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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Regarding assignment to struct
Date: Mon, 05 May 2025 06:34:49 -0700
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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.


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