[TUHS] C history question: why is signed integer overflow UB?

[Douglas McIlroy]

Idle thought; There's been mention of 1's complement. If overflow is
UB because of that possibility, maybe ==0 should be, too!

Doug

On Fri, Aug 15, 2025 at 2:44 PM John Levine <johnl at taugh.com> wrote:
>
> It appears that Luther Johnson <luther.johnson at makerlisp.com> said:
> >-=-=-=-=-=-
> >
> >I hear and understand what you're saying. I think what I'm trying to
> >point out, is that in C, as it was originally implemented, in
> >expressions "a + b", "a >> 1", "++a", C "does what the machine does".
>
> We just had the same argument in comp.arch and came to largely the same
> conclusion.  While overflow behavior on any particular machine may be
> predictable, there's no consistency from one machine to another,
> particularly back when there were still one's complement machines
> where people compiled C code (some of the Univac mainframes.)
>
> It isn't all that predictable even on a single machine.  I know several
> where overflow might or might not trap depending on a program-settable
> status bit.
>
> R's,
> John