Add system call

mansoda · #1 December 24th, 2010, 07:39

Hi,
I am a student studying FreeBSD Kernel. I want to ask how to add a system call to FreeBSD. Is there any detailed tutorial to follow?
Thanks indeed!

Juson

**DutchDaemon** · #2 December 24th, 2010, 17:32

Start here: http://www.freebsd.org/doc/en_US.ISO...opers-handbook

Zare · #3 December 24th, 2010, 19:50

The easiest (and AFAIK, preferred) way is to code a loadable kernel module which contains syscall code.
Search the google and find examples of skeletal, "hello world" style kernel module code, and Makefile example you'll use to compile it, kld skeleton is about 15 lines of code and Makefile is like two-three. Simple to begin with.

After you have a skeleton, make a variable definition that will hold your syscall number (don't use one that's already occupied, check /usr/src/sys/kern/syscalls.master), for instance

Code:

static int my_syscall_slot = 210;

Then define function that'll hold syscall's usable code

Code:

static int syscall_code(int something, char something_else, void *syscall_args)
{
   ...
}

syscall_args is mandatory, the first two are your optional arguments. Since this example uses two arguments for syscall, you need to define sysent for your syscall with 2.

Code:

static struct sysent my_syscall_sysent = { 2, syscall_code };

Then use SYSCALL_MODULE macro that'll declare the whole thing :

Code:

SYSCALL_MODULE(my_syscall, &my_syscall_slot, &my_syscall_sysent, put_the_name_of_the_module_entry_function_here, NULL);

So the args are : name (provisional), pointer to slot number, pointer to sysent, name of the kld entry function, optional callback argument for event handler.

Hope it helps, this is not tested and from the head. Last time i did a grounds up KLD was in days of FreeBSD 5.4, i might have missed something.

vdvluc · #4 December 29th, 2010, 03:22

You may want to look in /usr/share/examples/kld/syscall/ for example code.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
curiosity question, source port add or package add?	pgmrdlm	Installation and Maintenance of FreeBSD Ports or Packages	2	May 29th, 2010 16:45
dhclient and/or system can't add unroutable gateway	daikichi	Networking	1	May 4th, 2010 07:42
[Solved] system call explanation in the book	jronald	FreeBSD Development	2	March 21st, 2010 03:53
[Solved] how make a system call module?	MIDOSE	Userland Programming & Scripting	1	March 12th, 2010 10:06
SDL_Init() "Bad system call (core dumped)"	Eponasoft	Userland Programming & Scripting	0	August 8th, 2009 12:04