Handling fixed-length opaque-type arguments

For example, suppose you want to define a user-defined function named circle_area() that accepts a fixed-length opaque type named circle (which is defined in Internal representation of the circle opaque data type) and computes its area. The following CREATE FUNCTION statement registers the circle_area() function in the database:

CREATE FUNCTION circle_area(arg1 circle)
RETURNS FLOAT
EXTERNAL NAME '/usr/udrs/circle/circle.so'
LANGUAGE C;

Because circle is a fixed-length data type that cannot fit into an MI_DATUM structure, the following declaration of circle_area() specifies a pointer to the internal format of circle:

/* Valid C UDR declaration for fixed-length opaque-type
 * parameter
 */
mi_double_precision *circle_area(circle_ptr)
   circle_t *circle_ptr;

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

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

mi_double_precision *circle_area(circle)
   circle_t *circle;
{
   mi_double_precision *area;

   /* Allocate memory for mi_double_precision return 
    * value
    */
   area = mi_alloc(sizeof(mi_double_precision));

   /* Calculate circle area using radius from circle_t
    * structure and constant PI_CONSTANT (defined in
    * circle.h).
    */
   *area = (circle->radius * circle->radius) * PI_CONSTANT;

   return ( area )
}