());

                                    return 1;

                        }

            }

            else

                        printf("WSARecv() is OK!\n");

}

Now on the worker threads function where the application receives the IO post

it checks parameter lpNumberOfBytes of GetQueuedCompletionStatus for 0 also and then it closes the connection if it is

// First check to see if an error has occurred on the socket and if so

                        // then close the socket and cleanup the SOCKET_INFORMATION structure

                        // associated with the socket

                        if (BytesTransferred == 0)

When i tried it the first IOCP message posted the function GetQueuedCompletionStatus it always returning 0 in lpNumberOfBytes parameter because first WSARecv call was called with lpNumberOfBytesRecvd set to zero

I have tried to implement this code and i am having troubles to get the code working unless i choose to not use NULL in this parameter in the first call to WSARecv so that the GetQueuedCompletionStatus function returning value 0 in lpNumberOfBytes can be used as in case of all data recieved.

MSDN says to use NULL

lpNumberOfBytesRecvd [out]

A pointer to the number, in bytes, of data received by this call if the receive operation completes immediately.

Use NULL for this parameter if the lpOverlapped parameter is not NULL to avoid potentially erroneous results. This parameter can be NULL only if the lpOverlapped parameter is not NULL.

The code is not working well if i call WSARecv again (inside worker threads function) and all data were recieved this causes the GetQueuedCompletionStatus function to return immediatly without waiting even if INFINITE used and the worker threads enter in endless loop. No errors ever returned by GetQueuedCompletionStatus function it just acts as if there is more data but there is none.

WSARecv needs to be initiated with what value at buf size?

also should i use non blocking socket? or just normal blocking sockets since i use separate thread to accept connections so i think its fine because it won't freeze my GUI.