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'

Dmitry Rokosov
2022-08-31T00:33:10Z

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