Re: LINKIMAGE problem (beginner)

[Nigel Wade <nmw(at)ion.le.ac.uk>]

Reinhold Schaaf <Kakadu42@gmx.net> writes:

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: Hi there,
: 
: I want an IDL program call C subroutines. Since the CALL_EXTERNAL
: mechanism seems to provide no possiblities for typechecking (?) I want
: to use LINKIMAGE, where typechecking can (and must) be done. Everything
: is pretty nasty (I really don't like pointers on pointers on pointers
: ...), but anyway.
: 
: What I could not find out by myself is:
: 
: How can I create (in my C program) a (IDL-) scalar variable of any type,
: that is set to something and returned to IDL. I tried something like:
: 
: #include "export.h"
: #include "Linktest.h"
: 
: IDL_VPTR margins(int argc, IDL_VPTR argv[])
: {
: 
:   IDL_VPTR pvResult = IDL_Gettmp();
:   pvResult = IDL_CvtDbl(1, &pvResult);
:   return(pvResult);
: 
: }
: 
: 
: which should (in my understanding) create a temporary scalar of type
: undefined, convert it to double, and return it back to IDL.
: 
: However, after linking everything in with
: 
: LINKIMAGE, 'MARGINS', 'Linktest.dll', 1, 'margins', $
:              MAX_ARGS=0, MIN_ARGS=0
: 
: (I am working under NT)
: 
: and calling the function with
: 
: x = margins()
: 
: IDL responds:
: 
: % MARGINS: Variable is undefined: <UNDEFINED>.
: % Execution halted at:  $MAIN$
: 
: 
: I tried some variants of the above, none worked.
: 
: Any help?
: 
: 
: Thanks
: 
: Reinhold
: 

What your program does is to first allocate a variable (IDL_Gettmp) and
then attempt to convert that variable to double (IDL_CvtDbl).  Is that
what you really intended?

There are two problems with the code as it stands.  First, you attempt to 
convert a variable which is undefined (the temporary variable returned by 
IDL_Gettmp is undefined).  Second, you leave the temporary variable dangling 
when you assign the converted value to the variable result.  
IDL will tidy this up when the routine returns, but it is not good practice.

The reason that IDL is reporting the value of MARGINS as UNDEFINED is that
the value of the result which you return has no type, it is simply converted
from the undefined result of IDL_Gettmp.

If you want to make this simple example work then you can change the code
to something along the lines of:

#include "export.h"

IDL_VPTR margins(int argc, IDL_VPTR argv[]) {

   IDL_VPTR dummy = IDL_Gettmp();
   IDL_VPTR result;

   dummy->type = IDL_TYP_LONG;
   dummy->value.l = 123;

   result = IDL_CvtDbl(1,&dummy);
   IDL_Deltmp(dummy);

   return(result);
}

or, to convert an input argument

#include "export.h"

IDL_VPTR margins(int argc, IDL_VPTR argv[]) {

   IDL_VPTR result;

   if ( argc == 1 )
     result = IDL_CvtDbl(argc, argv);
   else {
     result = IDL_Gettmp();
     result->type = IDL_TYP_DOUBLE;
     result->value.d = -1.0;
   }

   return(result);
}


LINKIMAGE is a flexible method for interfacing to external code,
but you have to be sure you know what you are doing as it is very
easy to crash IDL.

-- 
-----------------------------------------------------------
Nigel Wade, System Administrator, Space Plasma Physics Group,
            University of Leicester, Leicester, LE1 7RH, UK 
E-mail :    nmw@ion.le.ac.uk 
Phone :     +44 (0)116 2523568, Fax : +44 (0)116 2523555