Hi Ahmed,

See the comments inline

#ifndef _LINUX_LIST_H // If not defined _LINUX_LIST_H macro

#define _LINUX_LIST_H // then define this macro

#include "linuxlist.h" // and include linuxlist.h header file

#endif // end of #ifndef

Now say in another file if u r not sure whether you have already included the "linuxlist.h" then you will again repeat above lines of code, assuming you have included it then certainly _LINUX_LIST_H macro has already been defined, so compiler will not include this file again. See comments below

#ifndef _LINUX_LIST_H // Since macro _LINUX_LIST_H has already defined

#define _LINUX_LIST_H // compiler will ignore this line

#include "linuxlist.h" // compiler will ignore this line, too. Hence no multiple inclusion of same header file

#endif

Please let me know whether I was clear with the explanation.

Thanks,

Amit

On Mon, Jul 20, 2015 at 3:33 PM, Ahmed Soliman <ahmedsoliman0x666@gmail.com> wrote:

currently I started reading through the linux kernel and I started
reading liunx/include/linux/list.h> I understood some of the functions
but still I dont know what does these lines of code do
#ifndef _LINUX_LIST_H
#define _LINUX_LIST_H
which exist at the very beginning of the file
I also noticed that there is many similar ifndefs in almost any .h
file in the kernel
note that I understand wnat does ifndef do bu I dont understand what
goal is it supposed to achieve at the beginning of the headerfile

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