Re: dynamic memory in call_external

[David Foster <foster(at)bial1.ucsd.edu>]

Mark Rivers wrote:
> 
> In article <88lnirufne.fsf@catspaw.jpl.nasa.gov>, Vapuser <vapuser@catspaw.jpl.nasa.gov> writes:
> >
> >
> >I have someone in my office who wants to know:
> >
> > Is it advisable to create and destroy memory within a CALL_EXTERNAL
> >routine?  (that is, can one safely use malloc and free?) Or must one
> >make the routine(s) in question LINKIMAGE routines and the idl memory
> >management routines (IDL_MEMAlloc, IDL_MEMfree and IDL_GetScratch)
> >available in that environment.
> 
> No, you should not create and destroy memory within CAll_EXTERNAL.  The reason
> is that when you use CALL_EXTERNAL you are passed only the address of the data
> storage part of the IDL variable.  You are not passed other important pieces of
> information for that variable, such as how big it is, what the data type is,
> etc.  If you create and destroy memory you will only change the pointer, but
> not the other descriptive information.  It might work OK if you are sure you
> won't change the size or type of the IDL variable, but I would not bet on it.
> 

I think we should be clear what we are talking about. I believe the
question was whether one can safely use malloc() and free() to
allocate and free memory *within* a CALL_EXTERNAL module, and the
answer to this is yes. But you cannot allocate memory for variables
that you intend to pass back to IDL (the well-known rule that you
have to allocate all arguments before passing them to CALL_EXTERNAL).

If we don't clarify this distinction I think people will get confused.

Dave
-- 

  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    David S. Foster         Univ. of California, San Diego
     Programmer/Analyst     Brain Image Analysis Laboratory
     foster@bial1.ucsd.edu  Department of Psychiatry
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                            La Jolla, CA  92037
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