利用udev在/dev下动态生成/移除设备文件

用udev在/dev/下动态生成设备文件，这样用户就不用手工调用mknod了。

利用的kernel API：

     class_create         :     创建class
     class_destroy         :     销毁class
     class_device_create     :     创建device
     class_device_destroy     :     销毁device

注意，这些API是2.6.13开始有的，在2.6.13之前，应当使用

class_simple_create
class_simple_destroy
class_simple_device_add
class_simple_device_remove

这一系列，也就是ldd3第14章描述的。 详见:

Output:
===========================================
[root@localhost dynamic_dev_node]# insmod ./dummy_dev.ko
[root@localhost dynamic_dev_node]# file /dev/dummy_dev0
/dev/dummy_dev0: character special (250/0)
[root@localhost dynamic_dev_node]# rmmod dummy_dev.ko
[root@localhost dynamic_dev_node]# file /dev/dummy_dev0
/dev/dummy_dev0: ERROR: cannot open `/dev/dummy_dev0' (No such file or directory)

 CODE:

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

#define THIS_DESCRIPTION "\
This module is a dummy device driver, it register\n\
\t\ta char device, and utilize udev to create/destroy \n\
\t\tdevice node under /dev/ dynamicallly."

MODULE_LICENSE("GPL");
MODULE_AUTHOR("albcamus ");
MODULE_DESCRIPTION(THIS_DESCRIPTION);

#define DUMMY_MAJOR 250
#define DUMMY_MINOR 0
#define DUMMY_NAME "dummy_dev"

/**
* the open routine of 'dummy_dev'
*/
static int dummy_open(struct inode *inode, struct file *file)
{
     printk("Open OK\n");
     return 0;
}

/**
* the write routine of 'dummy_dev'
*/
static ssize_t dummy_write(struct file *filp, const char *bp, size_t count, loff_t *ppos)
{
     printk("Don't Write!\n");
     return 0;
}

/**
* the read routine of 'dummy_dev'
*/
static ssize_t dummy_read(struct file *filp, char *bp, size_t count, loff_t *ppos)
{
     return 0;
}

/**
* the ioctl routine of 'dummy_dev'
*/
static int dummy_ioctl(struct inode *inode, struct file *filep,
             unsigned int cmd, unsigned long arg)
{

     return 0;
}

/**
* file_operations of 'dummy_dev'
*/
static struct file_operations dummy_dev_ops = {
     .owner = THIS_MODULE,
     .open = dummy_open,
     .read = dummy_read,
     .write = dummy_write,
     .ioctl = dummy_ioctl,
};

/**
* struct cdev of 'dummy_dev'
*/
struct cdev *my_cdev;
struct class *my_class;

static int __init my_init(void)
{
     int err, devno = MKDEV(DUMMY_MAJOR, DUMMY_MINOR);


     /* register the 'dummy_dev' char device */
     my_cdev = cdev_alloc();
     cdev_init(my_cdev, &dummy_dev_ops);

     my_cdev->owner = THIS_MODULE;

     err = cdev_add(my_cdev, devno, 1);
     if (err != 0)
         printk("dummy pci device register failed!\n");

     /* creating your own class */
     my_class = class_create(THIS_MODULE, "dummy_class");
     if(IS_ERR(my_class)) {
         printk("Err: failed in creating class.\n");
         return -1;
     }

     /* register your own device in sysfs, and this will cause udevd to create corresponding device node */
     class_device_create(my_class, NULL, MKDEV(DUMMY_MAJOR, DUMMY_MINOR), NULL, DUMMY_NAME "%d", DUMMY_MINOR );

     return 0;
}

static void __exit my_fini(void)
{
     printk("bye\n");

     cdev_del(my_cdev);
     //kfree(my_cdev); no use. because that cdev_del() will call kfree if neccessary.


     class_device_destroy(my_class, MKDEV(DUMMY_MAJOR, DUMMY_MINOR));
     class_destroy(my_class);

}

module_init(my_init);
module_exit(my_fini);

从linux内核2.6的某个版本之后，devfs不复存在，udev成为devfs的替代。相比devfs，udev有很多优势，在此就不罗嗦了，提醒一点，udev是应用层的东东，不要试图在内核的配置选项里找到它;)
   加入对udev的支持很简单，以作者所写的一个字符设备驱动为例，在驱动初始化的代码里调用class_create为该设备创建一个class，再为每个设备调用class_device_create创建对应的设备。大致用法如下：

   struct class *myclass = class_create(THIS_MODULE, "my_device_driver");
   class_device_create(myclass, NULL, MKDEV(major_num, 0), NULL, "my_device");

   这样的module被加载时，udev daemon就会自动在/dev下创建my_device设备文件。