Re: Does reading an uninitialized object have undefined behavior?

From	Spiros Bousbouras <spibou@gmail.com>
Newsgroups	comp.std.c
Subject	Re: Does reading an uninitialized object have undefined behavior?
Date	2023-08-31 18:18 +0000
Organization	To protect and to server
Message-ID	<S+3le+7=sc9SPmPL3@bongo-ra.co> (permalink)
References	(11 earlier) <868r9xz0ek.fsf@linuxsc.com> <5+eRe7cp3yQjL4=AX@bongo-ra.co> <86sf82ulmb.fsf@linuxsc.com> <KvVxh3+WExIyDnM+5@bongo-ra.co> <86zg28t563.fsf@linuxsc.com>

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On Wed, 30 Aug 2023 17:40:52 -0700
Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
> Spiros Bousbouras <spibou@gmail.com> writes:
> 
> > On Tue, 29 Aug 2023 04:35:40 -0700
> > Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
> >
> >> Spiros Bousbouras <spibou@gmail.com> writes:

[...]

> >> Not at all.  It's easy to write a specification that says what we
> >> want to do, along similar lines to what is said in the footnote
> >> about union member access in section 6.5.2.3
> >>
> >>    If the member used to access the contents of a union object
> >>    is not the same as the member last used to store a value in
> >>    the object, the appropriate part of the object representation
> >>    of the value is reinterpreted as an object representation in
> >>    the new type as described in 6.2.6 (a process sometimes called
> >>    "type punning").  This might be a trap representation.
> >
> > Works for me but it would be good to know that this is how compiler
> > writers actually understand  -fno-strict-aliasing .  [...]
> 
> No, it wouldn't.  Implementations follow the C standard, not
> the other way around.  Looking at what implementations do for
> the -fno-strict-aliasing flag is worse than a waste of time.

Actually the influence goes in both directions. In theory the standard is the
ultimate authority , in practice whatever C compilers one has access to. For
now the standard doesn't have something like  -fno-strict-aliasing  so if one
needs it then looking at what implementations do is the only option. But even
the standard committee should look at it and whether C programmers find it
useful to decide what around such lines (if anything) should go into the
standard.

> >> There isn't any reason to think malloc() should be writable in
> >> completely portable C.  That's the point of putting malloc() in
> >> the system library in the first place.  By the way, with type
> >> punning semantics mentioned above being the default, and with the
> >> alignment features added in C11, I think it is possible to write
> >> malloc() in portable C without needed any additional language
> >> changes.  But even if it isn't that is no cause for concern;  one
> >> of the principal reasons for having a system library is to
> >> provide functionality that the core language cannot express (or
> >> cannot express conveniently).
> >
> > One might want to experiment with different allocation algorithms
> > and it seems to me that this sort of thing is within the "remit" of
> > C. So ideally one should be able to write it in C [...]
> 
> You're conflating writing something in C and writing something
> in completely portable C.  It's already possible to do these
> things writing in C.

I wrote

    One might want to experiment with different allocation algorithms and it
    seems to me that this sort of thing is within the "remit" of C. So
    ideally one should be able to write it in C and prove , starting from the
    standard or precise specifications in compiler documentation , that it
    works correctly. I don't necessarily mean prove the correctness of the
    whole code but certain key parts.

.This doesn't conflate anything. One can do the writing but can one do the
proving or something close ?

-- 
vlaho.ninja/prog

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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

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