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Groups > comp.lang.c > #396323
| From | wij <wyniijj5@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: On Undefined Behavior |
| Date | 2026-01-10 00:08 +0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <43306b875d180e212420d6e78cb8c73d82ffc3ab.camel@gmail.com> (permalink) |
| References | <10j6qdt$3q9n4$1@dont-email.me> <20260109155448.00001428@yahoo.com> |
On Fri, 2026-01-09 at 15:54 +0200, Michael S wrote:
> On Thu, 1 Jan 2026 22:54:05 +0100
> highcrew <high.crew3868@fastmail.com> wrote:
>
> > 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.
It is UB, what the implement is irrevant.
The for loop above is equivalent to:
for (int i = 0; i <= 3; i++) {
if (table[i] == v) return 1;
}
if(table[i]==v) { // implement defined
return 1;
}
// implement defined
So, always returning 1 is correct compilation (no way exists_in_table(v) will return non-1).
> > 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!
> >
>
> Personally, I am not shocked by gcc behavior in this case. May be,
> saddened, but not shocked.
> I am shocked by slightly modified variant of it.
>
> struct {
> int table[4];
> int other_table[4];
> } bar;
>
> int exists_in_table(int v)
> {
> for (int i = 0; i <= 4; i++) {
> if (bar.table[i] == v)
> return 1;
> }
> return 0;
> }
>
> An original variant is unlikely to be present in the code bases that I
> care about professionally. But something akin to modified variant could
> be present.
> Godbolt shows that this behaviour was first introduced in gcc5. It was
> backported to gcc4 series in gcc 4.8
>
> One of my suspect code bases currently at gcc 4.7. I was considering
> moving to 5.3. In lights of that example, I likely am not going to
> do it.
> Unless there is a magic flag that disables this optimization.
>
I am also shocked many seemingly missed.
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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
Re: On Undefined Behavior Michael S <already5chosen@yahoo.com> - 2026-01-03 20:48 +0200
Re: On Undefined Behavior highcrew <high.crew3868@fastmail.com> - 2026-01-04 14:38 +0100
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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