Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.std.c > #6532

Re: Does reading an uninitialized object have undefined behavior?

Newsgroups comp.std.c
Date 2023-08-13 23:41 -0700
References (1 earlier) <87zg3pnuse.fsf@bsb.me.uk> <874jlxozzz.fsf@nosuchdomain.example.com> <87fs5hnipv.fsf@bsb.me.uk> <87a5vpnegz.fsf@nosuchdomain.example.com> <86a5uv95g7.fsf@linuxsc.com>
Message-ID <fcb2be8f-b346-421f-9804-5f94c93266b0n@googlegroups.com> (permalink)
Subject Re: Does reading an uninitialized object have undefined behavior?
From Martin Uecker <ma.uecker@gmail.com>

Show all headers | View raw

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.

Martin

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