#include <linux/module.h>	/* Needed by all modules */
#include <linux/kernel.h>	/* Needed for KERN_INFO */
#include <linux/init.h>		/* Needed for the macros */
#include <linux/moduleparam.h>
#include <linux/fs.h>
#include <asm/uaccess.h>	/* for put_user */

#define DRIVER_AUTHOR "Vurguuzko <vurguuz@gmail.com>"
#define DRIVER_DESC   "A simple driver"
#define DEVICE_NAME "vurguuz"
#define BUF_LEN 80		/* Max length of the message from the device */

static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
static int __init module_start(void);
static void __exit module_stop(void);

static int myint = 5;
static int maj_num;	/* Major number assigned to our device driver */
static char msg[BUF_LEN];	/* The msg the device will give when asked */
static char *msg_Ptr;

static struct file_operations fops = {
	.read = device_read,
	.write = device_write,
};

static int __init module_start(void)
{
	maj_num = register_chrdev(0, DEVICE_NAME, &fops);
	if (maj_num < 0)
	{
		printk(KERN_ALERT "Registering char device failed with %d\n", maj_num);
		return maj_num;
	}

	printk(KERN_INFO "Hello world. ");
	printk(KERN_INFO "myint = %d\n", myint);
	printk(KERN_INFO "I was assigned major number %d. To talk to\n", maj_num);
	printk(KERN_INFO "the driver, create a dev file with\n");
	printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, maj_num);
	return 0;
}

static void __exit module_stop(void)
{
	printk(KERN_INFO "Goodbye world.\n");
}

static ssize_t device_read(struct file *filp,	/* see include/linux/fs.h   */
			   char *buffer,	/* buffer to fill with data */
			   size_t length,	/* length of the buffer     */
			   loff_t * offset)
{
	int bytes_read = 0;

	sprintf(msg, "Random\n");
	msg_Ptr = msg;

	/*
	 * If we're at the end of the message, 
	 * return 0 signifying end of file 
	 */
	if (*msg_Ptr == 0)
		return 0;

	/* 
	 * Actually put the data into the buffer 
	 */
	while (length && *msg_Ptr) {

		/* 
		 * The buffer is in the user data segment, not the kernel 
		 * segment so "*" assignment won't work.  We have to use 
		 * put_user which copies data from the kernel data segment to
		 * the user data segment. 
		 */
		put_user(*(msg_Ptr++), buffer++);

		length--;
		bytes_read++;
	}

	/* 
	 * Most read functions return the number of bytes put into the buffer
	 */
	return bytes_read;
}

/*  
 * Called when a process writes to dev file: echo "hi" > /dev/hello 
 */
static ssize_t
device_write(struct file *filp, const char *buff, size_t len, loff_t * off)
{
	printk(KERN_ALERT "written \"%s\".\n", buff);
	return len;
}

module_param(myint, int, 0);
module_init(module_start);
module_exit(module_stop);

MODULE_LICENSE("GPL");
MODULE_AUTHOR(DRIVER_AUTHOR);
MODULE_DESCRIPTION(DRIVER_DESC);