Re: [Question] How to print size_t type variable?

[Masahiro Yamada]

Hi Geert,




On Mon, 15 Dec 2014 10:38:23 +0100
Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:

> Hi Yamada-san,
> 
> 
> On Mon, Dec 15, 2014 at 10:32 AM, Masahiro Yamada
> <yamada.m@xxxxxxxxxxxxxxxx> 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>) ??


Best Regards
Masahiro Yamada

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