Handling varying-length opaque-type arguments

Suppose that you want to create a user-defined function named image_id() that accepts a varying-length opaque type named image (which is defined in Internal representation for the image opaque data type) and returns its integer image identifier (img_id). The following CREATE FUNCTION statement registers the image_id() function in the database:

CREATE FUNCTION image_id(arg1 image)
RETURNS INTEGER
EXTERNAL NAME '/usr/udrs/image/image.so'
LANGUAGE C;

Because image is a varying-length data type, the following declaration of image_id() specifies an mi_bitvarying pointer as the parameter data type even though the function is registered to accept a value of type image:

/* Valid C UDR declaration for varying-length opaque-type
 * parameter
 */
mi_integer image_id(image)
   mi_bitvarying *image;

The following declaration of image_id() is invalid because it specifies an image pointer as the parameter data type:

/* INVALID declaration for varying-length opaque-type
 * parameter
 */
mi_integer image_id(image)
   image_t *image;

The following code fragment shows the implementation of the image_id() function.

#include <mi.h>
#include <ctype.h>
#include <image.h>

mi_integer image_id(image)
   mi_bitvarying *image;
{
   image_t *image_ptr;

   /* Obtain pointer to image_t structure, contained
    * within the data portion of the mi_bitvarying 
    * structure.
    */
   image_ptr = (image_t *)mi_get_vardata((mi_lvarchar *)image);

   return (image_ptr->img_id);
}