Re: [PATCH] scanf.3: Do not mention the ERANGE error

Alejandro Colomar
2022-12-09T19:33:31Z

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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?

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.

> 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.

>
> 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

--
<http://www.alejandro-colomar.es/>

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