what is the use of #ifndefs

[Amit Pandey]

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 at 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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