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);
}
> 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.
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
