Bug in Win32file WaitCommEvent ???

[Mark Hammond]

Grant Edwards wrote:
> I'm trying to figure out how WaitCommEvent() works in Python,
> and it looks like there's a problem when it's used in
> overlapped mode.
> 
> My understanding is that (in C) you:
> 
>  1) Call WaitCommEvent() and give it three parameters:
>      * File handle
>      * a pointer to where the output mask value is to be stored
>      * a pointer to an overlapped struct
>     
>  2) WaitCommEvent() returns immediately.
> 
>  3) Call one of the WaitXXX() functions to wait on the event in
>     the overloapped struct.
> 
>  4) When the event is triggered WaitXXX() returns. Check the
>     value stored via the output mask pointer passed in 1)
>     above.
> 
> [I've gleaned most of this from
> http://msdn.microsoft.com/library/en-us/devio/base/waitcommevent.asp]

I don't think this last step is correct.  Certainly the link you posted 
doesn't say this is true.  Looking further, I can't find *anything* that 
implies this is true.

KB Q175551 has sample code for Windows CE that uses overlapped IO, and 
it examines the mask immediately after the call.

Further, if what you say is correct, I would expect Python COM port 
based programs to regularly crash!  In your scenario, by the time the 
overlapped function ended up writing to our mask variable, that address 
would be invalid.  However, it would be pretty close to the top of the 
stack, so I would expect it to corrupt whatever was running when the 
mask was actually written.  I know a number of people have used these 
APIs to do serious serial port control - either directly, or via another 
wrapper.  I would be very surprised we survived this long.

Can you find an explicit reference to this?

Mark.