/in/foo.c: In function 'main':
/in/foo.c:6:15: warning: implicit declaration of function 'fun' [-Wimplicit-function-declaration]
  printf("%d", fun(x));
               ^~~
/in/foo.c: In function 'fun':
/in/foo.c:12:14: warning: 'return' with no value, in function returning non-void
  if (x == 1) return ;
              ^~~~~~
/in/foo.c:9:5: note: declared here
 int fun(int x)
     ^~~