From: "Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>
To: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: printf_arginfo_size_function error handling
Date: Wed, 24 Mar 2021 20:39:43 +0100 [thread overview]
Message-ID: <865294a1-542d-3ed2-314d-c8abf9b5df12@gmail.com> (raw)
In-Reply-To: <da225884-112d-0f66-91af-a1ad42266147@gmail.com>
Hi,
On 3/24/21 8:30 PM, Alejandro Colomar (man-pages) wrote:
> Hi,
>
> On 3/24/21 5:39 PM, Alejandro Colomar (man-pages) wrote:
>> Hi,
>>
>> I've been re-reading the docs for printf_register_specifier(), and
>> there's something in an example code that doesn't make much sense:
>>
>> <https://www.gnu.org/software/libc/manual/html_node/Printf-Extension-Example.html>
>> [[
>>
>> int
>> print_widget_arginfo (const struct printf_info *info, size_t n,
>> int *argtypes)
>> {
>> /* We always take exactly one argument and this is a pointer to the
>> structure.. */
>> if (n > 0)
>> argtypes[0] = PA_POINTER;
>> return 1;
>> }
>>
>> ]]
>>
>> In the code above there is a check that n>0, but:
>>
>> What can a user do if n<=0 ? There's no error reporting method, is there?
>> And, is it possible for such a case to happen? What to do there?
>>
>> I think either using an assert() or not checking at all might make more
>> sense (depending on how possible is n<=0 to happen). The current check
>> continues as if everything was fine, just without setting argtypes[0],
>> so if there's an error, it will be carried to some future stage, where
>> nasal daemons might unexpectedly happen.
>
> After reading the sources, I see a few things:
>
> - There's an undocumented error handling mechanism: you can return a
> negative value to signify an error. However, I couldn't understand what
> exactly happens when you report an error. Does the printf_function get
> called?
>
> - n seems to be nonnegative: it's called in three places
> (<stdio-common/vfprintf-internal.c:1821>,
> <stdio-common/printf-parsemb.c:316>, and stdio-common/printf-prs.c:94),
> and all of them make sure n is positive, so the test in the example is
> useless.
>
> - I could't understand how n>1 could be used from reading the
> (obfuscated) sources. And there's no documentation. Is it really
> supported? And how? Would you mind documenting that? Maybe with an
> example that makes use of the non-obvious features of this API.
>
> - Even though printf_arginfo_size_function supposedly handles the case
> of n>1, printf_function doesn't seem to handle it. It gets the array of
> arguments, but it doesn't get the size of the array, so the only valid
> assumption is that the array is of size 1; anything else might fail
> silently. If you use the misterious feature of multiple arguments for a
> single specifier, how do you guarantee that you'll receive as many slots
> in the array as you want? By checking it in the arginfo function and
> reporting an error if n is not big enough? Will that guarantee that
> printf_function is not called? Again, I see no documentation at all
> about this, and the sources are not the easiest to read.
>
If I understood correctly, the 3rd parameter argtypes of
printf_arginfo_size_function is an array, right?
So a more friendly prototype would be
typedef
int printf_arginfo_size_function(const struct printf_info *info,
size_t n,
int argtypes[static n],
int *size);
Also, what about the last 'size' argument of
printf_arginfo_size_function? Is it an array or just a pointer? If
it's a pointer, how do you pass the size of each of the arguments you
set in argtypes? Is the following prototype correct (I'm guessing)?:
typedef
int printf_arginfo_size_function(const struct printf_info *info,
size_t n,
int argtypes[static n],
int size[static n]);
Thanks,
Alex
--
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/
prev parent reply other threads:[~2021-03-24 19:39 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-24 16:39 Alejandro Colomar (man-pages)
2021-03-24 19:30 ` Alejandro Colomar (man-pages)
2021-03-24 19:39 ` Alejandro Colomar (man-pages) [this message]
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