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Groups > comp.lang.c > #399615
| From | Tim Rentsch <tr.17687@z991.linuxsc.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Constants and undefined behavior |
| Date | 2026-06-02 05:06 -0700 |
| Organization | A noiseless patient Spider |
| Message-ID | <86ik81cfk5.fsf_-_@linuxsc.com> (permalink) |
| References | (10 earlier) <10vh5f0$1g1ke$1@dont-email.me> <10vhgau$1iv7l$1@dont-email.me> <10vhkgl$1k67h$1@dont-email.me> <10vie7n$1r8io$1@kst.eternal-september.org> <10vm8tj$3uus7$7@dont-email.me> |
Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
> On 2026-06-01 00:54, Keith Thompson wrote:
>
>>> [...]
>>
>> Yes, a compiler can reduce (a + b) * 0 to just 0. But it's not
>> required to do so, and (INT_MAX + 1) * 0 still has undefined
>> behavior. Undefined behavior is determined by the rules of the
>> abstract machine *without* any adjustments permitted by the as-if
>> rule.
>
> This is something I really don't get in the actual C-logic...
>
> Using constants that can be determined at compile time is UB here,
> despite the '* 0' mathematically indicating an IMO clear semantics,
> but using variables is only UB possibly at runtime? [...]
There's an important distinction to make here. Consider this
program:
#include <limits.h>
int
foo(){
int zero = (INT_MAX+1)*0;
return zero;
}
int
main(){
return 0;
}
This program does not transgress the bounds of undefined behavior.
Even more than that, the program is strictly conforming, and must be
accepted by a conforming implementation.
Now let's change the program slightly:
#include <limits.h>
int
foo(){
static int zero = (INT_MAX+1)*0;
return zero;
}
int
main(){
return 0;
}
This program does transgress the bounds of undefined behavior. The
reason for the difference is that in the first program the semantics
of foo() is to evaluate the expression to be stored in 'zero' only
at runtime, whereas in the second program the semantics of foo() is
to evaluate the expression to be stored in 'zero' before program
startup (informally, "at compile time"). What matters is not
whether the offending expression /might/ be evaluated "at compile
time", but whether the offending expression /must/ be evaluated "at
compile time". Only in the second case is undefined behavior
inevitable (and thus it does not occur in the first program).
Fine point: strictly speaking, I believe the C standard allows even
the second program to complete translation phase 8 successfully, and
for any offending behavior to occur only when we actually try to run
the program. To say that another way, there is no requirement that
possible nasal demons be made manifest at any point before an actual
attempted execution. On the other hand, because that possibility is
there lurking in the background, there is no requirement that the
program be accepted, and could be rejected by a conforming compiler.
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Re: this girl calls c ugly "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-29 11:00 -0700
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Re: this girl calls c ugly BGB <cr88192@gmail.com> - 2026-05-28 14:07 -0500
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Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-29 19:09 +0100
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-05-29 22:00 +0200
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-29 22:14 +0100
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-29 12:09 -0700
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-29 17:05 +0100
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-05-29 18:34 +0200
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Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-29 18:53 +0100
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Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-29 13:56 -0700
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Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-29 15:52 -0700
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Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-30 02:03 +0100
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-29 19:02 -0700
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-30 12:12 +0100
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Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-30 16:43 -0700
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-05-31 03:37 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-30 19:53 -0700
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-06-02 12:16 +0200
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-31 11:47 +0200
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-06-02 12:55 +0200
Re: this girl calls c ugly Richard Harnden <richard.nospam@gmail.invalid> - 2026-05-31 09:12 +0100
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-31 11:49 +0200
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-31 11:10 +0100
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-31 13:18 +0200
Re: this girl calls c ugly James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-05-31 10:24 -0400
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-31 17:35 +0200
Re: this girl calls c ugly James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-05-31 12:46 -0400
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-31 22:24 +0200
Re: this girl calls c ugly James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-05-31 18:26 -0400
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-01 08:28 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-31 15:54 -0700
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-01 08:39 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-01 02:33 -0700
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-06-02 11:48 +0200
Re: this girl calls c ugly James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-06-02 06:37 -0400
Constants and undefined behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-02 05:06 -0700
Re: Constants and undefined behavior cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 16:28 +0000
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-02 05:35 -0700
Re: this girl calls c ugly Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-02 06:29 -0700
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-02 16:10 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-02 15:29 -0700
Re: this girl calls c ugly "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-06-02 13:59 -0700
Re: this girl calls c ugly cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 13:05 +0000
Parentheses (was: this girl calls c ugly) Bart <bc@freeuk.com> - 2026-06-02 14:38 +0100
Re: Parentheses (was: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 15:19 +0000
[OT] Fancy graphics (was Re: this girl calls c ugly) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-06-02 15:54 +0200
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) Bart <bc@freeuk.com> - 2026-06-02 15:19 +0100
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 15:19 +0000
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-06-02 17:39 +0200
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 16:36 +0000
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) scott@slp53.sl.home (Scott Lurndal) - 2026-06-02 21:33 +0000
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-06-02 14:43 -0700
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) ram@zedat.fu-berlin.de (Stefan Ram) - 2026-06-02 17:08 +0000
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 19:19 +0000
Re: [OT] Fancy graphics (was Re: this girl calls c ugly) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-02 15:39 -0700
Re: this girl calls c ugly scott@slp53.sl.home (Scott Lurndal) - 2026-06-02 15:10 +0000
Re: this girl calls c ugly cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 15:31 +0000
Re: this girl calls c ugly James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-05-31 10:15 -0400
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-31 16:29 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-31 03:45 -0700
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-31 04:02 -0700
Re: this girl calls c ugly Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-31 09:04 -0700
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-31 18:11 +0100
Re: this girl calls c ugly cross@spitfire.i.gajendra.net (Dan Cross) - 2026-05-31 19:34 +0000
Re: this girl calls c ugly Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-31 19:10 -0700
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-06-01 11:12 +0100
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-01 12:36 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-01 14:26 -0700
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-01 09:52 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-01 02:42 -0700
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-01 12:50 +0200
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-06-01 11:47 +0100
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-01 12:55 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-01 14:39 -0700
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-01 15:11 -0700
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-02 08:41 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-02 02:07 -0700
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-06-02 11:38 +0200
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-02 05:01 -0700
It is not futile to change the subject line (Was: this girl calls c ugly) gazelle@shell.xmission.com (Kenny McCormack) - 2026-06-02 12:39 +0000
Re: It is not futile to change the subject line (Was: this girl calls c ugly) gazelle@shell.xmission.com (Kenny McCormack) - 2026-06-02 12:42 +0000
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-02 11:46 +0200
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-06-02 11:09 +0100
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-02 05:25 -0700
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-06-02 14:20 +0100
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-02 15:12 -0700
Re: this girl calls c ugly Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-02 04:16 -0700
Re: this girl calls c ugly Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-01 15:23 -0700
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-01 16:06 -0700
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-06-01 23:24 +0100
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-06-02 11:35 +0200
Operator precedence in other (non-C, but "C-like") languages (Was: something about a girl) gazelle@shell.xmission.com (Kenny McCormack) - 2026-06-02 12:36 +0000
Re: this girl calls c ugly cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-01 11:04 +0000
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-01 14:04 +0200
Re: this girl calls c ugly cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-01 18:48 +0000
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-06-01 21:04 +0100
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-02 09:17 +0200
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-02 09:09 +0200
Re: this girl calls c ugly cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-02 12:07 +0000
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-06-02 14:37 +0200
Microcontroller software stacks (was Re: this girl calls c ugly) scott@slp53.sl.home (Scott Lurndal) - 2026-06-02 15:06 +0000
Re: this girl calls c ugly cross@spitfire.i.gajendra.net (Dan Cross) - 2026-05-31 19:11 +0000
Re: this girl calls c ugly Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-31 16:08 -0700
Re: this girl calls c ugly Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-05-31 16:32 -0700
Re: this girl calls c ugly Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-05-31 17:12 -0700
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-30 14:07 +0200
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-05-29 18:10 +0200
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-29 19:18 +0100
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-05-29 22:17 +0200
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-29 21:47 +0100
Re: this girl calls c ugly James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-05-29 15:57 -0400
Re: this girl calls c ugly Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-05-29 22:34 +0200
Re: this girl calls c ugly Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-05-29 23:18 +0000
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-30 01:26 +0100
Re: this girl calls c ugly Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-05-30 04:25 +0000
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-30 12:01 +0100
Re: this girl calls c ugly Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-05-31 00:29 +0000
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-05-31 10:59 +0100
Re: this girl calls c ugly Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-06-01 00:33 +0000
Re: this girl calls c ugly Bart <bc@freeuk.com> - 2026-06-01 02:26 +0100
Re: this girl calls c ugly David Brown <david.brown@hesbynett.no> - 2026-05-31 13:24 +0200
Re: this girl calls c ugly Bonita Montero <Bonita.Montero@gmail.com> - 2026-05-29 08:09 +0200
Re: this girl calls c ugly BGB <cr88192@gmail.com> - 2026-05-29 04:15 -0500
Re: this girl calls c ugly Bonita Montero <Bonita.Montero@gmail.com> - 2026-05-29 14:58 +0200
Re: this girl calls c ugly BGB <cr88192@gmail.com> - 2026-05-30 01:04 -0500
Re: this girl calls c ugly Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-05-29 23:20 +0000
Re: this girl calls c ugly Bonita Montero <Bonita.Montero@gmail.com> - 2026-05-30 11:18 +0200
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