#ifndef __KERNEL__
#define __KERNEL__
#endif
#ifndef MODULE
#define MODULE
#endif
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>

#define SUCCESS 0
#define DEVICE_NAME "kueng_char_dev"
#define BUF_LEN 50

static int Device_Open = 0;
static char Message[BUF_LEN];
static int Major;

static int mydev_open(struct inode *inode, struct file *file) {
    if (Device_Open) return -EBUSY;
    Device_Open = 1;
    return 0;
}

static int mydev_release(struct inode *inode, struct file *file) {
    Device_Open = 0;
    return 0;
}

static ssize_t mydev_read(struct file *file, char *buffer, size_t length, loff_t *f_pos) {
    return copy_to_user(buffer, Message, length);
}

static ssize_t mydev_write(struct file *file, const char *buffer, size_t length, loff_t *f_pos) {
    int len = BUF_LEN < length ? BUF_LEN : length;
    return copy_from_user(Message, buffer, len);
}

static struct file_operations fops = {
    .open = mydev_open,
    .release = mydev_release,
    .read = mydev_read,
    .write = mydev_write,
};

int init_module(void) {
    Major = register_chrdev(0, DEVICE_NAME, &fops);
    if (Major < 0) {
        printk("Register char device failed\n");
        return Major;
    }
    printk("Registered with major %d\n", Major);
    return 0;
}

void cleanup_module(void) {
    unregister_chrdev(Major, DEVICE_NAME);
}

MODULE_LICENSE("GPL");
MODULE_AUTHOR("KUENG");