Path: csiph.com!eternal-september.org!feeder.eternal-september.org!nntp.eternal-september.org!.POSTED!not-for-mail From: Keith Thompson Newsgroups: comp.lang.c Subject: Re: Regarding assignment to struct Date: Sun, 01 Mar 2026 19:36:03 -0800 Organization: None to speak of Lines: 52 Message-ID: <874imyaonw.fsf@example.invalid> References: <86plgo7ahu.fsf@linuxsc.com> <20250505111213.00004b55@yahoo.com> <20250505120145.000014f8@yahoo.com> <87jz6uhkgo.fsf@nosuchdomain.example.com> <867c2u4nbd.fsf@linuxsc.com> <87r012ftl1.fsf@nosuchdomain.example.com> <86ldgbeyab.fsf@linuxsc.com> MIME-Version: 1.0 Content-Type: text/plain Injection-Date: Mon, 02 Mar 2026 03:36:04 +0000 (UTC) Injection-Info: dont-email.me; posting-host="904fdc8ce99c4265d8d70c325ce3d825"; logging-data="847899"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1826DNbyj/P4v+8Ybhnbrqg" User-Agent: Gnus/5.13 (Gnus v5.13) Cancel-Lock: sha1:XG3YATV+d3lIvJG719eGuJxfIxg= sha1:Mme8Yx4HqqjAL5/uywuba6WtJo4= Xref: csiph.com comp.lang.c:396716 Tim Rentsch writes: > Keith Thompson writes: >> Tim Rentsch writes: >>> Keith Thompson writes: >>>> Andrey Tarasevich writes: >>>> [...] >>>> >>>>> #include >>>>> >>>>> struct S { int a[10]; }; >>>>> >>>>> int main() >>>>> { >>>>> struct S a, b = { 0 }; >>>>> int *pa, *pb, *pc; >>>>> >>>>> pa = &a.a[5], >>>>> pb = &b.a[5], >>>>> pc = &(a = b).a[5], >>>>> printf("%p %p %p\n", (void *) pa, (void *) pb, (void *) pc); >>>>> } >>>>> >>>>> This version has no UB. >>>> >>>> I believe it does. [...] >>> >>> If you look again carefully, I expect you will reach a >>> different conclusion. >> >> Please explain. Or don't. >> >> I explained, in text that you snipped, why I think it has UB. >> If there's a flaw in my reasoning, I invite you to point it out >> without being mysterious. > > I wasn't thinking about your reasoning or what flaw there may have > been in it. My intention was to suggest a course of action that > would be helpful rather than to point out any sort of mistake. But > since you ask, I think the mistake you made was jumping to a > conclusion without looking carefully enough at the problem. The mistake I made was failing to notice that the line with the assignment to pc ends with a comma operator rather than a semicolon. The code was deliberately obfuscated, and I fell for it. I acknowledged that nearly a year ago. Why bring it up now? I find it extremely annoying. -- Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com void Void(void) { Void(); } /* The recursive call of the void */