Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail From: Tim Rentsch Newsgroups: comp.lang.c Subject: Re: Loops (was Re: do { quit; } else { }) Date: Tue, 13 May 2025 18:38:32 -0700 Organization: A noiseless patient Spider Lines: 102 Message-ID: <86ecwsvunb.fsf@linuxsc.com> References: <20250415153419.00004cf7@yahoo.com> <86h62078i8.fsf@linuxsc.com> <20250504180833.00000906@yahoo.com> <86plggzilx.fsf@linuxsc.com> <86ldr4yx0x.fsf@linuxsc.com> <1000cs3$2234m$1@dont-email.me> <87sel8nqid.fsf@nosuchdomain.example.com> <86msbgw49b.fsf@linuxsc.com> <875xi4cevz.fsf@nosuchdomain.example.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Date: Wed, 14 May 2025 03:38:33 +0200 (CEST) Injection-Info: dont-email.me; posting-host="adcc0210618ce5c814d2d263cc71a7bf"; logging-data="2246136"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+2ENtE7x3/nzW4h8IUnYDUfZMbifaD+Hs=" User-Agent: Gnus/5.11 (Gnus v5.11) Emacs/22.4 (gnu/linux) Cancel-Lock: sha1:n868LTh3LverNEFicIGfqgRagYE= sha1:pNOl4WRrR2JBpTLN94671eI6nrw= Bytes: 5226 Keith Thompson writes: > Tim Rentsch writes: > >> Keith Thompson writes: > > [...] > >>> My personal interpretation is that this: >>> >>> void func(int arr[static 5]) { >>> } >>> >>> int main(void) { >>> int arr[10]; >>> func(arr+5); // OK >>> // func(arr+6); // UB >>> } >>> >>> is valid, because, for example, the last 5 elements of a 10-element >>> array object can be treated as a 5-element array object. gcc seems >>> to agree, based on the fact that it warns about func(arr+6) but >>> not about func(arr+5). >>> >>> This is a fundamental part of my mental model of C, but in a few >>> minutes of searching I wasn't able to find explicit wording in the >>> standard that supports it. >> >> In N1570, 6.7.6.3 p7. > > Did you mean to imply that that paragraph supports (or refutes) my > statement? [...] No. I posted the reference to say that the cited paragraph supports the conclusion that 'func(arr+6)' is undefined behavior. > """ > A declaration of a parameter as ??array of _type_?? shall > be adjusted to ??qualified pointer to _type_??, where the > type qualifiers (if any) are those specified within the [ and ] > of the array type derivation. If the keyword static also appears > within the [ and ] of the array type derivation, then for each call > to the function, the value of the corresponding actual argument > shall provide access to the first element of an array with at least > as many elements as specified by the size expression. > """ > > The question is whether, for example, the last 5 elements of a > 10-element array object can be treated as a 5-element array object. > If someone can cite wording in the standard that answers that > question, I'd appreciate it. (I'll be happier if the answer is yes.) To me it seems obvious that 6.7.6.3 p7 is meant to cover the case of 'func(arr+6)' as being undefined behavior. Note that 6.7.6.3 p7 doesn't say "array object", it says just "array". I believe the choice of wording is neither an accident nor an oversight. > Looking into this a bit more, I realize that the question doesn't > matter if there's no "static" keyword between the [ and ]. In that > case, the parameter is of pointer type, and the description of > pointer arithmetic (N1570 6.5.6p8) explicitly allows the pointer > to point to the i-th element of an array object. The wording for > [static N] is the only place I've seen (so far) that specifically > refers to the *first* element of an array object, raising the > question of whether a subobject of an array object is itself an > array object. Again, not an array object, just an array. > This might just be some slightly sloppy wording that was > introduced in C99 and never corrected. I draw the opposite conclusion. The wording of 6.7.6.3 p7 was carefully chosen so that it would cover cases like 'func(arr+6)'. > For example, given this code: > > ``` > void without_static(int arr[]) { > (void)arr[4]; > } > > void with_static(int arr[static 5]) { > (void)arr[4]; > } > > int main(void) { > int arr[10] = { 0 }; > without_static(arr+5); > with_static(arr+5); > } > ``` > > there's no problem with the call `without_static(arr+5)`, but the > call `with_static(arr+5)` has defined behavior if and only if the > last 5 elements of a 10-element array object can be treated as a > 5-element array object. That isn't what the C standard says. It just says "array", not "array object".