while( 1 ) {
  sprintf(portName, "COM%d", portNumber);
  h = CreateFile(portName,
                    GENERIC_READ | GENERIC_WRITE,
                     0 ,    // must be opened with exclusive-access
                    NULL, // no security attributes
                    OPEN_EXISTING, // must use OPEN_EXISTING
                     0 ,    // not overlapped I/O
                    NULL  // hTemplate must be NULL for comm devices
                    );
  if (h == INVALID_HANDLE_VALUE) {
    portNumber--;
    break;
  } else {
    CloseHandle(h);
    portNumber++;
  }
}