Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Regarding assignment to struct
Date: Mon, 05 May 2025 13:43:31 -0700
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Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
>> Andrey Tarasevich <noone@noone.net> writes:
>> [...]
>>
>>>   #include <stdio.h>
>>>
>>>   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", pa, pb, pc);
>>>   }
>>
>> [...]
>>
>> I think that code has undefined behavior.
>
> Right.  [*]
>
>> (a = b) is an rvalue that refers to an object of type struct S with
>> temporary lifetime.  pc holds the address of a subobject of that
>> temporary object.  The object reaches the end of its lifetime at the end
>> of the evaluation of the full expression.  You then print its value.
>
> Even if the printf() statement were replaced by
>
>    (void)pc;
>
> the behavior would be undefined, because the pointer held in pc
> becomes indeterminate as soon as the statement containing the
> assignment to pc completes.

Agreed.

> [*] Assuming C11 semantics.  At best inadvisable under C99
> semantics, and a constraint violation under C90 semantics.

What C90 constraint does it violate?  Both gcc and clang reject it
with "-std=c90 -pedantic-errors", with an error message "ISO C90
forbids subscripting non-lvalue array", but I don't see a relevant
constraint in the C90 standard.

I know that C11 introduced "temporary lifetime" to cover cases
like this.  In C99, the wording for the indexing operator implicitly
assumes that there's an array object; if there isn't, I'd argue the
behavior is undefined by omission.  I'm not aware of any relevant
change from C90 to C99.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */