Code

 Header...
#define PI 3.14159265358979

typedef struct ellipse {
  float centre[2];
  float axes[2];
  float orientation;
}Ellipse;

 void print_area(Ellipse el){
   ... 

»  float area;

»  area=PI*el.axes[0]*el.axes[1];
»  printf("The area is %f\n", area);
»  el.centre0=123.456;// Move the ellipse
»  el.centre1=-78.9;
}

 int main(){
   ... 

»  Ellipse ellie;

»  printf("Centre? (x,y)\n");
»  scanf("%f %f", &ellie.centre[0], &ellie.centre[1]);
 
  // NB: in a real program we should check
  // the axes are actually > 0 
»  printf("Length of axes ( > 0 )?\n");
»  scanf("%f %f", &ellie.axes[0], &ellie.axes[1]);

»  printf("Orientation to the vertical?\n");
»  scanf("%f", &ellie.orientation);

»  print_area(ellie);

»  return 0;
}