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

[Eric W. Biederman]

Linus Torvalds <torvalds@xxxxxxxx> writes:

> 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?

Only because the definition of the semantics of ``if'' is in terms of
comparisons with ``0'',  and I am familiar enough with the C
programming language that, that is how I read it.  It is still 
the case that because the comparison happens in pointer context the
``0'' referred  to is the null pointer constant.  

For some of us who are extremely familiar with C your argument is
confusing.  You make statements that sound like they are about the
definition of the C programming language when in fact they are
criticism of a given C programming style.  

Since I am already making distinctions 0 as the integer value and
0 as the pointer constant when 0 is implicitly introduced.  It is
really not confusing to me in the case of manifest constants.

> What's considered bad form is:
>  - assignments in boolean context (because of the confusion of "=" and 
>    "==")
>  - thinking the constant "0" is a pointer.

I would agree that using the constant "0" in a pointer context
when a more explicit NULL is bad form.  But "0" is the one
legal way in C to write the NULL pointer constant.

> There's no reason why "if (!ptr)" would be wrong. That has zero confusion 
> about 0 vs NULL.

For me it has exactly the same level of confusion as the cases that
are being fixed has.  I have to know the type to know if it is testing
against the NULL pointer constant or if it is testing against the
integer value zero.
 
> The confusion about "0" is that in traditional C it means two things: it 
> can either be an integer (the common case) or it can sometimes be a 
> pointer. That kind of semantic confusion is bad.

Either that or it can be called greater expressive power though fewer
concepts. 

I like the fact this allows cases like ``if (!ptr)'' and friends.
 
> 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.

see above.

> Problems arise when there is room for confusion, and that's when the 
> compiler should (and does) warn. If something is unambiguous, it's not 
> bad.

The compiler is compiling the correct code so the code is clearly not
ambiguous.  But since types are not always obvious to a person
staring at the code using the more explicit form of the constant
i.e. NULL or '\0' instead of 0 adds useful redundancy.

Hopefully that explains why I objected to the way you can out against
using ``0'' as the null pointer constant.

Eric
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