Re: What's in git.git

Linus Torvalds [off-list ref] writes:

I suspect that both could have been made to use NULL instead to indicate 
that no pointer exists.

As long as none of the following does not dereference the NULL
pointer that should work fine:

	- memchr(NULL, ch, 0);

        - memcmp(NULL, ptr, 0);

        - stream.next_in = NULL;
          stream.avail_in = 0;
          deflate(&stream, Z_FINISH);

On Mon, 26 Jun 2006, Junio C Hamano wrote:

As long as none of the following does not dereference the NULL
pointer that should work fine:

	- memchr(NULL, ch, 0);

        - memcmp(NULL, ptr, 0);

        - stream.next_in = NULL;
          stream.avail_in = 0;
          deflate(&stream, Z_FINISH);

Well, they clearly _shouldn't_, but bugs happen in libraries too.

(The reason they shouldn't is simple: replace the NULL pointer with a 
perfectly normal pointer at the end of a page, and if you access past the 
length of the memory area, you'd get the same SIGSEGV that a NULL pointer 
should give you).

So the question is how much to trust the libraries for a special case that 
has a clearly "Right Answer", but is probably unusual in practice.

(That said, "malloc(0)" can validly return NULL, so the NULL+<zero length> 
thing can really happen in real code even if it's not explicit).

If people want to be anally defensive, we should give it a valid pointer. 
If there are known buggy libraries, I wouldn't object to something 
explicit like

	#define DUMMYPTR ((void *)"")

and hide it that way - the code may be the exact same thing that I 
objected to, but now you're showing that you know that the cast is crud, 
and you're just wanting to pass in some non-NULL valid pointer that can be 
dereferenced but never written to.

Personally, I think NULL is much better, until somebody shows an actual 
buggy library.

			Linus

Hi,

On Mon, 26 Jun 2006, Linus Torvalds wrote:

	#define DUMMYPTR ((void *)"")

NULL_STRING describes better what you want.

Ciao,
Dscho