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IDL

Passing Structures

Passing Structures

IDL structure variables are stored in memory in the same layout that C uses. This makes it possible to pass IDL structure variables into CALL_EXTERNAL routines, as long as the layout of the IDL structure is known. To access an IDL structure from an external routine, you must create a C structure definition that has the exact same layout as the IDL structure you want to process.

For example, for an IDL structure defined as follows:

s = {ASTRUCTURE,zero:0B,one:0L,two:0.0,three:0D,four: intarr(2)}

the corresponding C structure would look like the following:

typedef struct { unsigned char zero; IDL_LONG one;
   float two; double three; short four[2];
} ASTRUCTURE;

Then, cast the pointer from argv to the structure type, as follows:

ASTRUCTURE* mystructure;
mystructure = (ASTRUCTURE*) argv[0];

The following routine, found in incr_struct.c, increments each field of an IDL structure of type ASTRUCTURE. This is implemented as a function with a natural C interface, and a second glue routine that implements the IDL portable convention, using the one with the natural interface to do the actual work:

 

#include <stdio.h>
#include "idl_export.h"
  /*
  * C definition for the structure that this routine accepts.The
  * corresponding IDL structure definition would look like this:
  *	s = {zero:0B,one:0L,two:0.,three:0D,four: intarr(2)}
  */
  typedef struct { unsigned char zero; IDL_LONG one;
  float two; double three; short four[2];
  } ASTRUCTURE;
  int incr_struct_natural(ASTRUCTURE *mystructure, IDL_LONG n)
  {
  /* for each structure in the array, increment every field */
  for (; n--; mystructure++) { mystructure->zero++; mystructure->one++; mystructure->two++; mystructure->three++; mystructure->four[0]++; mystructure->four[1]++;
  }
  return 1;
  }
  int incr_struct(int argc, void *argv[])
  {
  if (argc != 2) return 0;
  return incr_struct_natural((ASTRUCTURE*) argv[0], (IDL_LONG)
  argv[1]);
}

It is not possible to access structures with arbitrary definitions using the CALL_EXTERNAL interface. The system routine interface, discussed in Adding System Routines, does provide support for determining the layout of a structure at runtime.



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