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

[Simon Peter Nicholls]

Actually, I tried that first of all, using a COPYDATASTRUCT Storable.
When things didn't go smoothly, I tried to narrow down the problem by
direct passing of the CWString data.

However, this point about address space is a good one. In the link
below, it suggests that WM_COPYDATA is treated as a special case,
ensuring the COPYDATASTRUCT will be available in the receiving
process.

http://www.go4expert.com/forums/showthread.php?t=19730

Ah yes. Same function name, but magic stuff happening based on a magic
value. That ole ruse.

I'll go back to WM_COPYDATA for a fresh try tomorrow.

Cheers

On Wed, Jul 18, 2012 at 7:13 PM, Sylvain HENRY <hsyl20 at gmail.com> wrote:
> If you are doing inter-process communication, you should not send pointers
> as processes do not share the same address space.
>
> Use WM_COPYDATA to send a specific amount of data.
> http://msdn.microsoft.com/en-us/library/windows/desktop/ms649011%28v=vs.85%29.aspx
>
> I've read that SendMessage is blocking, so you can use withCWString
> (contrary to what I have previously written). As you have to pass the size
> of the data, you can even use withCWStringLen (and peekCWStringLen).
>
> Cheers
> Sylvain
>
> Le 18/07/2012 19:02, Simon Peter Nicholls a écrit :
>
>> It's inter-process by design.
>>
>> I have a small "single instance " C++ program that I'm porting to
>> Haskell. Once the first instance launched is up and running,
>> subsequent launches send their command line params through to the
>> single instance (file names). Similar to when a media player queues
>> files in a play list.
>>
>> The most recent extra clue I have is that if I wrap the SendMessage
>> function with my own C FFI version, my C function can both re-cast and
>> output the CWString I have sent, and can successfully invoke the real
>> SendMessage by overriding the CWString with a newly created L"blah"
>> string. It's just a shame it can't use the original!
>>
>> My next session will involve poking around at the two strings to learn
>> why they are treated differently.
>>
>> On Wed, Jul 18, 2012 at 6:33 PM, Sylvain HENRY <hsyl20 at gmail.com> wrote:
>>>
>>> Are you sending the message to a window of the same process?
>>>
>>> -Sylvain
>>>
>>> Le 18/07/2012 18:28, Simon Peter Nicholls a écrit :
>>>
>>>> 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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