[python-win32] win32com support for pointer in-parameters

[Mark Hammond]

Yeah, pywin32 will see a byref ui1 and pass the address of an integer 
filled with the value of the integer objects.  However, I'm not really 
sure what you are expecting this to do - passing the address of a Python 
integer object clearly isn't correct - or to put it another way - what 
exact are you hoping to pass as this parameter?

We could probably special-case VT_BYREF | VT_UI1 when a Python "buffer" 
object is passed and pass the address of that buffer - this already 
happens for a VT_ARRAY | VT_UI1 - but whether that would solve your 
problem depends on how you actually intend using that param...

Mark.

On 29/05/2013 4:26 AM, Jeremy Kloth wrote:
> I have been struggling with getting pointer parameters to work with the
> COM interface for VirtualBox.  In particular, the IFramebuffer
> interface.  I cannot seem to get the following to give the expected results:
>
> interface IFramebuffer : IDispatch
> {
>      // snipped to just the interesting definition
>      HRESULT RequestResize (
>          [in] ULONG aScreenId,
>          [in] ULONG aPixelFormat,
>          [in] BYTE * aVRAM,
>          [in] ULONG aBitsPerPixel,
>          [in] ULONG aBytesPerLine,
>          [in] ULONG aWidth,
>          [in] ULONG aHeight,
>          [out, retval] BOOL * aFinished
>      );
> };
>
> The type for aVRAM ends up being (correctly) VT_BYREF | VT_UI1.
>   Unfortunately PyIDispatch (incorrectly) converts the parameter to just
> VT_UI1.  The offending code path is: PyGatewayBase::Invoke() ->
> invoke_setup() -> PyCom_PyObjectFromVariant() -> VariantCopyInd().
>
> My not so informed understanding is that pythoncom considers all
> VT_BYREF parameters as out-type parameters whereas, in this case at
> least, it is not always such.  Indeed, in this case, the aVRAM parameter
> needs to be the *address* not the *value* for an object to implement
> IFramebuffer properly.
>
> Note that the object implementing IFramebuffer is passed as argument to
> another function much the same way as event listeners.
>
> I believe the fix involves PyIDispatch somehow using the type
> information to determine which parameters are not out-type and then to
> not dereference those.
>
> Any direction with this is greatly appreciated.
>
> Jeremy Kloth
>
>
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