Realizar el radio de una circulo mediante funciones

#include <stdio.h>
#include <math.h>

double circleArea(double radius)
{
  return M_PI * radius * radius;
}

double circleCircumference(double radius)
{
  return 2 * M_PI * radius;
}

int main()
{
  double radius = 2;
  double area = circleArea(radius);
  double circumference = circleCircumference(radius);
  printf("Area: %f \n", area);
  printf("Circumference: %f \n", circumference);
  return 0;
}