Language: C

#include<stdio.h>
#include<math.h>
 
struct Point{ double x,y; };

void koch(int,Point,Point);
 
int main(){
  Point a,b;
  int n;
 
  scanf("%d",&n);
 
  a.x = a.y = 0;
  b.x = 100;
  b.y = 0;
 
  printf("%8f %8f\n",a.x,a.y);
 
  koch(n,a,b);
 
  printf("%8f %8f\n",b.x,b.y);
  return 0;
}
 

 
void koch(int n, Point a, Point b){
   if(n == 0) return;
  Point s, t, u;
  double th = M_PI * 60.0 / 180.0;
 
  
  s.x = (2.0 * a.x + 1.0 * b.x) / 3.0;
  s.y = (2.0 * a.y + 1.0 * b.y) / 3.0;
  t.x = (1.0 * a.x + 2.0 * b.x) / 3.0;
  t.y = (1.0 * a.y + 2.0 * b.y) / 3.0;
  u.x = (t.x -s.x) * cos(th) - (t.y - s.y) * sin(th) + s.x;
  u.y = (t.x -s.x) * sin(th) + (t.y - s.y) * cos(th) + s.y;
 
  koch(n-1, a, s);
  printf("%8f %8f\n",s.x,s.y);
  koch(n-1, s, u);
  printf("%8f %8f\n",u.x,u.y);
  koch(n-1, u, t);
  printf("%8f %8f\n",t.x,t.y);
  koch(n-1, t, b);
}

int main(){
  Point a, b;
  int n;
 
  scanf("%d", &n);
 
  a.x = 0;
  a.y = 0;
  b.x = 100;
  b.y = 0;
 
  printf("%8f %8f\n",a.x, a.y);
  koch(n, a, b);
  printf("%8f %8f\n",b.x, b.y);
  
  return 0;
}

Compiler output:
main.c:6:15: error: unknown type name 'Point'; did you mean 'int'?
    6 | void koch(int,Point,Point);
      |               ^~~~~
      |               int
main.c:6:21: error: unknown type name 'Point'; did you mean 'int'?
    6 | void koch(int,Point,Point);
      |                     ^~~~~
      |                     int
main.c: In function 'main':
main.c:9:3: error: unknown type name 'Point'; use 'struct' keyword to refer to the type
    9 |   Point a,b;
      |   ^~~~~
      |   struct 
main.c:14:4: error: request for member 'x' in something not a structure or union
   14 |   a.x = a.y = 0;
      |    ^
main.c:14:10: error: request for member 'y' in something not a structure or union
   14 |   a.x = a.y = 0;
      |          ^
main.c:15:4: error: request for member 'x' in something not a structure or union
   15 |   b.x = 100;
      |    ^
main.c:16:4: error: request for member 'y' in something not a structure or union
   16 |   b.y = 0;
      |    ^
main.c:18:23: error: request for member 'x' in something not a structure or union
   18 |   printf("%8f %8f\n",a.x,a.y);
      |                       ^
main.c:18:27: error: request for member 'y' in something not a structure or union
   18 |   printf("%8f %8f\n",a.x,a.y);
      |                           ^
main.c:20:3: error: implicit declaration of function 'koch' [-Wimplicit-function-declaration]
   20 |   koch(n,a,b);
      |   ^~~~
main.c:22:23: error: request for member 'x' in something not a structure or union
   22 |   printf("%8f %8f\n",b.x,b.y);
      |                       ^
main.c:22:27: error: request for member 'y' in something not a structure or union
   22 |   printf("%8f %8f\n",b.x,b.y);
      |                           ^
main.c: At top level:
main.c:28:18: error: unknown type name 'Point'; did you mean 'int'?
   28 | void koch(int n, Point a, Point b){
      |                  ^~~~~
      |                  int
main.c:28:27: error: unknown type name 'Point'; did you mean 'int'?
   28 | void koch(int n, Point a, Point b){
      |                           ^~~~~
      |                           int
main.c:50:5: error: redefinition of 'main'
   50 | int main(){
      |     ^~~~
main.c:8:5: note: previous definition of 'main' with type 'int()'
    8 | int main(){
      |     ^~~~
main.c: In function 'main':
main.c:51:3: error: unknown type name 'Point'; use 'struct' keyword to refer to the type
   51 |   Point a, b;
      |   ^~~~~
      |   struct 
main.c:56:4: error: request for member 'x' in something not a structure or union
   56 |   a.x = 0;
      |    ^
main.c:57:4: error: request for member 'y' in something not a structure or union
   57 |   a.y = 0;
      |    ^
main.c:58:4: error: request for member 'x' in something not a structure or union
   58 |   b.x = 100;
      |    ^
main.c:59:4: error: request for member 'y' in something not a structure or union
   59 |   b.y = 0;
      |    ^
main.c:61:23: error: request for member 'x' in something not a structure or union
   61 |   printf("%8f %8f\n",a.x, a.y);
      |                       ^
main.c:61:28: error: request for member 'y' in something not a structure or union
   61 |   printf("%8f %8f\n",a.x, a.y);
      |                            ^
main.c:63:23: error: request for member 'x' in something not a structure or union
   63 |   printf("%8f %8f\n",b.x, b.y);
      |                       ^
main.c:63:28: error: request for member 'y' in something not a structure or union
   63 |   printf("%8f %8f\n",b.x, b.y);
      |                            ^