Re: Fwd: Question about container_of macro.

On Jan 18, 2008 9:45 AM, Manish Katiyar <[email protected]> wrote:
> Hi Laurent,
>
>
>
> On Jan 18, 2008 5:31 AM, Laurent Pinchart <[email protected]> wrote:
> >
> > On Thursday 17 January 2008, Alan Stern wrote:
> > > On Thu, 17 Jan 2008, Manish Katiyar wrote:
> > > > My question is instead of defining container_of as:
> > > >
> > > > #define container_of(ptr, type, member) ({ \
> > > >                 const typeof( ((type *)0)->member ) *__mptr = (ptr);
> > > >                 (type *)( (char *)__mptr - offsetof(type,member) );})
> > > >
> > > >
> > > > Why won't this simply work :
> > > >
> > > > #define container_of(ptr, type, member) ({ \
> > > >                         (type *)( (char *)(ptr) - offsetof(type,member)
> > > > );})
> > >
> > > Indeed, or even this:
> > >
> > > #define container_of(ptr, type, member) \
> > >                       ((type *) ((char *)(ptr) - offsetof(type,member)))
> >
> > If I'm not mistaken, that's because
> >
> > const typeof( ((type *)0)->member ) *__mptr = (ptr);
> >
> > will warn (or maybe even bail out ?) if ptr is not a pointer to a variable of
> > member's type.
> >
>
> But even if ptr is not a variable of member's type, in the second line
> we are force typecasting it to char * and then subtracting. I guess it
> will not warn. I tried in a simple program and it doesn't shout.
> Below is the program
>
> #include <stdio.h>
> #include <stddef.h>
>
> struct t_s{
>         int a;
>         int b;
> };
>
> #define container_of(ptr, type, member) \
>                                ((type *) ((char *)(ptr) -
> offsetof(type,member)))
>
> int main()
> {
>         struct t_s init;
>         char *c;
>         init.a = 1;
>         init.b = 2;
>
>         c = (char *)&(init.b);
>
>         printf("a = %d\n",(container_of(c,struct t_s,b))->a);
>         return 0;
> }
>
> /home/mkatiyar> gcc -Wall b.c
> b.c: In function 'main':
> b.c:19: warning: assignment from incompatible pointer type
> /home/mkatiyar> ./a.out
> a = 1

Sorry, I pasted the wrong command output........the program doesn't
give any warnings....

>
>
> Thanks
>
> > Laurent Pinchart
> >
>
>
>
> --
>
> Thanks & Regards,
> ********************************************
> Manish Katiyar  ( http://mkatiyar.googlepages.com )
> 3rd Floor, Fair Winds Block
> EGL Software Park
> Off Intermediate Ring Road
> Bangalore 560071, India
> ***********************************************
>



-- 
Thanks & Regards,
********************************************
Manish Katiyar  ( http://mkatiyar.googlepages.com )
3rd Floor, Fair Winds Block
EGL Software Park
Off Intermediate Ring Road
Bangalore 560071, India
***********************************************

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