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Message-ID: <20250409125505.GC2724612@port70.net>
Date: Wed, 9 Apr 2025 14:55:05 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Lénárd Szolnoki <cpp@...ardszolnoki.com>
Cc: musl@...ts.openwall.com
Subject: Re: strtod doesn't set errno to ERANGE for slightly
out-of-range hex float
* Lénárd Szolnoki <cpp@...ardszolnoki.com> [2025-04-09 11:38:01 +0100]:
> On 08/04/2025 23:25, Szabolcs Nagy wrote:
> > * Lénárd Szolnoki <cpp@...ardszolnoki.com> [2025-03-24 10:55:36 +0000]:
> > > Hi,
> > >
> > > #include <stdlib.h>
> > > #include <assert.h>
> > > #include <errno.h>
> > >
> > > int main() {
> > > const char* const input = "0x1.fffffffffffff8p+1023";
> > > double ret = strtod(input, NULL);
> > > assert(errno == ERANGE);
> > > }
> > >
> > > The input is slightly out of range, DBL_MAX is 1.fffffffffffffp+1023 (one
> > > fewer binary 1 in the mantissa).
> > >
> > > The assert fails with musl libc, passes with glibc.
...
> > the overflow threshold is not calculated precisely
> > for the hexfloat case. __floatscan returns an in
> > range long double value that is rounded to inf by
> > strtod, but there is no overflow check there.
>
> An other test case is with a different behaviour is
> "0x1.fffffffffffff00000000000000000000000000000000000000000000001p+1023"
>
> This is even more slightly larger than DBL_MAX, therefore it should still
> return inf and set errno to ERANGE.
>
in nearest rounding mode i'd expect DBL_MAX.
in upward rounding mode musl seems to return inf.
> For this one strtod returns DBL_MAX. What happens is that this rounds
> towards the value of DBL_MAX within long double, and the conversion from
> long double to double is exact, so it does not overflow. Sort of a double
> rounding problem, but at the edge of the range of double.
there is only a single rounding, and i think that's fine.
> >
> > i guess hexfloat overflow check should be fixed.
> >
> > >
> > > musl version: 1.2.5 (as shipped in Debian sid)
> > >
> > > Regards,
> > > Lénárd Szolnoki
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