linux_kernel/drivers/tty/n_null.c

// SPDX-License-Identifier: GPL-2.0
#include <linux/types.h>
#include <linux/errno.h>
#include <linux/tty.h>
#include <linux/module.h>

/*
 *  n_null.c - Null line discipline used in the failure path
 *
 *  Copyright (C) Intel 2017
 *
 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2
 *  as published by the Free Software Foundation.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 */

static int n_null_open(struct tty_struct *tty)
{
	return 0;
}

static void n_null_close(struct tty_struct *tty)
{
}

static ssize_t n_null_read(struct tty_struct *tty, struct file *file,
			   unsigned char __user * buf, size_t nr)
{
	return -EOPNOTSUPP;
}

static ssize_t n_null_write(struct tty_struct *tty, struct file *file,
			    const unsigned char *buf, size_t nr)
{
	return -EOPNOTSUPP;
}

static void n_null_receivebuf(struct tty_struct *tty,
				 const unsigned char *cp, char *fp,
				 int cnt)
{
}

static struct tty_ldisc_ops null_ldisc = {
	.owner		=	THIS_MODULE,
	.magic		=	TTY_LDISC_MAGIC,
	.name		=	"n_null",
	.open		=	n_null_open,
	.close		=	n_null_close,
	.read		=	n_null_read,
	.write		=	n_null_write,
	.receive_buf	=	n_null_receivebuf
};

static int __init n_null_init(void)
{
	BUG_ON(tty_register_ldisc(N_NULL, &null_ldisc));
	return 0;
}

static void __exit n_null_exit(void)
{
	tty_unregister_ldisc(N_NULL);
}

module_init(n_null_init);
module_exit(n_null_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Alan Cox");
MODULE_ALIAS_LDISC(N_NULL);
MODULE_DESCRIPTION("Null ldisc driver");