Reopening serial port fails if not closed properly with CloseHandle

Question

I am working with USB device on Windows that is seen as a virtual serial port. I can communicate with the device using CreateFile and ReadFile functions, but in some cases my application does not call CloseHandle (when my application in development crashes). After that all calls to CreateFile fail (ERROR_ACCESS_DENIED), and the only solution is to log in to my computer again. Is there any way to force closing the open handle (or reopening) programmatically?

Answer 1

This is certainly not normal. Windows automatically closes any handles that are left open after the process terminates. This must be a flaw in your USB device driver, although it is hard to see how it could mess this up. The ones that emulate serial ports are however notoriously lousy. Well, nothing much you can do but hope for a driver update from the manufacturer. Or a device from another manufacturer.

Answer 2

I agree with both previous posts.

1. This is not a normal situation.
2. Unplugging the USB device usually helps.

This problem is related to the glitches in the FTDI driver, which is responsible for implementing a virtual COM port. On the other hand those "glitches" are related to various malfunctions of the USB devices. (Of course this doesn't justify the FTDI driver).

BTW there're several other known problems with some FTDI drivers:

• Sometimes call to CloseHandle just hangs the calling thread.
• Sometimes also the application is still "visible" in the task manager, even after it's closed. Task manager can't terminate the application, and the debugger can't be attached to it. Its EXE file is locked (can't be erased).

Usually unplugging the USB device immediately helps in those situations. The FTDI driver, which seems to be "waiting for something" awakes.