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freg-alloc_chrdev_region.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <asm/uaccess.h>

static struct cdev freg_cdev; //An instance of a character device
static dev_t ndev; //The node of the device

/* "freg"设备的全局变量 */
static int freg_var = 0;

static ssize_t freg_read(struct file *filp, char __user *buf, size_t sz, loff_t *off)
{
	printk("In the freg_read() function!\n");
	/* 将freg_var从内核空间复制到用户空间 */
	if (copy_to_user(buf, &freg_var, sizeof(int))) {
		return - EFAULT;
	}
	printk("freg_read enter ,the data is %d\n",freg_var);
	return sizeof(int);
}

static ssize_t freg_write(struct file* filp, const char __user *buf, size_t count, loff_t* f_pos)
{
	printk("In the freg_write() function!\n");
	/* 将用户空间的数据复制到内核空间的freg_var */
	if (copy_from_user(&freg_var, buf, sizeof(int))) {
		return -EFAULT;
	}
	printk("freg_write enter ,the data is %d\n",freg_var);
	return sizeof(int);    
}

// file_operations is a very important data struct in character type device
struct file_operations freg_ops = {
	.owner = THIS_MODULE,
	.read  = freg_read,
	.write = freg_write,
};

// initialization function of the module
static int __init freg_init(void)
{
	int err;

	printk(KERN_ALERT"Initializing freg device.\n");

	cdev_init(&freg_cdev, &freg_ops); //initialize the device instance
	err = alloc_chrdev_region(&ndev, 0, 1, "freg"); //allocate the device node number dynamically
	if(err < 0)
		return err;
	printk("freg_init():major=%d, minor=%d\n", MAJOR(ndev), MINOR(ndev));
	err = cdev_add(&freg_cdev, ndev, 1);//register the char_dev into the system
	if(err < 0)
		return err;

	printk(KERN_ALERT"Succedded to initialize freg device.\n");

	return 0;
}

static void __exit freg_exit(void)
{
	printk(KERN_ALERT"Destroy freg device.\n");
	cdev_del(&freg_cdev); //unregister the char_dev from the system
	unregister_chrdev_region(ndev, 1); //free the device node number
}

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Fake Register Driver");

module_init(freg_init);
module_exit(freg_exit);