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Re: OT: One of Anton's papers makes Hacker News

From Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm>
Newsgroups comp.lang.forth
Subject Re: OT: One of Anton's papers makes Hacker News
Date 2016-03-07 19:19 -0500
Organization Aioe.org NNTP Server
Message-ID <20160307191942.6aae639e@_> (permalink)
References <87bn6vytsf.fsf@nightsong.com> <20160304210145.16c87863@_> <nbeo9g$1gb$2@dont-email.me> <20160305194309.147d27ed@_> <nbg248$i9m$4@dont-email.me>

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On Sun, 6 Mar 2016 01:45:45 -0000 (UTC)
Bernd Paysan <bernd.paysan@gmx.de> wrote:

> Am Sat, 05 Mar 2016 19:43:09 -0500 schrieb Rod Pemberton:

> >> A pointer comparison there was only an offset comparison,  
> > 
> > A pointer comparison is only an offset comparison for any x86 mode,
> > be it 16-bit RM, 16-bit PM, 32-bit PM, or 64-bit PM.  There are no
> > absolute addresses in the x86 architecture.  It's entirely
> > segmented.  16-bit RM used segment and offset, and PM uses a base
> > address plus an offset.   
> 
> But for 32- and 64-bit PM, there is no far model, so all pointers
> have the same base address.

No, each descriptor can have a different base address for 32-bit PM.
Correction, I'm not sure about base addresses for 64-bit PM.

> > What's your point?  
> 
> That pointer comparisons in C are allowed only to compare the offset,

Where is that requirement?

> > How do you expect a memory function, memmove, to work
> > with objects?  ... They work on memory.  
> 
> The terminology is not ours, but the one of the C standard.  They use 
> "object" for a lot of things, including memory regions, and it
> doesn't necessarily have a type.

Of course, one should not conflate an actual object in C with use
of the word "object" in the standard to identify different things.

> You think memmove should be well-defined and not UB.

Where is the UB?  See my reply to Anton.

> > None of this changes my point that Anton has a bad example for
> > something he didn't or doesn't understand in a formal paper.  
> 
> C99, page 85, semantics part 5 (last paragraph on the bottom of the
> page, continuing on page 86) lists a number of cases where pointer
> comparisons work, and "all others are undefined";

C99 6.5.8 sub 5 pg 85
C99 (12/01/1999 2nd Ed., ISO/IEC 9899:1999(E), not a draft)

"In all other cases, the behavior is undefined."

Interesting, ...

I don't see a way around that being UB.  It seems to apply
for both 'incomplete types' such as void as well as for
'object types'.

This seems to be an artifact of ANSI C's attempt to make
C portable to machines with non-contiguous memory.

There really should be an exception to that for void* on
contiguous memory machines to reduce UB complications.

If a compiler followed that and used it for an optimization,
it would break C in a number of ways.

Some of the ways that I think applying the UB pointer
comparison would break C are:

0) Use of 6.5.8 relational operators with NULL would not work.

1) The string and memory functions could not compare pointers,
and would need to find workarounds, as suggested by "as if"
in memmove()'s description implying use of a temporary buffer.

2) NULL pointer comparisons shouldn't work (but do).  NULL is
required to "compare unequal to a pointer to any object or
function" per C99 6.3.2.2 section for void.  If so, then NULL
would not "point to the same object" per C99 6.5.8 relational
operators.

Of course, 6.5.9 equality operators == and != provides an
exception allowing valid comparison with "null pointer
constants."  This would imply a contiguous memory machine
since the three most common ways of implementing NULL are:

#define NULL 0

#define NULL ((void *)0)

extern char __null[2];
#define NULL ((void*)&__null[1]) 

3) A NULL pointer shouldn't be passed for an empty string into
a function, unless there is an explicit check in the function
of the pointer for NULL while only using == or != from 6.5.9
conditional operators.  This would add excessive bloat and
is simply never done.

4) The inability to not compare pointers pointing to the same
object would also likely break all memory allocation functions,
and at most assuredly break realloc().

> Anton is right about what the C standard actually says,

No, he isn't.  Even though I was incorrect about void
pointer comparisons, the standard still provides a 100%
portable way to implement memmove() by using a temporary
buffer.  He doesn't have to use pointer comparisons.

Although, as mentioned in my reply to Anton, ANSI X3J11
committee member P.J.Plauger also used pointer comparisons
for implementing memmove() in his ANSI C library in his
book "The Standard C Library" which is the basis for
Dinkumware's C library.  It passes the Plum-Hall Validation
Suite for C.


Rod Pemberton

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Thread

OT: One of Anton's papers makes Hacker News Paul Rubin <no.email@nospam.invalid> - 2016-03-03 16:24 -0800
  Re: OT: One of Anton's papers makes Hacker News Julian Fondren <julian.fondren@gmail.com> - 2016-03-03 21:06 -0800
    Re: OT: One of Anton's papers makes Hacker News Ron Aaron <rambamist@gmail.com> - 2016-03-04 07:18 +0200
    Re: OT: One of Anton's papers makes Hacker News Paul Rubin <no.email@nospam.invalid> - 2016-03-03 21:33 -0800
      Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-04 17:52 +0000
    Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-04 12:32 +0000
      Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-04 18:20 +0000
        Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-05 02:23 +0000
          Re: OT: One of Anton's papers makes Hacker News hughaguilar96@gmail.com - 2016-03-04 19:06 -0800
            Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-05 20:32 -0500
          Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-06 11:28 +0000
            Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-06 18:02 +0000
              Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-06 23:55 +0000
            Re: OT: One of Anton's papers makes Hacker News hughaguilar96@gmail.com - 2016-03-06 14:48 -0800
              Re: OT: One of Anton's papers makes Hacker News Julian Fondren <julian.fondren@gmail.com> - 2016-03-07 03:36 -0800
            Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-07 19:19 -0500
              Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 17:40 +0000
        Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-05 03:35 -0600
          Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-05 13:37 +0000
            Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-05 15:42 -0600
              Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-06 00:50 +0000
                Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-05 20:33 -0500
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-06 02:01 +0000
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-06 11:57 +0000
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-07 07:35 +0000
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-07 12:40 +0000
                Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-07 19:20 -0500
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-08 00:49 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-06 02:20 -0600
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-07 00:35 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-07 02:53 -0600
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-07 16:07 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-08 05:17 -0600
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-08 22:01 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-09 03:14 -0600
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-09 11:17 +0000
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-09 23:58 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-10 04:45 -0600
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-10 11:39 +0000
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-10 13:29 +0000
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 16:02 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-10 08:22 -0600
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 15:49 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-12 13:35 -0600
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-10 21:58 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-11 06:11 -0600
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-12 00:05 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-12 04:23 -0600
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 12:05 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-12 08:36 -0600
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 16:57 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-12 13:44 -0600
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 14:29 +0000
                Re: OT: One of Anton's papers makes Hacker News "Elizabeth D. Rather" <erather@forth.com> - 2016-03-11 15:02 -1000
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 16:20 +0000
                Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-12 13:53 -0600
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-06 13:02 +0000
          Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-06 11:52 +0000
            Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-06 12:42 -0600
              Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-06 13:12 -0600
  Re: OT: One of Anton's papers makes Hacker News Ron Aaron <rambamist@gmail.com> - 2016-03-04 07:20 +0200
  Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-04 17:37 +0000
  Re: OT: One of Anton's papers makes Hacker News Julian Fondren <julian.fondren@gmail.com> - 2016-03-04 11:23 -0800
    Re: OT: One of Anton's papers makes Hacker News Andrew Haley <andrew29@littlepinkcloud.invalid> - 2016-03-09 03:24 -0600
  Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-04 21:01 -0500
    Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-05 13:51 +0000
      Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-05 19:43 -0500
        Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-06 01:45 +0000
          Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-07 19:19 -0500
            Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-12 17:24 +0000
              Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-13 12:27 +0000
                Re: OT: One of Anton's papers makes Hacker News raltbos@xs4all.nl (Richard Bos) - 2016-03-13 13:50 +0000
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-13 19:52 +0000
                Re: OT: One of Anton's papers makes Hacker News Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-03-13 16:08 -0400
                Re: OT: One of Anton's papers makes Hacker News Keith Thompson <kst-u@mib.org> - 2016-03-13 13:40 -0700
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-14 00:41 +0000
                Re: OT: One of Anton's papers makes Hacker News Keith Thompson <kst-u@mib.org> - 2016-03-13 18:23 -0700
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-14 02:59 +0000
                Re: OT: One of Anton's papers makes Hacker News Keith Thompson <kst-u@mib.org> - 2016-03-13 20:29 -0700
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-14 22:44 +0000
                Re: OT: One of Anton's papers makes Hacker News Keith Thompson <kst-u@mib.org> - 2016-03-14 16:23 -0700
                Re: OT: One of Anton's papers makes Hacker News Jan Coombs <jenfhaomndgfwutc@murmic.plus.com> - 2016-03-14 23:55 +0000
                Re: OT: One of Anton's papers makes Hacker News Keith Thompson <kst-u@mib.org> - 2016-03-14 17:19 -0700
                Re: OT: One of Anton's papers makes Hacker News Robert Wessel <robertwessel2@yahoo.com> - 2016-03-14 19:26 -0500
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-15 00:48 +0000
                Re: OT: One of Anton's papers makes Hacker News Keith Thompson <kst-u@mib.org> - 2016-03-14 22:30 -0700
                Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-15 07:23 +0000
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-15 10:09 +0000
                Re: OT: One of Anton's papers makes Hacker News Keith Thompson <kst-u@mib.org> - 2016-03-15 08:56 -0700
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-15 19:33 +0000
                Re: OT: One of Anton's papers makes Hacker News Rosario19 <Ros@invalid.invalid> - 2016-03-17 09:46 +0100
                Re: OT: One of Anton's papers makes Hacker News Jerry Stuckle <jstucklex@attglobal.net> - 2016-03-17 09:37 -0400
                Re: OT: One of Anton's papers makes Hacker News Rosario19 <Ros@invalid.invalid> - 2016-03-17 18:40 +0100
                Re: OT: One of Anton's papers makes Hacker News Jerry Stuckle <jstucklex@attglobal.net> - 2016-03-17 13:53 -0400
                Re: OT: One of Anton's papers makes Hacker News invalid <address@is.invalid> - 2016-03-17 18:00 +0000
                Re: OT: One of Anton's papers makes Hacker News Rosario19 <Ros@invalid.invalid> - 2016-03-17 18:37 +0100
                Re: OT: One of Anton's papers makes Hacker News Albert van der Horst <albert@spenarnc.xs4all.nl> - 2016-03-18 14:36 +0000
                Re: OT: One of Anton's papers makes Hacker News David Brown <david.brown@hesbynett.no> - 2016-03-14 09:17 +0100
                Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-14 22:56 +0000
                Re: OT: One of Anton's papers makes Hacker News Robert Wessel <robertwessel2@yahoo.com> - 2016-03-14 20:00 -0500
                Re: OT: One of Anton's papers makes Hacker News David Brown <david.brown@hesbynett.no> - 2016-03-15 11:22 +0100
                Re: OT: One of Anton's papers makes Hacker News raltbos@xs4all.nl (Richard Bos) - 2016-03-14 22:38 +0000
                Re: OT: One of Anton's papers makes Hacker News Randy Howard <rhoward.mx@EverybodyUsesIt.com> - 2016-03-14 01:40 -0500
    Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-06 11:00 +0000
      Re: OT: One of Anton's papers makes Hacker News Rod Pemberton <NoHaveNotOne@bcczxcfre.cmm> - 2016-03-07 19:19 -0500
        Re: OT: One of Anton's papers makes Hacker News Bernd Paysan <bernd.paysan@gmx.de> - 2016-03-08 01:02 +0000
  Re: OT: One of Anton's papers makes Hacker News Julian Fondren <ayrnieu@gmail.com> - 2016-03-14 07:29 -0700
    Re: OT: One of Anton's papers makes Hacker News anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2016-03-18 17:45 +0000

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