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

Zack Weinberg
2022-12-14T02:15:39Z

Alejandro Colomar <alx.manpages@gmail.com> writes:
> Okay, so %s and $[ are at least usable. Useful? I don't know. Probably
> fgets(3) and then either <string.h> or <regex.h> functions or taking
> unterminated strings (pointer plus length) is a much better idea.

Yeah, agreed.

>> The other design-level issue affects all of the numeric conversions:
>> if the result of (abstract, infinite-precision) numeric input conversion
>> does not fit in the variable supplied to hold the result of that conversion,
>> the behavior is undefined. The manpage says that you get an ERANGE error
>> in this case, and that may be the behavior _glibc_ guarantees (I don’t
>> actually know for sure), but in the modern era of compilers drawing
>> inferences from undefined behavior, a guarantee by one C library is
>> not good enough.
>
> This, to me, is enough to mark them as deprecated in the manual page. Anyway,
> deprecating something is not removing it. It's just saying "hey, you shouldn't
> be using that; it's bad, and don't expect that ISO C will keep it around next
> century".

In my lexicon “deprecated” is a very strong statement, possibly because
I’m used to seeing it in the context of standards where it means “we
think we should never have added this in the first place, there’s no
plausible way to fix it, but we have to keep it around for backward
compatibility.”

The scanf numeric conversions could be fixed with a one-sentence edit to
the C standard: change the last sentence of http://port70.net/~nsz/c/c11/n1570.html#7.21.6.2p10
from “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” to “If this object does not have an appropriate type, the
behavior is undefined. If the result of the conversion cannot be
represented in the object, the execution of the directive fails; this
condition is a matching failure.” And, even if the C committee doesn’t
want to make that change, open-source C libraries can and should do it
unilaterally, as a documented implementation extension. I think that’s
a better plan than declaring most uses of *scanf “deprecated.”

>> And, the scanf family *can* be used safely with
>> sufficient care — read entire lines of input with getline,
>
> If getline(3) _is necessary_ to be safe, then I would deprecate the stream
> functions, and keep only the "s" variants. Is it?

Oh, right, the _third_ headache with fscanf.

Yes, I think it would be fair to say that it is almost always a mistake
to use the scanf variants that read directly from a FILE. The issue
here is, at its root, that people new to C _expect_ a scanf call to read
an entire line of input, but it doesn’t. This is especially problematic
for interactive input — they try to use plain scanf to read numeric
input, don’t realize that `scanf("%d", &arg)` doesn’t consume the \n in
the terminal’s line buffer _after_ the number, and get very confused
when a subsequent getchar() reads that \n instead of the ‘y’ or ‘n’ they
were expecting as a response to the _next_ prompt. But it’s _also_ a
problem for error recovery, because scanf will stop in the middle of the
line when a matching failure occurs, and if you naively assumed it would
throw away the rest of the line, you get an error cascade.

The recommended practice to avoid this trap, is that you should use one
of the functions that _does_ read an entire line of input, i.e. fgets or
getline, and then parse the line as a string. It would make sense for
the [f]scanf manpage to say that.

>> In a more sober tone of voice I suggest this text for the manpage:
…
> That makes sense to me. Would you mind sending a patch? :)

I do not have time to do that anytime soon. Also, maybe glibc’s
behavior on numeric input overflow should be fixed first.

zw