Re: [PATCH] Use NULL instead of integer 0 in security/selinux/

[Andrea Arcangeli <andrea@suse.de>]

On Fri, Jul 09, 2004 at 11:23:52PM -0700, Linus Torvalds wrote:
> I really don't see the point of complaining about the fixes. There's just
> _no_ way to say that "0" is more readable than "NULL" in any of the cases.  
> I dare you - show _one_ case where a 0/NULL patch was wrong or even
> remotely debatable. I dare you.

I definitely agree.

Several years ago I once wrote a singificant piece of code for a projet
with #define NULL -1UL, this actually wasn't my choice but a requirement
of the project (the headers were pre-defined) but it worked perfectly
since we never did '!ptr' we always did 'ptr == NULL' instead (etc..).
So at runtime it has never been a problem because we coded with NULL !=
0 in mind. Of course I known normally NULL is always equal to 0 but I
didn't realize that defining NULL !=0 wasn't exactly the C language (I
learnt it later on the hard way in some mailing list, I believe at some
point I did patches like the one in this thread but claiming it to be a
bugfix, and not just a cleanup ;).

IIRC my argument about these patches being bugfixes, was about an
architecture with a valid page mapped at address 0, that wouldn't
generate a segfault. This is incidentally why we had to use NULL = -1
instead of NULL = 0. The answer I got at that time form some C guru is
that I would need to hack the compiler specifically for such achitecture
to accomodate for NULL = -1, so that '!ptr' will be the same as 'ptr ==
-1UL' (for pointers). In practice I think it has been a lot easier for
us to avoid using '!ptr' than to hack gcc...