Question

while I am creating a c++ header file, I declare the header file like;

/*--- Pencere.h ---*/
#ifndef PENCERE_H
#define PENCERE_H

I want to learn that why do I need to write underline.

Answer 1

You don’t need to use the underline, it’s just a convention to separate the header name and extension. You cannot use the literal . since that’s not valid in an identifier so you replace it with an underscore which is valid.

The reason you actually do it is as an include guard. The entire contents of the file are something like:

#ifndef PENCERE_H
    #define PENCERE_H
    // Your stuff goes here.
#endif

so that, if you accidentally include it twice:

#include "pencere.h"
#include "pencere.h"

you won’t get everything in it duplicated. The double inclusions are normally more subtle than that – for example, you may include pax.h and diablo.h in your code and pax.h also includes diablo.h for its purposes:

main.c:
    #include "pax.h"
    #include "diablo.h"
    // Other stuff

pax.h:
    #ifndef PAX_H
        #define PAX_H
        #include "diablo.h"
        // Other stuff
    #endif

diablo.h:
    #ifndef DIABLO_H
        #define DIABLO_H
        typedef int mytype;
    #endif

In this case, if the include guards weren’t there you would try to compile the line typedef int mytype; twice in your program. Once for main.c -> pax.h -> diablo.h and again for main.c -> diablo.h.

With the include guards, the pre-processor symbol DIABLO_H is defined when main.c includes diablo.h so the #define and typedef are not processed.

This particular mapping of header files to #define names breaks down in the situation where you have dir1/pax.h and dir2/pax.h since they would both use PAX_H. In that case, you can use a scheme like DIR1_PAX_H and DIR2_PAX_H to solve the problem.