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From: antispam@fricas.org (Waldek Hebisch)
Newsgroups: comp.lang.c
Subject: Re: Regarding assignment to struct
Date: Tue, 6 May 2025 17:36:36 -0000 (UTC)
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Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
> 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", (void *) pa, (void *) pb, (void *) pc);
>>   }
>>
>> This version has no UB.
> 
> I believe it does.  pc points to an element of an object with
> temporary lifetime.  The value of pc is then used after the object
> it points to has reached the end of its lifetime.  At that point,
> pc has an indeterminate value.
> 
> N3096 6.2.4p2: "If a pointer value is used in an evaluation after
> the object the pointer points to (or just past) reaches the end of
> its lifetime, the behavior is undefined. The representation of a
> pointer object becomes indeterminate when the object the pointer
> points to (or just past) reaches the end of its lifetime."

Note commas above.  Assignment to pc and call to printf are parts
of a single expression, so use of pc is within lifetime of the
temporary object.

-- 
                              Waldek Hebisch