Re: Does reading an uninitialized object have undefined behavior?

From	Kaz Kylheku <864-117-4973@kylheku.com>
Newsgroups	comp.std.c
Subject	Re: Does reading an uninitialized object have undefined behavior?
Date	2023-08-16 20:03 +0000
Organization	A noiseless patient Spider
Message-ID	<ubja3a$3e365$1@dont-email.me> (permalink)
References	<87zg3pq1ym.fsf@nosuchdomain.example.com> <864jlfj34p.fsf@linuxsc.com> <871qgjlqe9.fsf@nosuchdomain.example.com>

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On 2023-08-03, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>> Repeating the question stated in the Subject line:
>>
>> Does reading an uninitialized object [always] have undefined
>> behavior?
>
> Thank you for taking the time to write that.
[ ... ]
> I'm not criticizing the author of the standard for making this mistake.
> Stuff happens.  It was likely a result of an oversight during the
> transition from C90 to C99.

[Supersede attempt to reduce quoted material.]

I would be in favor of a formal model of what "uninitialized" means
which could be summarized as below.

Implementors wishing to develop tooling to catch uses of uninitialized
data can refer to the model; if their tooling diagnoses only
what the model deems undefined, then the tool can be integrated
into a conforming implementation.

- Certain objects are unintialized, like auto variables without
  an initializer, or new bytes coming from malloc or realloc.

- What is undefined behavior is when an uninitialized value is used
  to make a control-flow decision, or when it is output, or otherwise
  passed to the host environment.

- The formal model defines "uninitialized" in terms of there being,
  in the abstract semantics, a "shadow value" corresponding to every
  byte of a value, and that shadow value indicates whether the
  corresponding byte is initialized or not.

- Shadow values propagate across copies, accesses and calculations.

- No special exception is needed for unsigned, other than that
  it doesn't have trap representations.

- This would be undefined:

  {
    int uninited;
    int *p = &uninited;
    int v = * (unsigned char *) p;

    if (v) ...   // undefined here

    printf("%d\n", v); // undefined

  No special blessing is required for unsigned char to access
  the object. The resulting value keeps carrying the shadow byte
  which indicates that it is uninitialized, and so when it is output,
  or used for a control flow decision, the behavior is undefined.

  memcpy can be written without outputting the bytes being copied,
  and without allowing their value sto control flow.

  If a structure is copied with memcpy, and has uninitialized padding,
  the shadow value models says that the destination object now
  has uninitialized padding.

- When a value is obtained by accessing an object which has one
  or more uninitialized bytes, the corresponding bytes of the
  value are uninitialized.

- When a calculation has any operands that have one or more
  uninitialized bytes, all bytes of the resulting value
  are uninitialized.

  E.g. if there is an int *p, which is used to access a value *p,
  where the low-order byte is initialized, then the low order
  byte of *p is  initialized; the other bytes are uninitialized.
  But in the value *p + 0, the entire value is uninitialized.
  Implementations following the model don't have to track individual
  bits or bytes through calculations. This could apply to type
  conversions. e.g. tif *p is of type unsigned char, and
  refers to an uninitialized byte, then the entire promoted
  int (or possibly unsigned int) value is uninitialized:
  all four bytes (or however many) of it.

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

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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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