There are two ways by which you can include header file in c++ lan.
1.<stdio.h>:- this is valid for own subroutine and function for a particular program.
2."stdio.h","conio.h", "string.h" and so on this is valid for a entire a project where one or more
programs are include under a master project... and ur question for strlen() this function is defined
in <STRING.H> so u go for include this header file .