On Mon, 2011-01-24 at 13:19 +0100, Pierre Joye wrote:
>
> NULL is for pointers where 0 is for integer-like.
>
> Testing if a ptr is NULL should be done by testing for NULL or not
> NULL.
>
> While compilers tolerate *ptr = 0 by casting 0 to NULL, any other
> runtime check must use NULL. That's K&R 101.
The standard (quoting ISO/IEC 9899: TC2, but identical in other
versions) states
6.3.2.3 Pointers
3 An integer constant expression with the value 0, or such an
expression cast to type void *, is called a null pointer
constant.55) If a null pointer constant is converted to a
pointer type, the resulting pointer, called a null pointer, is
guaranteed to compare unequal to a pointer to any object or
function.
[...]
6 Any pointer type may be converted to an integer type. Except
as previously specified, the result is implementation-defined.
and
7.17 Common definitions <stddef.h>
3 The macros are
NULL
which expands to an implementation-defined null pointer constant
[...]
So casting (int)0 to a (void*) results to a NULL pointer and is valid.
The other way round is not specified, but implementation defined. By
that Pierre's change is correct. Out of curiosity: I'm not aware of any
implementation (which is in use) where this would be invalid. Anybody
knows one?
Sidenote: I think that one is even stricter in C++, as it binds the
value of 0 to the NULL pointer. (Stroustrup suggests doing const int
NULL 0; in §5.1.1 of his book, but I don't have the standard at
hand) ... but C++ doesn't matter here.
johannes
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