Re: Does reading an uninitialized object have undefined behavior?

From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.std.c
Subject	Re: Does reading an uninitialized object have undefined behavior?
Date	2023-08-15 21:06 -0700
Organization	A noiseless patient Spider
Message-ID	<864jkz7hrm.fsf@linuxsc.com> (permalink)
References	(2 earlier) <874jlxozzz.fsf@nosuchdomain.example.com> <87fs5hnipv.fsf@bsb.me.uk> <87a5vpnegz.fsf@nosuchdomain.example.com> <86a5uv95g7.fsf@linuxsc.com> <fcb2be8f-b346-421f-9804-5f94c93266b0n@googlegroups.com>

Show all headers | View raw

Martin Uecker <ma.uecker@gmail.com> writes:

> On Sunday, August 13, 2023 at 2:00:45?AM UTC+2, Tim Rentsch wrote:
>
>> Keith Thompson <Keith.S.T...@gmail.com> writes:
>>
>>> I think the right way for C to permit NaT-like bits is, as Kaz
>>> suggested, to define "indeterminate value" in terms of provenance,
>>> not just the bits that make up its current representation.  [...]
>>
>> This idea is fundamentally wrong.  NaT bits are associated with
>> particular areas of memory, which is to say objects.  The point
>> of provenance is that non-viability is associated with /values/,
>> not with objects.  Once an area of memory acquires an object
>> representation, the NaT bit or NaT bits for that memory are set
>> to zero, end of story.  Note also that NaT bits are independent
>> of what type is used to access an object - if the NaT bit is set
>> then any access is illegal, no matter what type is used to do the
>> access.  By contrast, provenance is used in situations where
>> non-viability is associated with values, not with objects.  But
>> values are always type dependent;  a pointer object that holds
>> a value that has been passed to free() is "indeterminate" when
>> accessed as a pointer type, but perfectly okay to access as an
>> unsigned char type.  The two kinds of situations are essentially
>> different, and the theoretical models used to characterize the
>> rules in the two kinds of situations should therefore be
>> correspondingly essentially different.
>
> One could still consider the idea that "indeterminate" is an
> abstract property that yields UB during read even for types
> that do not have trap representations.   There is no wording
> in the C standard to support this, but I would not call this
> idea "fundamentally wrong".  You are right that this is different
> to provenance provenance which is about values.  What it would
> have in common with pointer provenance is that there is hidden
> state in the abstract machine associated with memory that
> is not part of the representation.  With effective types there
> is another example of this.

My preceding comments were meant to be only about NaT bits (or
NaT-like bits) and provenance.  There is an inherent mismatch
between the two, as I have tried to explain.  It is only the idea
that provenence would provide a good foundation for defining the
semantics of "NaT everywhere" that I am saying is fundamentally
wrong.

I understand that you want to consider a broader topic, and that,
in the realm of that broader topic, something like provenance
could have a role to play.  I think it is worth responding to
that thesis, and am expecting to do so in a separate reply (or
new thread?) although probably not right away.

Back to comp.std.c | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-20 22:16 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 16:33 +0100
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-21 11:56 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 20:54 +0100
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-21 14:26 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 23:39 +0100
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-12 17:00 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-13 23:41 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-15 21:06 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-15 22:40 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 23:13 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-17 07:08 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-18 12:44 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-19 05:04 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-19 01:36 -0700
Re: Does reading an uninitialized object have undefined behavior? Richard Damon <Richard@Damon-Family.org> - 2023-08-19 09:18 -0400
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-19 11:12 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-18 20:20 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-19 05:23 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-18 22:56 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-18 12:52 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-26 19:25 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-27 08:31 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-29 04:35 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-30 19:53 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-30 17:40 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-31 18:18 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-05 05:39 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-05 17:03 -0700
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-09-07 17:09 +0200
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-07 17:19 +0100
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-09-08 23:12 +0200
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-08 22:31 +0100
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-07-22 06:40 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-07-22 06:03 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-25 21:53 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 11:11 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-07-21 17:42 +0000
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-07-24 07:53 +0200
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-25 21:57 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-03 13:13 -0700
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-08-03 15:20 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-05 01:15 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 09:19 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 19:51 +0000
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 20:03 +0000
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-08-16 13:43 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 21:08 +0000

csiph-web