This is xnu-8019. See this file in:
/*
 * Copyright (c) 1997-2006 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/*-
 * Copyright (c) 1982, 1986, 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)tty_conf.c	8.4 (Berkeley) 1/21/94
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/tty.h>
#include <sys/conf.h>

#ifndef MAXLDISC
#define MAXLDISC 8
#endif

#define l_noopen        ((l_open_t *)  &enodev)
#define l_noclose       ((l_close_t *) &enodev)
#define l_noread        ((l_read_t *)  &enodev)
#define l_nowrite       ((l_write_t *) &enodev)
#define l_norint        ((l_rint_t *)  &enodev)

static l_ioctl_t        l_noioctl;
static l_start_t        l_nostart;

/*
 * XXX it probably doesn't matter what the entries other than the l_open
 * entry are here.  The l_noioctl and ttymodem entries still look fishy.
 * Reconsider the removal of nullmodem anyway.  It was too much like
 * ttymodem, but a completely null version might be useful.
 */
#define NODISC(n) \
	{ l_noopen,	l_noclose,	l_noread,	l_nowrite, \
	  l_noioctl,	l_norint,	l_nostart,	ttymodem }

struct  linesw linesw[MAXLDISC] =
{
	/* 0- termios */
	{ ttyopen, ttylclose, ttread, ttwrite,
	  l_noioctl, ttyinput, ttwwakeup, ttymodem },
	NODISC(1),              /* 1- defunct */
	                        /* 2- NTTYDISC */
	{ ttyopen, ttylclose, ttread, ttwrite,
	  l_noioctl, ttyinput, ttwwakeup, ttymodem },
	NODISC(3),              /* TABLDISC */
	NODISC(4),              /* SLIPDISC */
	NODISC(5),              /* PPPDISC */
	NODISC(6),              /* loadable */
	NODISC(7),              /* loadable */
};

const int nlinesw = sizeof(linesw) / sizeof(linesw[0]);

static struct linesw nodisc = NODISC(0);

#define LOADABLE_LDISC 6
/*
 * ldisc_register: Register a line discipline.
 *
 * discipline: Index for discipline to load, or LDISC_LOAD for us to choose.
 * linesw_p:   Pointer to linesw_p.
 *
 * Returns: Index used or -1 on failure.
 */
int
ldisc_register(int discipline, struct linesw *linesw_p)
{
	int slot = -1;

	if (discipline == LDISC_LOAD) {
		int i;
		for (i = LOADABLE_LDISC; i < MAXLDISC; i++) {
			if (bcmp(linesw + i, &nodisc, sizeof(nodisc)) == 0) {
				slot = i;
			}
		}
	} else if (discipline >= 0 && discipline < MAXLDISC) {
		slot = discipline;
	}

	if (slot != -1 && linesw_p) {
		linesw[slot] = *linesw_p;
	}

	return slot;
}

/*
 * ldisc_deregister: Deregister a line discipline obtained with
 * ldisc_register.  Can only deregister "loadable" ones now.
 *
 * discipline: Index for discipline to unload.
 */
void
ldisc_deregister(int discipline)
{
	if (discipline >= LOADABLE_LDISC && discipline < MAXLDISC) {
		linesw[discipline] = nodisc;
	}
}

/*
 * Do nothing specific version of line
 * discipline specific ioctl command.
 */
static int
l_noioctl(__unused struct tty *tp, __unused u_long cmd, __unused caddr_t data,
    __unused int flags, __unused struct proc *p)
{
	return ENOTTY;
}

static void
l_nostart(__unused struct tty *tp)
{
}