/in/foo.c: In function 'main':
/in/foo.c:15:12: error: 'a' undeclared (first use in this function)
  for(int i=a;i<=b;i++)
            ^
/in/foo.c:15:12: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:15:17: error: 'b' undeclared (first use in this function)
  for(int i=a;i<=b;i++)
                 ^
/in/foo.c:20:6: error: 'j' undeclared (first use in this function)
   if(j<=sqrt(i))
      ^