Linux kernel 2.4 programming problem

["Federico Paparoni" <federico.paparoni@gmail.com>]

Hi all,
i'm trying to create a simple system call in a kernel 2.4 distribution.
I read guide/articles like this

http://irfanhabib.wordpress.com/2006/10/01/adding-a-system-call-linux-24x/

All the articles say the same things, but the implementation of my new
system call doesn't work.
I create a new entry in arch/i386/kernel/entry.S
I create a new entry in include/asm/unistd.h
I create a new function with the asmlinkage prefix in a source file
already defined (in ipc/sem.c for example)

When i create my user program, it doesn't work. Firstly when i try to
compile i receive a problem like this

: undefined reference to 'errno'

Then I include errno.h (also if i don't understand this step) and the
compilation works, but the syscall neverworks, because the call always
return -1.

Any help?
Any tutorial FOR DUMMY MAN that someone already used?

Cheers

Federico