Untitled

#include<stdio.h>
#include<stdlib.h>

/* I prefer this style of struct definition */

typedef struct {
  int len;
} square_t;

typedef struct {
  double radius;
} circle_t;

typedef enum { 
  SQUARE, 
  CIRCLE
} shape_type_t;

typedef struct {
  /* Ordinary C unions are UNSAFE; they do not provide any way to
     determine what the type of the current content is. Therefore, we
     must keep track of it manually with the 'type' member. */
  shape_type_t type;
  union {
    square_t square;
    circle_t circle;
  };
} shape_t;

void scale(shape_t *, double);

int main(void) {
  shape_t s, *r;

  /* s is statically allocated shape */
  s.type = SQUARE;
  s.square.len = 10;

  /* r is 2 dynamically allocated shapes in an array */
  r = (shape_t *) malloc(sizeof(shape_t)*2);
  if (r == NULL) exit(1);  /* always check result of malloc */
  
  r[0].type = CIRCLE;
  r[0].circle.radius = 3.14;

  r[1].type = SQUARE;
  r[1].square.len = 2;

  /* function call example */
  scale(&s, 2);
    
  free(r);

  return 0;
}

void scale(shape_t *p, double factor) {
  /* Discriminate type */
  if (p->type == SQUARE) {
    /* Use pointer-to-struct syntax for access */
    p->square.len *= factor;
  } else if (p->type == CIRCLE) {
    p->circle.radius *= factor;
  }
}