c语言函数如何声明

 1 #include 
 2 #define PI 3.14159
 3  int main()
 4  {
 5       //函数原型声明
 6      double circle(double radius);     
 7      double r, area;
 8      scanf("%lf", &r);
 9      area = circle(r);
10printf("area=%.2f\n", area);
11      return 0;
12   }
13   double circle(double radius)
14   {
15
16       return PI * radius * radius;
17   }