/in/foo.c: In function 'main':
/in/foo.c:9:25: error: 'is_prime' undeclared (first use in this function)
    9 |                         is_prime=0;
      |                         ^~~~~~~~
/in/foo.c:9:25: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:18:17: error: expected declaration or statement at end of input
   18 |                 }
      |                 ^