InstallComCallback on Win7

I seem to have found a slight issue using InstallComCallback() between WinXP and Win7 target machines.

I developed my initial software on WinXP.  The following two bits of code worked fine in WinXP with an eventMask = LWRS_RXFLAG | LWRS_RECEIVE.  But when I distributed the final software to Win7 machines, I was observing the callback getting called twice each time.

Here's my callback:

// Callback scanner COM function
void BarcodeScanner(int scannerPortNumber, int eventMask, void *callbackdata)
{
	// re-init the output buffer
	memset(scannerData,0,SCANNER_MAX_BYTES);
	// go get the buffer
	ComRdTerm(scannerPortNumber,scannerData,SCANNER_MAX_BYTES,SCANNER_EOS);

	// raise the data flag
	scannerDataFlag = TRUE;

	// empty the buffer
	FlushInQ(scannerPortNumber);
}

And here's my use of InstallComCallback():

void InitScannerComm(void)
{
	int notifyCount = SCANNER_MIN_BYTES;	
	int eventChar = SCANNER_EOS; 	// terminator charactor
	//int eventMask = LWRS_RXFLAG | LWRS_RECEIVE;	// callback event won't happen unless the eventChar is received after notifyCount characters
	int eventMask = LWRS_RXFLAG;	// TODO: the InstallComCallback() function seems to be calling the callback twice each time, even though I get a verified buffer of only N number of bytes
	int dataBits = SCANNER_DATABITS;
	int stopBits = SCANNER_STOPBITS;
	int inputQueueSize = SCANNER_MAX_BYTES;
	int parity = SCANNER_PARITY;
	int comportRing = 0;
	int i;
	// close port if it's already open:
	CloseCom(scannerPortNumber);
	// open COM port one time
	OpenComConfig(scannerPortNumber,scannerDeviceName,scannerBaudRate,parity,dataBits,stopBits,inputQueueSize,-1);

	// Make sure Serial buffers are empty
	FlushInQ(scannerPortNumber);
	FlushOutQ(scannerPortNumber);
	
	// look up how many COM options there are in the control
	GetNumListItems(tabPage2Handle,TABPANEL2_COMPORT,&comportRing);
	// set up callback function for COM
	for (i=1; i<=comportRing; i++)
	{
		if (i != scannerPortNumber)
			// disable all the other callbacks
			InstallComCallback(i, 0, notifyCount, eventChar, BarcodeScanner, NULL);
		else
			// enable the port that's been selected
			InstallComCallback(i, eventMask, notifyCount, eventChar, BarcodeScanner, NULL);
	}
}

A bit of background... the device sending COM traffic is a barcode scanner from Motorola/Symbol.  I have it set up in a virtual COM mode.  Using RealTerm in hex display mode, I can confirm that this hardware is set up correctly, i.e., it is sending what it should be (a string in the form: "012345" with a CR at the end).

And I know that the string is correct in that the callback gets called.  The puzzling thing is that in Win7 only, the callback gets called twice and then it's done.  The second time through, the string is all nulls.

My fix was to eliminate the LWRS_RECEIVE option in the eventMask (i.e., remove the notifyCount threshold).  Phew!

Is this a bug?