/* https://cirosantilli.com/linux-kernel-module-cheat#automatically-create-character-device-file-on-insmod */

#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/fs.h> /* register_chrdev, unregister_chrdev */
#include <linux/module.h>
#include <linux/seq_file.h> /* seq_read, seq_lseek, single_release */

#define NAME "lkmc_character_device_create"

static int major = -1;
static struct cdev mycdev;
static struct class *myclass = NULL;

static int show(struct seq_file *m, void *v)
{
	seq_printf(m, "abcd");
	return 0;
}

static int open(struct inode *inode, struct file *file)
{
	return single_open(file, show, NULL);
}

static const struct file_operations fops = {
	.llseek = seq_lseek,
	.open = open,
	.owner = THIS_MODULE,
	.read = seq_read,
	.release = single_release,
};

static void cleanup(int device_created)
{
	if (device_created) {
		device_destroy(myclass, major);
		cdev_del(&mycdev);
	}
	if (myclass)
		class_destroy(myclass);
	if (major != -1)
		unregister_chrdev_region(major, 1);
}

static int myinit(void)
{
	int device_created = 0;

	/* cat /proc/devices */
	if (alloc_chrdev_region(&major, 0, 1, NAME "_proc") < 0)
		goto error;
	/* ls /sys/class */
	if ((myclass = class_create(THIS_MODULE, NAME "_sys")) == NULL)
		goto error;
	/* ls /dev/ */
	if (device_create(myclass, NULL, major, NULL, NAME "_dev") == NULL)
		goto error;
	device_created = 1;
	cdev_init(&mycdev, &fops);
	if (cdev_add(&mycdev, major, 1) == -1)
		goto error;
	return 0;
error:
	cleanup(device_created);
	return -1;
}

static void myexit(void)
{
	cleanup(1);
}

module_init(myinit)
module_exit(myexit)
MODULE_LICENSE("GPL");