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

Ian Abbott
2022-12-12T10:17:17Z

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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