great alignement
On Wed, Aug 20, 2008 at 8:32 PM, Clark Cox <[EMAIL PROTECTED]> wrote:
> On Wed, Aug 20, 2008 at 8:06 PM, Michael Ash <[EMAIL PROTECTED]> wrote:
>> On Wed, Aug 20, 2008 at 8:34 PM, Douglas Davidson <[EMAIL PROTECTED]> wrote:
>>> Well, after all, zero is zero, how much difference can it make? Quite a
>>> bit, as it turns out, since in 64-bit one of them is four bytes of zero, and
>>> the other is eight bytes of zero. If you're just comparing against NULL, it
>>> doesn't matter, but if you're using it in something where size counts--say,
>>> a list of vararg arguments--then it matters a lot. It's not easy to debug,
>>> though, because who would think that you need to distinguish one NULL from
>>> another?
>>
>> It is a little known fact that when passing NULL (and by extension nil
>> or Nil) as a parameter to a vararg function, you *must* cast it to the
>> appropriate pointer type to guarantee correct behavior.
>>
>> Interestingly, Apple's vararg methods which use nil as a terminator
>> (such as dictionaryWithObjectsAndKeys:) make no mention of this in
>> their documentation, and have a great deal of officially sanctioned
>> sample code which doesn't use such a cast. (And none of my code uses
>> it either.) I suppose Apple must be implicitly making a stronger
>> guarantee about the pointer-ness of nil than the C language makes
>> about NULL.
>
> GCC does that for you (i.e. the NULL defined by GCC is already typed
> to a pointer):
>
> [EMAIL PROTECTED]:~]% cat test.c
> #include <stdio.h>
>
> int main() {
>
> printf("sizeof(NULL) == %zu\n", sizeof(NULL));
> return 0;
> }
> [EMAIL PROTECTED]:~]% cc test.c -arch i386 && ./a.out
> sizeof(NULL) == 4
> [EMAIL PROTECTED]:~]% cc test.c -arch x86_64 && ./a.out
> sizeof(NULL) == 8
>
> --
> Clark S. Cox III
> [EMAIL PROTECTED]
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--
-mmw
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