foo.c: In function ‘main’:
foo.c:6:9: error: ‘cin’ undeclared (first use in this function)
    6 |         cin>>a>>b>>c;
      |         ^~~
foo.c:6:9: note: each undeclared identifier is reported only once for each function it appears in
foo.c:9:17: error: ‘cout’ undeclared (first use in this function)
    9 |                 cout<<a<<b<<c<<endl;
      |                 ^~~~
foo.c:9:32: error: ‘endl’ undeclared (first use in this function)
    9 |                 cout<<a<<b<<c<<endl;
      |                                ^~~~
foo.c:17:29: error: ‘z’ undeclared (first use in this function)
   17 |                 cout<<b<<c<<z<<endl;
      |                             ^