Hi, Dave,

Thank you for the kind explanation and I think it will help much for me.

I searched down the uClib as you told me and could locate the kernel sys_write function.

In my case, the arch is sparc, and it is defined in fs/read_write.c as you said(in linux kernel source).

I've checked that

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf, size_t, count)

is expanded to

=> asmlinkage long sys_write(unsigned int fd, const char __user * buf, size_t count)

and I was able to use prom_printf (write to uart) to print what I tried to print in busybox with printf.

Now I can see busybox printf at least through the uart port though it's not yet shown on the LCD(which I'm trying to map to tty1 and the shell).

Thank you!

Chan

From : "Dave Hylands" <dhylands@gmail.com>
Sent : 2014-03-13 02:54:40 ( +09:00 )
To : Kim Chan <ckim@etri.re.kr>
Cc : Mohan L <l.mohanphy@gmail.com>, kernelnewbies@kernelnewbies.org <kernelnewbies@kernelnewbies.org>
Subject : Re: where is the printf source for busybox?

Hi,

On Tue, Mar 11, 2014 at 11:55 PM, Kim Chan <ckim@etri.re.kr> wrote:

Wow, that was it. I'm happy to hear that because I thought it should end up with a system call anyway.

Then isn't there some codes making the system call? Or is it that the compiler understands that 'write' is a system call and inserts the assembly code for calling it by itself?

In uclibc, the write source code is found here:
http://git.uclibc.org/uClibc/tree/libc/sysdeps/linux/common/write.c

How the syscall is actually implemented is architecture specific, and will be in one of the directories here:
http://git.uclibc.org/uClibc/tree/libc/sysdeps/linux

For example, for ARM, with EABI, then I believe that it winds up here:
http://git.uclibc.org/uClibc/tree/libc/sysdeps/linux/arm/syscall-eabi.S

The SWI instruction transfers control to the kernel.

On the kernel side, how the syscalls get handled is also architecture specific, but it will generally wind up at a function called sys_FUNCTION, however the generation of the function name is usually hidden behind a macro.

So sys_write in the kernel winds up being declared here:
http://lxr.linux.no/#linux+v3.13.5/fs/read_write.c#L514

--
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com