Re: [PATCH] printf: add __printf attribute

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Sat, Dec 06, 2025 at 12:52:53PM -0500, Tamir Duberstein wrote:
> On Sat, Dec 6, 2025 at 12:49 PM Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:
> > On Sat, Dec 06, 2025 at 12:13:34PM -0500, Tamir Duberstein wrote:
> > > On Sat, Dec 6, 2025 at 11:11 AM Andy Shevchenko
> > > <andriy.shevchenko@linux.intel.com> wrote:
> > > > On Sat, Dec 06, 2025 at 08:19:09AM -0500, Tamir Duberstein wrote:

...

> > > > > -static void
> > > > > +static void __printf(2, 3)
> > > >
> > > > 3?!
> > > >
> > > > I think it should be (2, 0). Yes, the both users call it with "%p..." in format
> > > > string, but the second parameter tells compiler to check the variadic
> > > > arguments, which are absent here. Changing 'const void *p' to '...' will align
> > > > it with the given __printf() attribute, but I don't know if this what we want.
> > >
> > > The second parameter is the first-to-check, it is not specific to
> > > variadic arguments. Using 0 means that no arguments are checkable, so
> > > the compiler only validates the format string itself and won’t
> > > diagnose mismatches with `p`. This works whether or not we later
> > > change `const void *p` to `...`.
> >
> > Yes, but this is fragile. As I explained it works only because we supply
> > the format string stuck to "%p", anything else will require reconsidering
> > the function prototypes. So, strictly speaking this should be (2, 0) if
> > we leave const void *p as is.
> >
> I believe this is not correct. As I said, 0 means "do not check
> arguments" so only the format string will be checked. See the existing
> uses of this annotation in this file:
> 
> static void __printf(7, 0)
> do_test(struct kunit *kunittest, const char *file, const int line, int
> bufsize, const char *expect,
> int elen, const char *fmt, va_list ap)
> 
> and
> 
> static void __printf(6, 7)
> __test(struct kunit *kunittest, const char *file, const int line,
> const char *expect, int elen,
> const char *fmt, ...)
> 
> as you can see, 0 is used only when the arguments are not in the
> function prototype at all. When variadic arguments are present, N+1 is
> used.

Yes to all what you said. And how does it object what I said? In the case
you are trying to add __print(2, 3) the 3rd one is *not* a variadic argument.
If you make it to be variadic, I will agree with __print(2, 3). Before that
it doesn't look right to me even if it works.

-- 
With Best Regards,
Andy Shevchenko