Date: Mon, 01 Mar 2004 14:39:06 -0800 From: Tim Kientzle <tim@kientzle.com> To: Erik Trulsson <ertr1013@student.uu.se> Cc: current@freebsd.org Subject: Re: NULL vs 0 vs 0L bikeshed time Message-ID: <4043BB8A.3010601@kientzle.com> In-Reply-To: <20040301173231.GA61430@falcon.midgard.homeip.net> References: <xzpoergtt6z.fsf@dwp.des.no> <200403011707.i21H7bY96897@lakes.dignus.com> <20040301173231.GA61430@falcon.midgard.homeip.net>
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Erik Trulsson wrote:
> On Mon, Mar 01, 2004 at 12:07:37PM -0500, Thomas David Rivers wrote:
>>>Mark Murray <mark@grondar.org> writes:
>>>
>>>>I'd like to commit the following patch. It makes sure that for C
>>>>and the kernel, NULL is a ((void *)0)
>>>
>>>This is not correct, because it makes NULL unusable for function
>>>pointers; you can assign 0 to a function pointer, but not (void *)0.
>>
>> That assignment seems to work... I thought (void *) was assignable to
>> any function pointer... (Isn't (void *) assignable to any pointer?)
>
> To any _object_ pointer, not to a function pointer. Null pointers are
> special however.
I was curious about this, so I looked it up.
ANSI/ISO 9899-1990, Section 6.2.2.3:
"An integral constant expression with the value
0, or such an expression cast to type void * is
called a null pointer constant. If a null pointer
constant is assigned to ... a pointer, the
constant is converted to a pointer of that type.
Such a pointer ... is gauranteed to compare
unequal to a pointer to any object or function."
There's some earlier text about void* assignments
that seems to restrict assigning void* to function pointers.
The conclusion I draw from this:
int (*f)(void);
void *p;
f = p; /* NOT LEGAL: Can't assign void* to function ptr */
f = (void *)0; /* LEGAL: Can assign NULL to function ptr */
f = 0; /* LEGAL: Can assign NULL to function ptr */
Tim Kientzle
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