Re: Null pointer check elimination

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Mon, Nov 14, 2005 at 04:16:43PM -0800, Janis Johnson wrote:
> On Mon, Nov 14, 2005 at 11:56:16PM +0100, Gabriel Dos Reis wrote:
> > "Michael N. Moran" <mike@mnmoran.org> writes:
> > SEGFAULT is not a behaviour defined by the language.  It is *just* one
> > form of undefined behaviour.  If you execute that function, it might
> > reformat your harddrive and that woud be fine -- though I know of no
> > compiler that does that on purpose.  But, the point is that your
> > program in erring in the outer space.
> > 
> > |      // dereference: access to object
> > |      // If a is null, then SEGFAULT
> > |      *a	= 0;
> > 
> > Again, that may or may not happen. 
> 
> Some operating systems load a page of zeroes at address zero, so the
> dereference of a null pointer has no visible effect; a different form
> of undefined behavior.  DYNIX/ptx did that by default, and I think
> that HP-UX does it.  I much prefer a segfault.

(I'm about to reveal just how old I am).

Actually, there was a time when this was just the way C worked: you
got the tape from Bell Labs and ran it on your PDP-11, and people wrote
C programs that relied on *NULL returning an all-zero object (sometimes
by accident).  Every C implementation in the world worked this way
at one time.  It was the same on early VAX BSD releases, and then a
linker option was added to decide whether the zero page was all-zero or
inaccessible, and then SEGV became the default but the option (of an
all-zero page) was still needed to make old software work.

(We're talking 1980-82 time frame here).