For comparison, syscalls in NetBSD all have the following prototype:

	int syscall(struct proc *p, void *v, register_t *retval);

	So you want to shove the return value in *retval. What you return() from
	this routine is the error code to be returned. libc glue takes care of
	setting errno as needed for you.

	I'm not 100% how linux syscalls work, but I'd expect they are about the
	same.

linux syscalls return the value as the return value rather
than through *retval.  That means that you're limited to 
retirning `int' values.  ISTR that errors are returned 
by negating by subtracting the errno from -1.

Eduardo