Re: Regarding assignment to struct

On 06/05/2025 21:22, Scott Lurndal wrote:

David Brown <david.brown@hesbynett.no> writes:

On 06/05/2025 19:36, Waldek Hebisch wrote:

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.
>

>
I must admit I had not noticed that detail.

 That would get an immediate downcheck during review for exactly
that reason.

Of course.  In fact, if someone presented such code for review (and assuming I noticed the commas!) I'd have to consider whether it was done maliciously, intentionally deceptively, due to incompetence, or smart-arse coding.  In all my C coding experience, I can't recall ever coming across a single situation when I thought the use of the comma operator was appropriate in the kind of code I work with.
Other people, projects, and teams work with different standards, different requirements, and different kinds of code - there are a lot of things that are common practice in some C coding that are strongly rejected in my field (and perhaps vice versa).  So I am not suggesting that the comma operator is always bad in C - just that it is pretty much always bad in my line of work.
And of course Andrey was using it here to make a specific point in a discussion about C details, rather than real-life code.