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

Sylvain HENRY hsyl20 at gmail.com
Wed Jul 18 17:56:43 CEST 2012


[As I've just registered to the list, I don't have the original message. 
That's why I answer to Brent insted of Simon]

By using "withCWString", your CWString may be freed before the message 
is sent. Use "newCWString" instead and "free" in your receiving code.


Le 18/07/2012 17:47, Brent Yorgey a écrit :
> Hi,
> 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.
> -Brent
> 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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