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

[Simon Peter Nicholls]

I've tried that without joy. Have reposted over at Haskell cafe, as
per Brent's advice. Thanks to both of you.

On Wed, Jul 18, 2012 at 5:56 PM, Sylvain HENRY <hsyl20 at gmail.com> wrote:
> Hi,
>
> [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.
>
> Cheers
> Sylvain
>
> 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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>>
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