foo.c: In function 'fun':
foo.c:5:2: error: too few arguments to function 'fun'
  fun(x/n);
  ^
foo.c:2:5: note: declared here
 int fun(int x,int n){
     ^
foo.c:6:2: error: 'cout' undeclared (first use in this function)
  cout<<x%n;
  ^
foo.c:6:2: note: each undeclared identifier is reported only once for each function it appears in
foo.c: In function 'main':
foo.c:11:2: error: 'cin' undeclared (first use in this function)
  cin>>x>>n;
  ^
foo.c: In function 'fun':
foo.c:7:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

自豪的采用 HydroJudger 进行评测(github.com/hydro-dev/HydroJudger)