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Message-ID: <d6f24ca6-6800-4759-8dc7-2be40938b12d@lenardszolnoki.com>
Date: Wed, 9 Apr 2025 11:38:01 +0100
From: Lénárd Szolnoki <cpp@...ardszolnoki.com>
To: musl@...ts.openwall.com
Cc: nsz@...t70.net
Subject: Re: strtod doesn't set errno to ERANGE for slightly
out-of-range hex float
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.
>
> this looks valid (might depend on long double
> representation).
Good point, maybe I should have used strtof. Anyway, I am testing on x86_64 Linux, with
80bit long double.
>
> 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.
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.
>
> 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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