From: Ian Abbott <abbotti@mev.co.uk>
To: Alejandro Colomar <alx.manpages@gmail.com>,
Alejandro Colomar <alx@kernel.org>
Cc: linux-man@vger.kernel.org, GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH] scanf.3: Do not mention the ERANGE error
Date: Mon, 12 Dec 2022 10:07:13 +0000 [thread overview]
Message-ID: <5af4f708-337f-fddf-9a2d-e0e4602d3a72@mev.co.uk> (raw)
In-Reply-To: <06f70d09-a258-7d6d-4a98-6a89ed761849@gmail.com>
On 09/12/2022 19:33, Alejandro Colomar wrote:
> Hi Ian,
>
> On 12/9/22 20:28, Ian Abbott wrote:
>> On 09/12/2022 18:59, Alejandro Colomar wrote:
>>> On 12/8/22 13:34, Ian Abbott wrote:
>>>> The `scanf()` function does not intentionally set `errno` to `ERANGE`.
>>>> That is just a side effect of the code that it uses to perform
>>>> conversions. It also does not work as reliably as indicated in the
>>>> 'man' page when the target integer type is narrower than `long`.
>>>> Typically (at least in glibc) for target integer types narrower than
>>>> `long`, the number has to exceed the range of `long` (for signed
>>>> conversions) or `unsigned long` (for unsigned conversions) for `errno`
>>>> to be set to `ERANGE`.
>>>>
>>>> Documenting `ERANGE` in the ERRORS section kind of implies that
>>>> `scanf()` should return `EOF` when an integer overflow is encountered,
>>>> which it doesn't (and doing so would violate the C standard).
>>>>
>>>> Just remove any mention of the `ERANGE` error to avoid confusion.
>>>>
>>>> Fixes: 646af540e467 ("Add an ERRORS section documenting at least
>>>> some of the errors that may occur for scanf().")
>>>> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
>>>> Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
>>>
>>> I see. How about saying something like "it may also fail for any of
>>> any errors that functions used to perform the conversions may fail"?
>>
>> It depends what you mean by "fail". These errors do not make scanf
>> return EOF.
>
> Just to clarify. Does scanf(3) _never_ fail (EOF) due to ERANGE? Or is
> it that ERANGE sometimes makes it fail, sometimes not?
The glibc implementation certainly doesn't return EOF when ERANGE is
detected. __vfscanf_internal() in stdio-common/vfscan-internal.c does
not contain any code to deal with ERANGE - it's just a side-effect of
the calls to __strtol_internal(), __strtoul_internal(),
__strtoll_internal(), or __strtoull_internal().
> If it's the former, I agree with your patch. When a function hasn't
> reported failure, errno is unspecified.
>
> If it's the latter, I'd write something about it.
For the glibc implementation, it's the former.
>> Technically, the behavior is undefined if the result of the conversion
>> cannot be represented in the object being assigned to by scanf. (In
>> the case of glibc, that probably results in either the integer object
>> being set to a truncated version of the input integer, or the integer
>> object being set to a truncated version of LONG_MIN or LONG_MAX,
>> depending on the actual number.)
>
> Hmm, UB. Under UB, anything can change, so error reporting is already
> unreliable. If EOF+ERANGE can _only_ happen under UB, I'd rather remove
> the paragraph. Please confirm.
Yes, it is UB as per C17 7.21.6 paragraph 10: "[...] Unless assignment
suppression was indicated by a *, the result of the conversion is placed
in the object pointed to by the first argument following the format
argument that has not already received a conversion result. If this
object does not have an appropriate type, or if the result of the
conversion cannot be represented in the object, the behavior is undefined."
>> Setting errno to 0 before calling scanf and expecting errno to have a
>> meaningful value when scanf returns something other than EOF is bogus
>> usage.
>
> Yep, that's bogus.
>
>
> Cheers,
>
> Alex
Best regards,
Ian
--
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prev parent reply other threads:[~2022-12-12 10:07 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20221208123454.13132-1-abbotti@mev.co.uk>
2022-12-09 18:59 ` Alejandro Colomar
2022-12-09 19:28 ` Ian Abbott
2022-12-09 19:33 ` Alejandro Colomar
2022-12-09 21:41 ` Zack Weinberg
2022-12-11 15:58 ` Alejandro Colomar
2022-12-11 16:03 ` Alejandro Colomar
2022-12-12 2:11 ` Zack Weinberg
2022-12-12 10:21 ` Alejandro Colomar
2022-12-14 2:13 ` Zack Weinberg
2022-12-14 10:47 ` Alejandro Colomar
2022-12-14 11:03 ` Ian Abbott
2022-12-29 6:42 ` Zack Weinberg
2022-12-29 6:39 ` Zack Weinberg
2022-12-29 10:47 ` Alejandro Colomar
2022-12-29 16:35 ` Zack Weinberg
2022-12-29 16:39 ` Alejandro Colomar
2022-12-12 15:22 ` Ian Abbott
2022-12-14 2:18 ` Zack Weinberg
2022-12-14 10:22 ` Ian Abbott
2022-12-14 10:39 ` Alejandro Colomar
2022-12-14 10:52 ` Ian Abbott
2022-12-14 11:23 ` Alejandro Colomar
2022-12-14 14:10 ` Ian Abbott
2022-12-14 16:38 ` Joseph Myers
2022-12-12 10:07 ` Ian Abbott [this message]
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