Importing nested structures?

[skerr(at)herma.ucsd.edu (Dr. Stephen S. Kerr)]

Dear IDL gurus,

A colleague and I are very confused about the proper use of the
IDL_MakeStruct and IDL_ImportNamedArray routines.  Specifically,
what do you do with the "type" field of an IDL_STRUCT_TAG_DEF
when structures are nested?  The IDL 4 Advanced Development Guide
is a bit vague on this point.  It says it is "either a pointer to
another structure type definition, or a simple IDL type cast to
void (e.g., (void *) IDL_TYP_BYTE)."

For a nested structure, this suggests it could either be another
IDL_STRUCT_TAG_DEF defined previously, or (void *) IDL_TYP_STRUCT.
The strange thing is, both work in IDL 4, at least for the sample
program I have appended below.  My colleague discovered that NULL
also works in IDL 5.

Does anyone know what the correct syntax is that will work in both
IDL 4 and IDL 5?

Regards,
S. Kerr

-------------------- sample program appended ------------------------

#include <stdio.h>
#include "/usr/local/rsi/idl/external/export.h"

int main(int argc, char **argv)
{
  static IDL_LONG one = 1;

  static IDL_STRUCT_TAG_DEF substruct_tags[] = {
    {"P", 0, (void *) IDL_TYP_INT},
    {"Q", 0, (void *) IDL_TYP_INT},
    {0}
  };

  static IDL_LONG matrix_dims[] = {2,2,2};
  static IDL_LONG substruct_dims[] = {1,1};
  static IDL_STRUCT_TAG_DEF s_tags[] = {
    { "MATRIX", matrix_dims, (void *) IDL_TYP_FLOAT},
    { "SUBSTRUCT", substruct_dims, substruct_tags},
    /* should we use (void *) IDL_TYP_STRUCT instead of substruct_tags? */
    {0}
  };

  typedef struct substruct {
    short int p;
    short int q;
  } SUBSTRUCT;

  typedef struct data_struct {
    float mat [2] [2];
    SUBSTRUCT sub;
  } DATA_STRUCT;

  static DATA_STRUCT s_data, *s_new_data;
  void *s;
  IDL_VPTR v;
  int i,j;

  IDL_Init(0,&argc,argv);

  s_data.mat[0][0] = s_data.mat[1][1] = 1.0;
  s_data.mat[0][1] = s_data.mat[1][0] = 0.0;
  s_data.sub.p = -999;
  s_data.sub.q = +1001;
  s = IDL_MakeStruct("FOO", s_tags);

  printf("Almost all folks.\n");

  v = IDL_ImportNamedArray("FOO",1,&one,IDL_TYP_STRUCT,
			   (UCHAR *) &s_data, 0, s);

  s_new_data = (DATA_STRUCT *) v->value.s.arr->data;
  for (i=0;i<2;i++)
    for (j=0;j<2;j++)
      printf("%f\n",s_new_data->mat[i][j]);
  printf("%i\n", s_new_data->sub.p);
  printf("%i\n", s_new_data->sub.q);

  printf("That's all folks.\n");
}

---------------------- end of sample program ------------------------------