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Re: Passing Structures with Pointers with Call_External [message #40545 is a reply to message #40456]

Wed, 11 August 2004 16:44

Peter Mason
Messages: 145
Registered: June 1996

Senior Member

PeterOut wrote:
...
> Just a follow up question I thought of. Is it possible to return a C
> structure with pointers to IDL and have IDL interpret it correctly?

No, I can't see that working. An array in an IDL structure is "right
there" in the structure's data block; it does not hang off a
memory-pointer. (...Except for a string scalar which might be considered
to be an array of sorts but is really a special kind of thing in IDL - it
hangs off a descriptor.) If you're thinking "IDL pointer" rather than
memory-pointer then again - you can't. For some reason, RSI have not
exposed any functions to work with IDL pointers in external code, and
digging around in exports.h doesn't reveal anything interesting. (Note I
haven't got IDL6.1 yet so I don't know if this is still the case.)

I sense a certain regret in abandoning the structure approach? :-)
I imagine that you have ancillary frame information that you'd like to keep
together with the primary data, or something like that?
If you really would like to keep all this info in a single IDL structure
array then I think that you could work with it in a C routine if you don't
mind getting your hands a bit dirty. What I'm thinking here is accessing
the structure data in a raw sense. Your C CALL_EXTERNAL routine gets
passed a memory pointer to the structure array's data block and can
interpret it any way that it likes. In your particular example, you could
just cast it to an int pointer *IP. You'd have to track which frame you
were at. Given that the DATA array has to be the same size in each
structure element, you could calculate a frame-size as IFS=IP[0]*IP[1] + 2.
To get at the DATA of frame I, use a float pointer *FDATA and set it to
FDATA=(float *)(I*IFS + 2).
If you go with this approach you will have to take great care in how you
define the structure in IDL. Your example from the other day just had
LONGs and FLOATs so all elements were 4 bytes long and the structure had no
padding bytes. If the structure got more complicated then things could get
a lot messier.

Cheers
Peter Mason

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[Message index]

		Re: Passing Structures with Pointers with Call_External By: MajorSetback on Wed, 11 August 2004 07:34
		Re: Passing Structures with Pointers with Call_External By: MajorSetback on Wed, 11 August 2004 06:53
		Re: Passing Structures with Pointers with Call_External By: MajorSetback on Wed, 11 August 2004 06:53
		Re: Passing Structures with Pointers with Call_External By: Bob[2] on Wed, 11 August 2004 05:40
		Re: Passing Structures with Pointers with Call_External By: Peter Mason on Tue, 10 August 2004 16:12
		Re: Passing Structures with Pointers with Call_External By: Peter Mason on Wed, 11 August 2004 16:44

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