Re: kfree(0) - ok?

[Kyle Moffett]

On Aug 15, 2007, at 06:20:27, Jan Engelhardt wrote:
> On Aug 15 2007 11:58, Rene Herman wrote:
> > > > NULL is not 0 though.
> > > It is.  Its representation isn't guaranteed to be all-bits-zero,
> >
> > He said the null _pointer_ isn't guaranteed to be all-bits zero.  
> > And it isn't. Read the standard or the faq.
>
> 0 is all-bits-zero.
> NULL is 0. ("It is.", above)
>
> Transitively, this would make NULL all-bits-zero.  I might have  
> missed something, though, perhaps that the cast to void* makes it  
> intransitive.  But leave it at whatever the standard says.
>
> > but the constant value 0 when used in pointer context is always a  
> > null pointer (and in fact the standard requires that NULL be  
> > #defined as 0 or a cast thereof).

Irrespective of whatever the standard says, EVERY platform and  
compiler anybody makes nowadays has a NULL pointer value with all  
bits clear.  Theoretically the standard allows otherwise, but such a  
decision would break so much code.  Linux especially, we rely on the  
uninitialized data to have all bits clear and we depend on that  
producing NULL pointers; if a NULL pointer was not bitwise exactly 0  
then the test "if (some_ptr != NULL)" would fail and we would start  
dereferencing garbage.

Cheers,
Kyle Moffett

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