ok - this is often confused. the c faq has a *big* section on this...
NULL should be defined as 0.
0 cast to a pointer type is defined as an undefined pointer and can be
implemented by the compiler as any number (including -1, or something
that will be assured to cause a tlb/pagetable miss). Therefore, both the
compare with zero and the straight "!" usage and the compare with NULL
will give the same results on any system that is ANSI-C. There are no
problems with portability, even on systems that have non-zero null
pointers.
(Note that it has been a while since I've gone through the C faq, so I
could well be wrong... :)
Pete
On Wed, 27 May 1998, Andrea Arcangeli wrote:
> On Wed, 27 May 1998, Glynn Clements wrote:
>
> >
> >Andrea Arcangeli wrote:
> >
> >> > But for testing null pointers, I use `!'. In this context, it seems
> >>
> >> BAD. I worked in envinronments where NULL is ((void *)-1UL).
> >
> >That's _really bad_. ANSI C requires stdio.h to define NULL as being
> >equivalent to zero.
>
> Hmm, this is true in user land. Is it still true in kernel (not Linux)
> developement?
>
> >> If you use ! to check for a NULL pointer you make your code not
> >> portable.
> >
> >Using ! to check for a NULL pointer is entirely portable.
>
> If so excuse me, but for safe I will continue to use == NULL instead of !
> ;-).
>
> Andrea[s] Arcangeli
>
Pete Ryland Home phone: +61 2 9697 9262 Mobile: 014 035 802
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