How do I regain access to a COM port after access is denied?

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C++

COM

MS-Access

Here is some code which simply opens a com port and should allow me to do things with it, however it does not because the COM port returns with "Access Denied" as you can see I am trying to do everything to clear up that message for the next run in order to free up the port to no avail. The port remains frozen if some error occurs.... any idea how to free up a port, restart it or something in order to regain access to the port after an error has occurred without needing to unplug, replug it? thanks for your help!

DCB dcb;
    b->h1 = CreateFile(b->port,GENERIC_READ | GENERIC_WRITE,
    0,    // comm devices must be opened w/exclusive-access
    NULL, // no security attributes
    OPEN_EXISTING, // comm devices must use OPEN_EXISTING
    0,    // overlapped I/O
    NULL);



if (b->h1 == INVALID_HANDLE_VALUE)
    {
        //Major Error Need to Prepare for next run & Exit with error
        //Clear Comm Error Flag and also reset for I/O
        DWORD MsgID = GetLastError();
        char *TextSize;
        FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, 0, MsgID, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),(LPTSTR) &TextSize, 0, 0);

        DWORD dwErrorFlags;
        COMSTAT ComStat;
        int result;
        BOOL fSuccess2;

        ClearCommError(b->h1, &dwErrorFlags, &ComStat );
        ClearCommBreak(b->h1);

        //  Fill in some DCB values and set the com state:
        //  57,600 bps, 8 data bits, no parity, and 1 stop bit.
        dcb.BaudRate = b->boud;     // set the baud rate
        dcb.ByteSize = 8;             // data size, xmit, and rcv
        dcb.Parity = NOPARITY;        // no parity bit
        dcb.StopBits = ONESTOPBIT;    // one stop bit
        SetCommState(b->h1, &dcb);
        //Port is still open so try to restart the comm port with those new settings above.
        fSuccess2 = SetCommState(b->h1, &dcb);
        //Do a complete purge of the comm now.
        PurgeComm(b->h1, PURGE_TXABORT | PURGE_RXABORT | PURGE_TXCLEAR | PURGE_RXCLEAR);
        //Now Close Port
        CloseHandle(b->h1);
        b->h1 = NULL;
        b = NULL;
        a = NULL;
        hSock = NULL;
        hSock2 = NULL;
        ComSock.Close();
        ClosePort();

        return 2;

    }

[EDIT Jochen Arndt: Copied from solution]

The idea is to be able to "Force" close the port, the problem is another thread may have the port open and I want to close it, without having access to the other instance that encountered an error.

For example, someone might pull out the chord during transmission, if we encounter the error, now we want to clear up the port, so everything would be fine on the next run, I am specifically interested in these functions as they seem to help with this process but they didnt do much to resolve the "Access Denied" error:

ClearCommError(b->h1, &dwErrorFlags, &ComStat );
ClearCommBreak(b->h1);
PurgeComm(b->h1, PURGE_TXABORT | PURGE_RXABORT | PURGE_TXCLEAR | PURGE_RXCLEAR);

Another option I looked at while investigating was the use of DevCon.exe which is not what I am looking for, I am specifically looking to restart the com port using code, not plugging in & unplugging the device, etc....

Thanks for your help & advice!