From: J Wunsch <j@uriah.heep.sax.de>
Message-Id: <199509291911.UAA04787@uriah.heep.sax.de>
Subject: Re: Questions on system calls
To: humprey@linux1.dlsu.edu.ph
Date: Fri, 29 Sep 1995 20:11:39 +0100 (MET)
Cc: freebsd-hackers@FreeBSD.org (FreeBSD hackers),
        freebsd-questions@FreeBSD.org, joerg_wunsch@uriah.heep.sax.de
Reply-To: freebsd-questions@FreeBSD.org
In-Reply-To: <199509290958.CAA08560@freefall.freebsd.org> from "owner-freebsd-hackers@freefall.freebsd.org" at Sep 29, 95 02:58:19 am
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As owner-freebsd-hackers@freefall.freebsd.org wrote:
> 
> We're currently developing a software on FreeBSD platform, and generally, 
> we are not really familiar with system calls.  We were hoping you'd take 
> time out to answer these questions.  Thanks!
> 
> 1. Are system calls preemptive? 

I'm not exactly sure what you're meaning here, but if you're asking
whether the scheduler could switch processes while one is inside a
system call -- yes! this is a basic principle on every Unix
installation.

> 2. How do we create additional system calls? 

What exactly would you need it for?  It's normally not a good idea to
do it.  Of course, you've got the source, so you can do what you want,
but it's not the recommended procedure.

Perhaps you would better use an existing system call and hook up a new
driver?

> 3. Do we have to recompile the whole kernel if we create additional
> system calls? 

Normally yes, but you can also build a "loadable kernel module" (LKM).
Sample code for how to do this is under /usr/share/examples.  Note
that on FreeBSD 2.0.5, all Makefiles under this directory are missing,
and the source files are not buildable out of the box.  So you might
wish to grab more recent example files, e.g. from ftp.cdrom.com under
FreeBSD-current.

-- 
cheers, J"org

joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/
Never trust an operating system you don't have sources for. ;-)