RE: where is the printf source for busybox?

[Kim Chan <ckim@xxxxxxxxxx>]

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@xxxxxxxxx>
Sent : 2014-03-13 02:54:40 ( +09:00 )
To : Kim Chan <ckim@xxxxxxxxxx>
Cc : Mohan L <l.mohanphy@xxxxxxxxx>, kernelnewbies@xxxxxxxxxxxxxxxxx <kernelnewbies@xxxxxxxxxxxxxxxxx>
Subject : Re: where is the printf source for busybox?

Hi,

On Tue, Mar 11, 2014 at 11:55 PM, Kim Chan <ckim@xxxxxxxxxx> 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

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