linux-kernel - Re: [Question] How to print size

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Message-ID: <CAMuHMdUaSOr2iDHzWhkKXwwxdvHhpL1C_UR_tw9B1ALhebbdtQ@mail.gmail.com>
Date:	Mon, 15 Dec 2014 12:00:25 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Masahiro Yamada <yamada.m@...panasonic.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Andrew Murray <amurray@...-data.co.uk>,
	linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [Question] How to print size_t type variable?

Hi Yamada-san,

On Mon, Dec 15, 2014 at 11:51 AM, Masahiro Yamada
<yamada.m@...panasonic.com> wrote:
> Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> On Mon, Dec 15, 2014 at 10:32 AM, Masahiro Yamada
>> <yamada.m@...panasonic.com> wrote:
>> > I read through Documentation/printk-formats.txt
>> >
>> > It clearly says to use "%zu" or "%zx" to print size_t variables,
>> > but I still have a question.
>> >
>> >
>> > Assume we have code something like:
>> >
>> >     printk("%zx", (size_t)10);
>> >
>> >
>> > I think this code works fine as long as it includes
>> > the compiler-provided <stddef.h>.
>> >
>> > In the kernel space, however, <stddef.h> is never included.
>> > Instead, size_t is defined by include/linux/types.h
>> > and include/uapi/asm-generic/posix_types.h.
>> >
>> >
>> > That is, size_t is defined independently from the compiler you are using,
>> > although the compiler still decides which variable type is expected for the "%zx" format.
>>
>> That's correct.
>>
>> > This causes compiler warnings for some compilers.
>> >
>> > On bare-metal m68k toolchains, for example, size_t is "unsignd long",
>> > whearas it is "unsigned int" on kernel.org m68k toolchains.
>> >
>> >
>> > I see such warnings when I built the kernel with bare-metal m68k toolchains.
>> >
>> >
>> > $ git describe
>> > v3.18
>> > $ make ARCH=m68k  CROSS_COMPILE=m68k-elf-  defconfig all
>> >   HOSTCC  scripts/basic/fixdep
>> >   HOSTCC  scripts/kconfig/conf.o
>> >   SHIPPED scripts/kconfig/zconf.tab.c
>> >   SHIPPED scripts/kconfig/zconf.lex.c
>> >   SHIPPED scripts/kconfig/zconf.hash.c
>> >   HOSTCC  scripts/kconfig/zconf.tab.o
>> >   HOSTLD  scripts/kconfig/conf
>> > *** Default configuration is based on 'multi_defconfig'
>> > kernel/time/Kconfig:163:warning: range is invalid
>> > #
>> > # configuration written to .config
>> > #
>> >
>> >   [ snip ]
>> >
>> >   LD      init/mounts.o
>> >   CC      init/initramfs.o
>> > init/initramfs.c: In function 'populate_rootfs':
>> > init/initramfs.c:635:5: warning: format '%zd' expects argument of type 'signed size_t', but argument 2 has type 'ssize_t' [-Wformat]
>>
>> Please use a compiler configured for Linux, i.e. m68k-linux-*.
>
>
> Yes, I can use it, but I am still curious.
>
> Do we have a good reason to keep this limitation?
>
> (All the problem I could see for using GCC that was not configured for Linux
> was just the printk-related warnings.)
>
>
> Instead of hard-coding the size_t type,
> can we use compiler-provided __SIZE_TYPE__ (or include <stddef.h>) ??

Note that cris and s390 used __SIZE_TYPE__ in kernel headers before,
but it caused other compiler warnings due to a mismatch in the base types
for size_t and ssize_t, cfr. https://lkml.org/lkml/2012/8/12/36.
AFAIK, there's no __SSIZE_TYPE__.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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