what is the use of #ifndefs

[anish singh]

On Mon, Jul 20, 2015 at 3:03 AM, 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
>

It makes sure that the file doesn't get included twice.Suppose
you have a.c and a.h file and  you have #include "thisfile"(where
you have that #ifndef thing) in both a.c and a.h file then it will
be included only once. Because once .c or .h includes it then
that particular #define is already defined so next time it will
not execute when someone tries to include it again.

Hope that helps.

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