[Question] How to print size_t type variable?

From: Masahiro Yamada
Date: Mon Dec 15 2014 - 04:33:22 EST

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
$ 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

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