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Re: Returning A Variable Length struct to IDL from C [message #27856 is a reply to message #27675]

Mon, 05 November 2001 18:16

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

Greetings--

K Banerjee <kbanerje2@home.com> writes:

> The next two lines are used to create the return value to IDL:
>
> void *psDef = IDL_MakeStruct(NULL, vbHeaderTags);
> IDL_VPTR ivReturn = IDL_ImportArray(1, ilDims, IDL_TYP_STRUCT,
> (UCHAR *) theHeaderActual, releaseMemory, psDef);

Here is your first problem. IDL_ImportArray only works when you
import static data, not dynamically allocated. Basically ImportArray
only works once per piece of memory per IDL session.

> IDL_STRUCT_TAG_DEF vbHeaderTags[] =
> {
> {"VERS", 0, (void *) IDL_TYP_STRING},
> {"USERHEADER", dims_user_header, (void *) IDL_TYP_STRING},
> {"BYTEOFFSET", 0, (void *) IDL_TYP_LONG},
> {0}
> };

Here (in my opinion) is your second problem: trying to do something
too complicated within a DLM function. I personally think that while
it's not impossible, trying to manipulate complex data structures
within a DLM are very *close* to impossible and are really
unnecessary.

In my opinion, the proper approach is to have your DLM function do the
dirty work, and construct only the simplest of data, and then have a
wrapper function, written in IDL itself, compose it into something
more complex. What that means in this case, is to use positional or
keyword arguments to return three variables (VERS, USERHEADER [an
array of IDL strings], and BYTEOFFSET). Then in your wrapper function
you can easily compose these variables into a structure.

Even returning simple data values is not particularly simple, at least
not in my experience. Be sure you test your program for memory leaks
afterwards.

Craig

--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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[Message index]

		Returning A Variable Length struct to IDL from C By: K Banerjee on Mon, 05 November 2001 11:05
		Re: Returning A Variable Length struct to IDL from C By: Nigel Wade on Wed, 07 November 2001 02:00
		Re: Returning A Variable Length struct to IDL from C By: Craig Markwardt on Tue, 06 November 2001 08:28
		Re: Returning A Variable Length struct to IDL from C By: Richard Younger on Tue, 06 November 2001 08:27
		Re: Returning A Variable Length struct to IDL from C By: Richard Younger on Tue, 06 November 2001 07:58
		Re: Returning A Variable Length struct to IDL from C By: Nigel Wade on Tue, 06 November 2001 05:36
		Re: Returning A Variable Length struct to IDL from C By: Nigel Wade on Tue, 06 November 2001 04:22
		Re: Returning A Variable Length struct to IDL from C By: Craig Markwardt on Mon, 05 November 2001 18:16

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