Adding a new object file to kernel source code and use it's function

[mhdjavanmardi]

Hi all,

I have an object file foo.o which provide a function foo().
when I use this object file with simple main.c as shown below it works correctly.

#main.c
include "foo.h"

int main()
{
        foo();
        return 0;
}

I use gcc for compile & link.

gcc -o foo foo.o main.c

Goal:
I want to use foo() in init/main.c of my linux-2.6.35 kernel source code.

What I've done:
I added foo.o and foo.h to the kernel/init/ directory, modified my init/main.c to use foo(), and added foo.o to obj-y in init/Makefile as shown below.

obj-y := main.o version.o mounts.o foo.o

Problem:
I get these errors when I make the kernel..

init/built-in.o: In function `foo1':
foo.c:(.text+0xe3): undefined reference to `printf'
foo.c:(.text+0xf8): undefined reference to `printf'
foo.c:(.text+0x19f): undefined reference to `printf'
foo.c:(.text+0x1b8): undefined reference to `printf'
init/built-in.o: In function `foo2':
foo.c:(.text+0x1e6): undefined reference to `printf'
init/built-in.o:foo.c:(.text+0x1ff): more undefined references to `printf' follow
init/built-in.o: In function `foo3':
foo.c:(.text+0xcb0): undefined reference to `putchar'
foo.c:(.text+0xe29): undefined reference to `stdout'
foo.c:(.text+0xe3c): undefined reference to `fputc'
foo.c:(.text+0xe51): undefined reference to `stdout'
foo.c:(.text+0xe63): undefined reference to `fputc'
foo.c:(.text+0xe76): undefined reference to `puts'
.
.
.

I know the problem arises from Makefile, but I don't know how to solve it.
I would appreciate any help!
Thanks!

[Rubberman]

You have some studying to do. Writing kernel code is NOT the same as normal user code, which is what you have written. There are documents and books online that will help, but you have a steep learning curve to overcome before you can do this effectively. Note that the standard library functions as printf, fputc, and such DO NOT WORK with kernel code!

[mhdjavanmardi]

Thank you Rubberman for your answer.
How can I convert some code to work with kernel code?? can you offer me some documents or books to do this?

[Rubberman]

Available in ebook form, and in many cases for free online:

Linux Kernel Development, Author: Robert Love, Publisher: Addison-Wesley
Linux Device Drivers, Author: Jonathan Corbet, Publisher: O'Reilly

Google searches should find them PDQ.