Re: NULL pointer arithmetic issues

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2020-02-24 21:24, Kamil Rytarowski wrote:
> On 24.02.2020 21:18, Mouse wrote:
> > > > If we use 0x0, it can be a valid pointer.
> >
> > > > If we use NULL, it's not expected to work and will eventually
> > > > generate a syntax erro.
> >
> > Then someone has severely broken compatability with older versions of
> > C.  0x0 and (when one of the suitable #includes has been done) NULL
> > have both historically been perfectly good null pointer constants.
> >
> > Also...syntax error?  Really?  _Syntax_ error??  I'd really like to see
> > what they've done to the grammar to lead to that; I'm having trouble
> > imagining how that would be done.
>
> The process of evaluation of the NULL semantics is not a recent thing.
>
> Not so long time, still in the NetBSD times, it was a general practice
> to allow dereferencing the NULL pointer and expect zeroed bytes over there.
>
> We still maintain compatibility with this behavior (originated as a hack
> in PDP11) in older NetBSD releases (NetBSD-0.9 Franz Lisp binaries
> depend on this).

Really? I thought we usually do not have anything mapped at address 0 to 
explicitly catch any dereferencing of NULL pointers.

But yes, on the PDP11 this was/is not the case. Memory space is too 
precious to allow some of it to be wasted for this... (Even if there are 
a comment about it in 2.11BSD, bemoaning this fact...)

  Johnny

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Johnny Billquist                  || "I'm on a bus
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