/in/foo.cc: In function 'int length(const char*)':
/in/foo.cc:5:19: error: 'strlen' was not declared in this scope
  return strlen(str);
                   ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:15:5: error: 'cin' was not declared in this scope
     cin.getline(s1, changdu);
     ^~~
/in/foo.cc:18:5: error: 'cout' was not declared in this scope
     cout<<length(s1)<<" "<<length(s2)<<endl;
     ^~~~
/in/foo.cc:18:40: error: 'endl' was not declared in this scope
     cout<<length(s1)<<" "<<length(s2)<<endl;
                                        ^~~~