printf_arginfo_size_function error handling

[Alejandro Colomar (man-pages)]

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.

Thanks,

Alex

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/