Re: System call problem

[linux-os]

On Sat, 26 Feb 2005, Josef E. Galea wrote:

> Hi,
>
> I am implemeting a new system call for a project I'm working on. I added the 
> system call to the file arch/i386/kernel/process.c and added the relevant 
> entries in the files arch/i386/entry.S and include/asm-i386/unistd.h. My 
> system call is made up of only two lines, a printk statement, and a return 
> statement which gets the value of a field that I added to the task_struct 
> structure.
>
> I compiled and booted the kernel and am trying to build a user space 
> application that uses my system call, however gcc is returning this error:
> /tmp/cc4zgzUr.o(.text+0x4e): In functiono `get_rmt_paging':
> : undefined reference to `errno'
>
> Can anyone help me with this?
>
> Thanks
> Josef

You can't use kernel headers in user-mode functions. For one thing,
they bring in undefined stuff.

Your user mode code can, of course get the definition of errno
by #include <errno.h>. That's the errno that exists in every user-
mode 'C' program-developed process space. However, if get_rmt_paging
is not one of your functions, you are in trouble by mixing up
user-mode and kernel-mode headers.

Normally, the return value of a kernel function is checked and
if it's negative, the positive equivalent is put into the global
variable errno and then the return value is changed to -1. This
is where the user-mode reference to errno occurs.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.10 on an i686 machine (5537.79 BogoMips).
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