[python-win32] USB CDC-class support with win32file, exception processing issues

[Mark Hammond]

I'm afraid I don't have many insights.  Using overlapped IO may solve your
problem of not knowing if an error occurred or not, but I don't think it
will solve your general problem.  One thing that might be worth looking at
are "device notifications", which may be able to inform you of the removal -
see win32\Demos\win32gui_devicenotify.py in build 211 for an example (I
think that only will see mass-storage devices, but if you dig around MSDN
you may have luck)

Cheers,

Mark


> -----Original Message-----
> From: python-win32-bounces at python.org [mailto:python-win32-
> bounces at python.org] On Behalf Of Ron Jackson
> Sent: Tuesday, 3 June 2008 8:30 AM
> To: python-win32 at python.org
> Subject: [python-win32] USB CDC-class support with win32file, exception
> processing issues
> 
> When USB communication device class devices are installed in Windows
> they appear as COMxxx ports, just like legacy serial ports. They are
> different in that many RS-232 device parameters don't apply (such as
> baud rate, handshaking, etc). Also, since they are USB devices they
> may be suspended while remaining connected (as when the computer goes
> into sleep mode), or may be unplugged at any time.
> 
> I am creating a simple Python communications library to support
> USB-CDC devices, ignoring the unused legacy features but adding
> suspend and disconnect support. I have found some problems with
> exception processing that is making this problematic.
> 
> win32file.WriteFile works well. If the device is suspended I get
> pywintypes.error (31, 'WriteFile', 'A device attached to the system is
> not functioning.'). If the device is unplugged I get pywintypes.error
> (1167, 'WriteFile', 'The device is not connected.').
> 
> I can find out if there are any characters in the input buffer using
> win32file.ClearCommError. Its exception reporting half-way works. If
> the device is suspended I get pywintypes.error (31, 'ClearCommError',
> 'A device attached to the system is not functioning.'). But if the
> device is unplugged I get no error reported at all.
> 
> win32file.ReadFile provides neither of these errors. Either suspended
> or unplugged just causes ReadFile to hang. I can get ReadFile to
> return if I set a timeout, but no error is reported so I don't know if
> it returned because the device hasn't sent the data yet, or because
> the device is suspended or disconnected.
> 
> Is there some way to get ReadFile to report errors 31 and 1167 like
> WriteFile does? Or is there some other way to determine port status as
> "attached but not functioning" or "not connected"?
> 
> Thanks for the help!
> 
>   -- Ron
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