/in/foo.c: In function 'f':
/in/foo.c:19:6: error: 'X' undeclared (first use in this function)
  if (X!=0)
      ^
/in/foo.c:19:6: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:23:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^