Am Freitag, dem 27.02.2026 um 20:54 +0100 schrieb David Brown:
> On 27/02/2026 19:57, Martin Uecker via Gcc wrote:
> > Am Freitag, dem 27.02.2026 um 19:40 +0100 schrieb David Brown:
> > > On 27/02/2026 18:51, Martin Uecker via Gcc wrote:
>
> <snip>
>
> > > No, it does not trigger a warning with those values. (It will if
> > > someone changes MAX_POINTS to 250000, but that's the point of the
> > > -Wvla-larger-than=512000.)
> >
> >
> > You are right, thank you. I was somehow confusing this with
> > something else.
> >
>
> You showed that code in your other post:
>
> void foo(int n)
> {
> if (n > 100) return;
> char buf[n];
> foo(n);
> }
>
> I would say that definitely deserves a warning. Either make the
> parameter unsigned, or check if for negative values.
In any case, the warning one gets
warning: argument to variable-length array may be too large
is then a bit confusing.
But the problem is that I like to use signed types to be able to use the
sanitizer to find overflow eerrors, and I can also use the sanitizer to find
negative sizes for VLAs. So I peronally find the warning for the negative
part unhelpful.
> (Maybe the
> potential infinite recursion should have a warning too, but I'm guessing
> that's an unintentional blip!)
Yes ;-)
>
> > >
> > >
> > > <https://godbolt.org/z/xPo7ocWfr>
> > >
> > >
> > > One thing I have noticed about all this is the weird error messages.
> > > With the code "foo" above, but the -Wvla warning enabled, the error
> > > message is :
> > >
> > > """
> > > warning: ISO C90 forbids variable length array 'v' [-Wla]
> > > """
> > >
> > > That is despite using -std=c23.
> > >
> > > Even stranger, it changes when using "const int n = MAX_POINTS;" (in
> > > code like this, "n" should be declared "const") :
> > >
> > > """
> > > warning: ISO C90 forbids array 'v' whose size cannot be evaluated [-Wla]
> > > """
> > >
> > > The same warning is given if "const int n = MAX_POINTS;" is moved to
> > > file-scope - even though that requires that the initialiser for "n" can
> > > not only be evaluated, but must be a constant expression and the
> > > compiler knows the value of "n" can never be changed (without UB).
> > >
> > >
> > >
> > > So there is scope for improvement on the wording of the warnings here!
> >
> > I agree. Do you have a suggestion?
> >
> > Martin
> >
>
> How about just :
>
> warning: Variable length array 'v' [-Wvla]
>
> You will only see the warning if you specifically enable it - you are
> getting the warning because /you/ choose to enable the warning, not
> because ISO C90 forbids it. (And ISO C90 does not "forbid" VLAs - it
> just doesn't have the concept.)
>
> The same message could be given if "n" is "const" - it does not seem to
> me that there should be a difference here.
>
> Alternatively, you could have two levels of the warning - -Wvla=1 which
> warns on VLAs with a size not known until runtime, and -Wvla=2 which
> also warns on arrays with sizes known at compile-time but which are
> still VLAs in C. (Perhaps following the C++ rules, to give an
> optimisation-independent definition.) For consistency, -Wvla alone
> would default to -Wvla=2.
>
> Then you would have two warnings :
>
> warning: Dynamically sized variable length array [-Wvla=1]
>
> warning: Const sized array is variable length array in C [-Wvla=2]
Makes sense!
Note there is a proposal to make those constants true integer constants:
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3693.htm
Martin
>
>
>
