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Re: CALL_EXTERNAL puzzle (still) ?

After trying to follow this discussion, there are a few comments which
might be helpfull to you:

> >  However, why can't I pass a pointer?
> Good question - I would have liked to be able to exploit
> pointers inside external code, but RSI has (at the moment) 
> specifically forbidden that:
>     Direct access to pointer and object reference heap 
>     variables (types IDL_TYP_PTR and IDL_TYP_OBJREF, 
>     respectively) is not allowed. 
> (External dev. guide, "Heap Variables")
> So, although the pointer is passed, you cannot do anything
> with it. Please, RSI? Why not some function  like
>         IDL_VPTR IDL_GetHeapVariable(HVID)

There is a way! At least I think :-))
When developing an intermediate DLL to control a framegrabber I ran into
the following problems:


The ID of the framegrabber was of an unknown structure (protected by the
manufacturer for future development..... sounds like RSI:-)), but I
wanted to "know" inside IDL...... and pass the pointer to that unknown
structure back into DLL for other procedures.....


I defined a pointer in a structure 

	info={DevPointer : Ptr_New()}

Beacause you cannot change it in the DLL (as RSI told us) I thought I
might just as well let it be returned by the Dll!

Then the C-code looked a bit like this:

Unknown_ID WINAPI FG_OpenDevice( LONG lArgc, LPVOID lpvArgv)
	LPLONG				lplArgv = NULL;
	CHAR				szMsg[256];

	lplArgv				= (LPLONG)lpvArgv;

    /* Try to open the device */

if (OpenDevice(&DevicePointer) != 0) {
	return -200;
	return DevicePointer;

where the Unknown_ID was that protected structure. The IDL code to get
the DevPointer was like:

Ptr_Free, info.DevPointer
info.DevPointer = Ptr_New(Call_External('idl_fg.dll', 'FG_OpenDevice'))

When wanting to pass the DevPointer as a pointer to the DLL (as I didn't
know the structure of the Unknown_ID this was the only way) I passed the
dereferenced pointer !!!!by reference!!!! like

lResult = Call_External('idl_fg.dll', 'FG_SomeProcedure',
(*(info.DevPointer)), $

Then IDL passes in fact the pointer to the DLL, just what I wanted!!

> >And if I want to pass a pointer, and
> >print the value of another pointer just before the CALL_EXTERNAL, why is the
> >wrong one passed?
> These are how your "bad luck" arose, apparently. Probably the
> print statement caused the text to be stored right after the
> space where the first pointer was located? Expect *no* 
> consistency from platform to platform, or IDL version to IDL
> version!

When developing the application where I used the above mentioned
technique, at one point everything seemed to work perfectly! Foolproof
so to say. The only thing was that when I started the application for
the second time in one IDL session, it crashed. Restarting IDL made it
work again! After three nights of how, why, hell etc... I found out that
some akward ID of a widget! was used for the ID of a framebuffer and
fortunately (or unfortunately:-((( whichever way you look at it) these
where the same when starting a fresh IDL session!!! The message: if
things work, it is not always so that things are running the way you
think they do!

Hopefully all this is not to much (crap), and maybe a bit usefull.

A last note (It's almost weekend:-):

When passing (BOLD) structures (BOLD) by reference, IDL does not pass
the normal pointer to the DLL. In fact it makes a copy of the structure
and passes the pointer to that copy into the operand. Therefore only the
copy is changed and not the original! A workaround is to make a copy
yourself before passing, and assigning the (altered) copy back to the
structure after your DLL-call. Example:

tmp_RefVolt = (*ptr).RefVolt ; Here I expect a value

Result = Call_External('idl_fg.dll', 'FG_GetRefVoltage', $
        (*ptr).InputSource, tmp_RefVolt, Value=[1b,0b])

(*ptr).RefVolt = tmp_RefVolt

In these cases it is again better to let the DLL return the wanted
values as shown above. When passing dereferenced pointers by refernce
thing work as expected so e.g.


Ptr_Free, info.image

lResult = Call_External('idl_fg.dll', 'FG_GetFrame', $
		(*(info.Image)), Value=[0b])

fills the array with the captured image (provided the DLL is written

Enough for now, have a nice weekend!


Armand J.L. Jongen                               Academic Medical Centre
                                                 Laser Centre
Phone   +31-20-5667419            \\||||//       Meibergdreef 9
Fax     +31-20-6975594            | ~  ~ |       1105 AZ  Amsterdam
E-mail  a.j.jongen@amc.uva.nl    [| o  o |]      The Netherlands