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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Regarding assignment to struct
Date: Tue, 06 May 2025 13:06:02 -0700
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antispam@fricas.org (Waldek Hebisch) writes:
> 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.

Right, I didn't see the commas and assumed semicolons.  Changing the
formatting would have made the point much clearer.

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