/in/foo.c: In function 'fun':
/in/foo.c:6:2: error: too few arguments to function 'fun'
  fun(a/b);
  ^~~
/in/foo.c:2:5: note: declared here
 int fun(int x,int n){
     ^~~
/in/foo.c:7:2: error: 'cout' undeclared (first use in this function)
  cout<<x%n;
  ^~~~
/in/foo.c:7:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c: In function 'main':
/in/foo.c:12:2: error: 'cin' undeclared (first use in this function)
  cin>>x>>n;
  ^~~
/in/foo.c: In function 'fun':
/in/foo.c:8:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^