Preprocessor #ifndef

Question

Assume I have a.h which includes the following:

Assume I also have b.

Answer 1

It's normal that most header start with

#ifndef _HEADERFILENAME_H_
#define _HEADERFILENAME_H_

and end with the following line:

#endif

If you include a header two times, the second time your programm won't include the full header again because of the #ifndef, #define and #endif