On Tue, 6 Jul 2021 at 16:39, Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> wrote:
>
> On Sat, 2021-07-03 at 16:36 +0100, Jonny Grant wrote:
> >
> >
> > On 16/06/2021 14:36, Xi Ruoyao wrote:
> > > On Wed, 2021-06-16 at 14:01 +0100, Jonny Grant wrote:
> > >
> > > > Chris Latner also mentioned integer overflow being undefined, that
> > > > crops up too. There's no easy solution right, we need to hand write
> > > > code the checks? It's human-error prone if we need to manually code
> > > > each check. throwing in C++, or handling in C.
> > > >
> > > > if(N >= INT_MAX)
> > > > {
> > > > throw std::overflow_error("N >= INT_MAX would overflow in for
> > > > loop");
> > > > }
> > > >
> > > > for (i = 0; i <= N; ++i)
> > > > {
> > > > // ...
> > > > }
> > >
> > > For debugging use -fsanitize=undefined.
> > >
> > > And this is buggy anyway, no matter if there is an UB:
> > >
> > > for (unsigned i = 0; i <= N; i++)
> > > make_some_side_effect_without_any_undefined_behavior(i);
> > >
> > > If N may be UINT_MAX, this is not UB, but a dead loop. Programming is
> > > just human-error prone, even if you use "some programming language
> > > claimed to be able to eliminate many human errors" (I'll not say its
> > > name, to prevent a flame war).
> > >
> > Hi Xi
> >
> >
> > Checking the UINT_MAX would at least prevent the continual running of
> > any such buggy loop where it increments right? and the code within the
> > loop does not modify 'i'
> >
> > for (unsigned i = 0; (i <= N) && (i != UINT_MAX); i++)
> > make_some_side_effect_without_any_undefined_behavior(i);
>
> Even if i is signed, it will still "work" if you modify the &&
> expression a little:
>
> for (int i = 0; i != UINT_MAX && i < N; i++)
> make_some_side_effect_without_any_undefined_behavior(i);
>
> The problem is, now the behavior when N == UINT_MAX is same with when N
> == (UINT_MAX - 1). This can really puzzle someone who will call your
> function.
>
> If I'm designing this function I'd make it to interpret N as [0, N),
> instead of [0, N]:
>
> // Do something for each integer in [0, N).
> void do_something(int N)
> {
> for (int i = 0; i < N; i++)
> do_something_once(i);
> }
That's good, specify the limit.
>
> > Is there any way to have a way to make loop variables like this 'i'
> > const within the body of the loop, to avoid accidental changing of 'i'
> > by the body of the loop
>
> I don't think there is one in C. Perhaps, maybe use some "nasty"
> macros.
Probably I could use this pattern to avoid the risk of code modifying
the loop counter by accident.
for (int loop_var = 0; loop_var < N; loop_var++)
{
const int i = loop_var;
printf("i: %d\n", i);
}
Jonny
