Question

I have an application wherein serial I/O is conducted with an attached USB device via a virtual COM port. When surprise removal of the device is detected, what would be the best way to stop the serial I/O. Should I simply close the port? Or, should there be a global variable, which is maintained to indicate the presence of the device, that should be checked in each serial I/O function prior to attempting to transmit/receive data? Or, should it be a combination of the two, or something else? Thanks.

Answer 1

I’m assuming you are running Windows.
This depends on how you have designed your communication flow.

I have a BasePort object where I have derived a COMPort object (and many other communication objects). The COMPort object creates one TXThread and RXThread class. These threads are waiting for the “OVERLAP” to signal that the read or write operation finished with WaitForMultipleObjects().

The TXThreads goes to sleep if there is nothing to do and wakes up by the TXWrite function (the data between main process and thread goes through a trhead safe FIFO buffer).

In this case they also need to wait for an event signal that the port has closed, so they actually can cancel any pending operations and exit (the treads exits and gets deleted).

To detect if the USB port is connectd/disconneted I listen for the Windows message DEVICE_CHANGE. If the port is disconnected I set the event and waits for the threads to exit before the Port class deletes and closes the port.

I have found this approach very reliable and safe. It’s the core in a communication platform I designed for over 8 years ago and still kicking.