Path: csiph.com!usenet.pasdenom.info!aioe.org!.POSTED!not-for-mail
From: "Rod Pemberton" <dont_use_email@xnohavenotit.cnm>
Newsgroups: comp.lang.forth
Subject: Re: CASE mis-understanding?
Date: Thu, 27 Feb 2014 16:23:50 -0500
Organization: Aioe.org NNTP Server
Lines: 42
Message-ID: <op.xbylp0b95zc71u@localhost>
References: <52e54d17.461102185@news.demon.co.uk> <85ha7kg45j.fsf@junk.nocrew.org> <4P6dnWTTv7tKspLOnZ2dnUVZ_sGdnZ2d@supernews.com> <7xlhwwela4.fsf@ruckus.brouhaha.com> <530f5e60$0$25041$e4fe514c@dreader37.news.xs4all.nl>
NNTP-Posting-Host: CNsg4fVcCsvs3UaOgZtQCw.user.speranza.aioe.org
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii; format=flowed; delsp=yes
Content-Transfer-Encoding: 7bit
X-Complaints-To: abuse@aioe.org
User-Agent: Opera Mail/12.16 (Linux)
X-Notice: Filtered by postfilter v. 0.8.2
Xref: csiph.com comp.lang.forth:28820

On Thu, 27 Feb 2014 10:48:48 -0500, Albert van der Horst  
<albert@spenarnc.xs4all.nl> wrote:
> In article <7xlhwwela4.fsf@ruckus.brouhaha.com>,
> Paul Rubin  <no.email@nospam.invalid> wrote:
>> Andrew Haley <andrew29@littlepinkcloud.invalid> writes:

>>> How is that semantically different from a null pointer?  Surely NIL is
>>> a null pointer, however it is represented.
>>
>> Each list node is a discriminated union (member of a sum type) that's
>> either NIL, or a pair consisting of a value and a pointer to the next
>> node.  So in the case where it's NIL, that's not a pointer.  With a
>> static type system, if the list has a type like List<A>, the NIL for
>> that list would also have that type, and any attempt to dereference it
>> would be caught at compile time.
>
> So instead of a runtime system that gives an error:
> "attempt to derefence a nil pointer" the user is obliged to put
> some code in to give that error himself. Big advantage -;)
>
> Really, on paper and properly implemented, the null pointer in C is
> a fairly decent nil value. Properly implemented in my book means
> that an access through the pointer must be trapped.
>

AFAIK, there's no requirement in C to prevent reading or writing to the
same address as the NULL pointer.  AFAIK, there is no requirement to trap
accesses to it's location either.  The requirement is that no C objects
must be located *at* the NULL pointer address.  The NULL is for comparing
a pointer in C against NULL to determine if the pointer was set or not.
I know for a fact that many C compilers implement NULL as zero and also
allow reading and writing to address zero via a C pointer.  The NULL
pointer's address is zero for many C implementations, but can be any
location as long as no other C objects are located at that address, i.e.,
NULL can be non-zero.  One of the original authors of the ANSI C
specification posted an example of how to do that many years ago.  C
compilers which allow writing to the same address as NULL is usually
because the NULL pointer value is zero *and* there is memory mapped
device or data space located there that must be accessable.


Rod Pemberton