[Haskell-beginners] How do I marshall a pointer over SendMessage LPARAM or WPARAM?

Brent Yorgey byorgey at seas.upenn.edu
Wed Jul 18 17:47:40 CEST 2012


Just a meta-comment: this doesn't seem like a beginner question to
me. =) Perhaps someone on this list will know the answer (and there's
nothing wrong with asking), but for such a specific question you may
have better luck posting to haskell-cafe or StackOverflow.


On Wed, Jul 18, 2012 at 03:14:46PM +0200, Simon Peter Nicholls wrote:
> I'm new to Haskell, and have had some good success with FFI so far,
> but using Win32's sendMessage to send a pointer in LPARAM or WPARAM is
> resulting in access violations at the other end.
> Is there some issue with my pointer conversions? Am I hitting some
> restriction, or missing some compiler options?
>  It's driving me pretty crazy, after a very nice start to using Haskell.
> Some "sending" code:
>             Foreign.C.String.withCWString "frustrator" $ \s -> do
>                 let wParam = System.Win32.Types.castPtrToUINT s ::
> System.Win32.Types.WPARAM
>                 Graphics.Win32.sendMessage wnd Graphics.Win32.wM_APP wParam 0
> wndProc "receiving" code:
>     | wmsg == Graphics.Win32.wM_APP = do
>         s <- peekCWString $ System.Win32.Types.castUINTToPtr wParam
>         putStrLn s
>         return 0
> The string will not be seen.
> Some extra notes:
> I can get wndProc messages and integral data generally.
> The pointer values match textually at both ends when "shown" to stdout.
> At the sending side I can pass the CWString to a regular FFI function
> call just fine, and castUINTToPtr will give me back a functioning Ptr
> for that call.
>  I have also tried sending to & receiving from a working C++ program,
> without success. Access violations are reported when receiving, though
> again the address matches up. Silence from Haskell as before, when C++
> is sending.
> I found someone else having an issue here:
> http://osdir.com/ml/haskell-cafe@haskell.org/2009-11/msg00731.html but
> no solution unfortunately.
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