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

[Alexandre Oliva <aoliva@redhat.com>]

On Jul 10, 2004, Linus Torvalds <torvalds@osdl.org> wrote:

> On Fri, 9 Jul 2004, Eric W. Biederman wrote:
>> 
>> Does this mean constructs like:
>> ``if (pointer)'' and ``if (!pointer)'' are also outlawed.

> Of course not.

> Why should they be?

Err...  Because the conditional expression is implicitly compared with
0 [6.8.4.1]/#2.  If 0 is not to be used explicitly in pointer
contexts, why should it be ok to use it implicitly?

> What's considered bad form is:
[snip]
>  - thinking the constant "0" is a pointer.

> There's no reason why "if (!ptr)" would be wrong.

[6.5.3.3]/#5 defines the result of the logical negation operator
based on the result of comparing the expression with 0.

> But it has nothing to do with the _value_ zero, or testing pointers for
> being non-NULL. The value zero is not about semantic confusion, it's just 
> a bit pattern. And testing pointers is not ambiguous: when you test a 
> pointer, it's _un_ambigiously checking that pointer for NULL.

I don't see why (!ptr) is any more confusing than (ptr != 0), and why
(ptr != NULL) would be any clearer.  Is `ptr != 0' one of the cases
that are not bad?

-- 
Alexandre Oliva             http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}