[Question] How to print size_t type variable?

[Question] How to print size_t type variable?

[Masahiro Yamada]

Hi experts,

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.
 
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]




Any advice?
How to fix this issue?




Best Regards
Masahiro Yamada

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

[Geert Uytterhoeven]

Hi Yamada-san,


On Mon, Dec 15, 2014 at 10:32 AM, Masahiro Yamada
<yamada.m@jp.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-*.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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

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@linux-m68k.org> wrote:

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


Best Regards
Masahiro Yamada

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

[Geert Uytterhoeven]

Hi Yamada-san,

On Mon, Dec 15, 2014 at 11:51 AM, Masahiro Yamada
<yamada.m@jp.panasonic.com> wrote:
> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> On Mon, Dec 15, 2014 at 10:32 AM, Masahiro Yamada
>> <yamada.m@jp.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@linux-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

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

[Masahiro Yamada]

Hi Geert,


On Mon, 15 Dec 2014 12:00:25 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> Hi Yamada-san,
> 
> On Mon, Dec 15, 2014 at 11:51 AM, Masahiro Yamada
> <yamada.m@jp.panasonic.com> wrote:
> > Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> >> On Mon, Dec 15, 2014 at 10:32 AM, Masahiro Yamada
> >> <yamada.m@jp.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__.


I notice __kernel_(s)size_t has the same width as "long".
(The kernel does not support LLP64.  We just have to take LP64 into account.)

Perhaps, we should have hard-coded

typedef __kernel_ulong_t __kernel_size_t;
typedef __kernel_long_t __kernel_ssize_t;

and used "%lx" to print (s)size_t type variables.

I think we do not have a good reason to use "%zx",
although it might be too late.



Best Regards
Masahiro Yamada

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

[Geert Uytterhoeven]

Hi Yamada-san,

On Tue, Dec 16, 2014 at 4:05 AM, Masahiro Yamada
<yamada.m@jp.panasonic.com> wrote:
> On Mon, 15 Dec 2014 12:00:25 +0100
> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> On Mon, Dec 15, 2014 at 11:51 AM, Masahiro Yamada
>> <yamada.m@jp.panasonic.com> wrote:
>> > Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> >> On Mon, Dec 15, 2014 at 10:32 AM, Masahiro Yamada
>> >> <yamada.m@jp.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__.
>
> I notice __kernel_(s)size_t has the same width as "long".
> (The kernel does not support LLP64.  We just have to take LP64 into account.)

On 64-bit, __kernel_(s)size_t are indeed (unsigned) long
On 32-bit, __kernel_(s)size_t is usually (unsigned) int for historical
reasons.
Cfr. include/uapi/asm-generic/posix_types.h:

/*
 * Most 32 bit architectures use "unsigned int" size_t,
 * and all 64 bit architectures use "unsigned long" size_t.
 */
#ifndef __kernel_size_t
#if __BITS_PER_LONG != 64
typedef unsigned int    __kernel_size_t;
typedef int             __kernel_ssize_t;
typedef int             __kernel_ptrdiff_t;
#else
typedef __kernel_ulong_t __kernel_size_t;
typedef __kernel_long_t __kernel_ssize_t;
typedef __kernel_long_t __kernel_ptrdiff_t;
#endif
#endif

The are a few exceptions (e.g. avr32 and blackfin)

> Perhaps, we should have hard-coded
>
> typedef __kernel_ulong_t __kernel_size_t;
> typedef __kernel_long_t __kernel_ssize_t;
>
> and used "%lx" to print (s)size_t type variables.
>
> I think we do not have a good reason to use "%zx",
> although it might be too late.

In hindsight, it would have been better for (s)size_t to match (unsigned) long,
removing the need for %z.
However, that was realized only after 64-bit architectures were introduced.
It's definitely to late to change this for existing 32-bit architectures.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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