/in/foo.c: In function 'main':
/in/foo.c:4:2: error: 'cin' undeclared (first use in this function)
  cin>>a>>b>>c;
  ^~~
/in/foo.c:4:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:4: error: 'cout' undeclared (first use in this function)
    cout<<b<<a<<endl;
    ^~~~
/in/foo.c:7:16: error: 'endl' undeclared (first use in this function)
    cout<<b<<a<<endl;
                ^~~~