linux-kernel - Re: [jic23-iio:testing 124/129] drivers/iio/accel/msa311.c:993:24: warning: format specifies type 'unsigned char' but the argument has type 'unsigned int'

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Message-ID: <20220831002405.m3j5sug2rz7bdz5s@Rockosov-MBP>
Date:   Wed, 31 Aug 2022 03:24:05 +0300
From:   Dmitry Rokosov <ddrokosov@...rdevices.ru>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
CC:     kernel test robot <lkp@...el.com>, <llvm@...ts.linux.dev>,
        <kbuild-all@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [jic23-iio:testing 124/129] drivers/iio/accel/msa311.c:993:24:
 warning: format specifies type 'unsigned char' but the argument has type
 'unsigned int'

Hello Jonathan and Andy,

Sorry for such a late response, a couple of days ago my daughter was born.
So I couldn't reach my laptop :)

Please find my thoughts below.

> > > >> drivers/iio/accel/msa311.c:993:24: warning: format specifies type 'unsigned char' but the argument has type 'unsigned int' [-Wformat]
> > >                                               "msa311-%hhx", partid);
> > >                                                       ~~~~   ^~~~~~
> > >                                                       %x
> > >    1 warning generated.
> 
> > >    992                msa311->chip_name = devm_kasprintf(dev, GFP_KERNEL,
> > >  > 993                                                   "msa311-%hhx", partid);
> 
> > I'm thinking intent here was to limit range of what was printed. Maybe better to use
> > local u8 variable or cast?
> >
> > I can fix it up if that's fine with you - or even better send me a patch that fixes
> > it however you prefer!
> 
> Looking back at what Linus said about those specifiers, I would rather
> go with simple %x or %02x.
> 
> P.S. Surprisingly many C developers don't know the difference between
> %hhx and %02x, which is easy to check by
> 
>   char a = -1;
>   printf("%hhx <==> %02x\n", a, a);
>   a = 217;
>   printf("%hhx <==> %02x\n", a, a);

Thank you for pointing to Linus answer. I have explored it at the link:

https://lore.kernel.org/lkml/CAHk-=wgoxnmsj8GEVFJSvTwdnWm8wVJthefNk2n6+4TC=20e0Q@mail.gmail.com/

Actually, Linus described one exception to this rule, which I have
in my patch. I have an integer which I want to print as a char.
I see that Linus mentions it's a bad idea. I agree with that. But
currently %hhx => %02x replacement breaks the requested behavior, %02x
will not shrink integer value to char. I want to say, maybe it's better
just cast the value to u8 type and print as %x. What do you think? I can
prepare such a patch.

P.S. Andy's example to show the difference between %hhx and %02x makes
more clear why such a replacement is not acceptable here.

Output:
ff <==> ffffffff
d9 <==> ffffffd9

-- 
Thank you,
Dmitry