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| From | Tim Rentsch <tr.17687@z991.linuxsc.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: On Undefined Behavior |
| Date | 2026-01-09 01:42 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <86ms2nm89u.fsf@linuxsc.com> (permalink) |
| References | <10j6qdt$3q9n4$1@dont-email.me> |
highcrew <high.crew3868@fastmail.com> writes:
> Hello,
>
> While I consider myself reasonably good as C programmer, I still
> have difficulties in understanding undefined behavior.
> I wonder if anyone in this NG could help me.
>
> Let's take an example. There's plenty here:
> https://en.cppreference.com/w/c/language/behavior.html
> So let's focus on https://godbolt.org/z/48bn19Tsb
>
> For the lazy, I report it here:
>
> int table[4] = {0};
> int exists_in_table(int v)
> {
> // return true in one of the first 4 iterations
> // or UB due to out-of-bounds access
> for (int i = 0; i <= 4; i++) {
> if (table[i] == v) return 1;
> }
> return 0;
> }
>
> This is compiled (with no warning whatsoever) into:
>
> exists_in_table:
> mov eax, 1
> ret
> table:
> .zero 16
>
>
> Well, this is *obviously* wrong. And sure, so is the original code,
> but I find it hard to think that the compiler isn't able to notice it,
> given that it is even "exploiting" it to produce very efficient code.
>
> I understand the formalism: the resulting assembly is formally
> "correct", in that UB implies that anything can happen.
> Yet I can't think of any situation where the resulting assembly
> could be considered sensible. The compiled function will
> basically return 1 for any input, and the final program will be
> buggy.
>
> Wouldn't it be more sensible to have a compilation error, or
> at least a warning? The compiler will be happy even with -Wall -Wextra
> -Werror.
>
> There's plenty of documentation, articles and presentations that
> explain how this can make very efficient code... but nothing
> will answer this question: do I really want to be efficiently
> wrong?
>
> I mean, yes I would find the problem, thanks to my 100% coverage
> unit testing, but couldn't the compiler give me a hint?
>
> Could someone drive me into this reasoning? I know there is a lot of
> thinking behind it, yet everything seems to me very incorrect!
> I'm in deep cognitive dissonance here! :) Help!
The important thing to realize is that the fundamental issue here
is not a technical question but a social question. In effect what
you are asking is "why doesn't gcc (or clang, or whatever) do what
I want or expect?". The answer is different people want or expect
different things. For some people the behavior described is
egregiously wrong and must be corrected immediately. For other
people the compiler is acting just as they think it should,
nothing to see here, just fix the code and move on to the next
bug. Different people have different priorities.
After observing that, I think the right question is something like
"Given that compilers act in these surprising ways, how should I
protect my code so that it doesn't fall prey to the death-by-UB
syndrome, or what can I do to diagnose a possibly death-by-UB
situation when a strange bug crops up?" I don't pretend to have
good answers to these questions. The best advice I can give
(besides seeking help from others with more experience) is to be
persistent, and to realize that the skills needed for combating a
death-by-UB syndrome are rather different from the skills needed
for regular programming. I have been in the situation of being
made responsible for finding and correcting a death-by-UB kind of
symptom, and what's worse in programming environment where I
didn't have a great deal of familiarity or experience. Despite
those drawbacks the bug got diagnosed and fixed, and I attribute
that result mostly to tenacity and by being willing to consider
unusual or unfamiliar points of view.
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On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-01 22:54 +0100
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-02 00:26 +0200
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-01 23:57 +0100
Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-02 22:56 +0000
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Re: On Undefined Behaviour Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 21:42 +0000
Re: On Undefined Behaviour candycanearter07 <candycanearter07@candycanearter07.nomail.afraid> - 2026-01-07 06:40 +0000
Re: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-04 16:58 -0500
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-05 08:49 +0100
Re: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-01 17:49 -0500
Re: On Undefined Behavior antispam@fricas.org (Waldek Hebisch) - 2026-01-02 05:53 +0000
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-02 17:38 +0100
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-03 13:30 +0100
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-02 10:31 +0100
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-02 17:51 +0100
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-03 13:42 +0100
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-03 14:42 +0100
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-03 17:51 +0100
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 00:20 +0100
Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-02 22:52 +0000
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-03 23:47 +0100
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-04 12:58 +0100
Re: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-03 07:53 -0800
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 00:15 +0100
NULL dereference in embedded [was: On Undefined Behavior] highcrew <high.crew3868@fastmail.com> - 2026-01-04 00:25 +0100
Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-03 18:59 -0500
Re: NULL dereference in embedded [was: On Undefined Behavior] scott@slp53.sl.home (Scott Lurndal) - 2026-01-04 15:51 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] David Brown <david.brown@hesbynett.no> - 2026-01-05 08:55 +0100
Re: NULL dereference in embedded [was: On Undefined Behavior] Andrey Tarasevich <noone@noone.net> - 2026-01-03 17:24 -0800
Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 02:19 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-03 21:31 -0500
Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 04:52 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-04 13:00 -0500
Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-04 21:22 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-04 16:53 -0500
Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-05 00:16 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-05 06:41 -0500
Re: NULL dereference in embedded [was: On Undefined Behavior] David Brown <david.brown@hesbynett.no> - 2026-01-05 09:07 +0100
Re: NULL dereference in embedded [was: On Undefined Behavior] scott@slp53.sl.home (Scott Lurndal) - 2026-01-04 15:56 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] Andrey Tarasevich <noone@noone.net> - 2026-01-03 18:44 -0800
Re: NULL dereference in embedded [was: On Undefined Behavior] David Brown <david.brown@hesbynett.no> - 2026-01-04 17:16 +0100
Re: NULL dereference in embedded [was: On Undefined Behavior] antispam@fricas.org (Waldek Hebisch) - 2026-01-06 13:08 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-06 21:59 +0000
Re: NULL dereference in embedded [was: On Undefined Behavior] Andrey Tarasevich <noone@noone.net> - 2026-01-07 20:48 -0800
Re: NULL dereference in embedded [was: On Undefined Behavior] Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-08 23:56 +0000
Re: On Undefined Behavior Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-01-03 23:14 +0000
Re: On Undefined Behavior "Paul J. Lucas" <paul@lucasmail.org> - 2026-01-03 17:10 -0800
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 12:51 +0100
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-05 15:39 +0100
Re: On Undefined Behavior "Paul J. Lucas" <paul@lucasmail.org> - 2026-01-06 18:08 -0800
Re: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-07 11:25 +0100
Re: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-07 06:31 -0500
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-07 14:10 +0200
Re: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-09 01:42 -0800
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-09 14:36 +0200
Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-09 20:14 +0000
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-10 18:19 +0200
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-10 18:41 +0200
Re: On Undefined Behavior Kaz Kylheku <046-301-5902@kylheku.com> - 2026-01-13 23:31 +0000
Re: On Undefined Behavior antispam@fricas.org (Waldek Hebisch) - 2026-01-14 03:57 +0000
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-14 10:47 +0200
Re: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 14:49 +0000
Re: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-10 17:08 +0000
Re: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-11 11:48 -0800
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-11 22:52 +0200
Re: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-11 22:53 -0800
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 11:44 +0200
Re: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 20:29 -0500
Re: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 05:29 -0800
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-09 15:54 +0200
Re: On Undefined Behavior wij <wyniijj5@gmail.com> - 2026-01-10 00:08 +0800
UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 16:28 +0200
Re: UB or not UB? was: On Undefined Behavior bart <bc@freeuk.com> - 2026-01-12 15:58 +0000
Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 20:08 +0200
Re: UB or not UB? was: On Undefined Behavior scott@slp53.sl.home (Scott Lurndal) - 2026-01-12 20:02 +0000
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 21:09 -0500
Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-13 11:31 +0200
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:21 -0500
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:19 -0500
Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-14 09:35 +0100
Re: UB or not UB? was: On Undefined Behavior antispam@fricas.org (Waldek Hebisch) - 2026-01-14 17:23 +0000
Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-14 12:53 -0800
Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-14 14:43 -0800
Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 11:45 +0100
Re: UB or not UB? was: On Undefined Behavior James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-01-15 06:16 -0500
Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-15 04:04 -0800
Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 13:56 +0100
Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 05:34 -0800
Re: UB or not UB? was: On Undefined Behavior scott@slp53.sl.home (Scott Lurndal) - 2026-01-15 15:10 +0000
Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-15 16:23 +0100
Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 21:54 +0000
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 21:58 -0500
Re: UB or not UB? was: On Undefined Behavior Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-01-13 22:02 -0800
Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 14:24 +0000
Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-14 16:48 +0200
Re: UB or not UB? was: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-12 08:03 -0800
Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 19:36 +0200
Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-01-12 12:03 -0800
Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-12 22:41 +0200
Re: UB or not UB? was: On Undefined Behavior David Brown <david.brown@hesbynett.no> - 2026-01-13 09:12 +0100
Re: UB or not UB? was: On Undefined Behavior pa@see.signature.invalid (Pierre Asselin) - 2026-01-13 20:19 +0000
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:20 -0500
Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-02-03 21:53 -0800
Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 23:53 +0000
Re: UB or not UB? was: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-14 08:06 +0000
Re: UB or not UB? was: On Undefined Behavior Andrey Tarasevich <noone@noone.net> - 2026-01-13 08:11 -0800
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:10 -0500
Re: UB or not UB? was: On Undefined Behavior Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-01 22:53 -0800
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-13 22:20 -0500
Re: UB or not UB? was: On Undefined Behavior "James Russell Kuyper Jr." <jameskuyper@alumni.caltech.edu> - 2026-01-12 20:35 -0500
Re: UB or not UB? was: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-13 11:07 +0200
Re: On Undefined Behavior Tristan Wibberley <tristan.wibberley+netnews2@alumni.manchester.ac.uk> - 2026-01-13 20:37 +0000
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