Re: where is the printf source for busybox?

[Dave Hylands <dhylands@xxxxxxxxx>]

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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