/in/foo.cc: In function 'void Concat(char*, char*)':
/in/foo.cc:12:22: error: 'strlen' was not declared in this scope
  int len1 = strlen(s1);
                      ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:26:11: error: 'gets_s' was not declared in this scope
  gets_s(s1);
           ^