what is the use of #ifndefs

[navych@126.com (Navy)]

On Tue, Jul 21, 2015 at 11:04:15AM +0530, Amit Pandey wrote:
> 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
> >
> > _______________________________________________
> > Kernelnewbies mailing list
> > Kernelnewbies at kernelnewbies.org
> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> >

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Your explanation seems to be wrong.
The code below should in "linuxlist.h" ,other than in other files which include "linuxlist.h".
#ifndef _LINUX_LIST_H    
#define _LINUX_LIST_H     
...
#endif