fops.c

static ssize_t
mydev_read(struct file *filp, char __user *buf, size_t count, loff_t *pos)
{
	char tmp[] = "Kernel says hello";
	printk("[%s][%d]\n", __func__, __LINE__);
	copy_to_user(buf, tmp, sizeof(tmp));
	*pos = 0;
	return 0;
}

static ssize_t
mydev_write(struct file *filp, const char __user *buf,
        size_t count, loff_t *pos)
{
	char tmp[128] = {0};
	printk("[%s][%d]\n", __func__, __LINE__);
	copy_from_user(tmp, buf, sizeof(tmp));
	printk("%s", tmp);
	return count;
}

static int mydev_open(struct inode *inode, struct file *filp)
{
	printk("[%s][%d]\n", __func__, __LINE__);
	return 0;
}

static int
mydev_release(struct inode *inode, struct file *filp)
{
	printk("[%s][%d]\n", __func__, __LINE__);
	return 0;
}

static struct file_operations 
  mydev_fops = {
	.owner = THIS_MODULE,
	.open = mydev_open,
	.release = mydev_release,
	.read = mydev_read,
	.write = mydev_write,
	.unlocked_ioctl = mydev_ioctl,
};