利用udev、sys动态创建linux设备结点

作者：刘洪涛， 华清远见嵌入式学院讲师。

在Linux2.6内核中，devfs被认为是过时的方法，并最终被抛弃，udev取代了它。Devfs的一个很重要的特点就是可以动态创建设备结点。那我们现在如何通过udev和sys文件系统动态创建设备结点呢？

下面通过一个实例，说明udev、sys动态创建设备结点的方法。注意代码中红色的部分是为了实现动态创建设备结点添加的。
         #include <linux/module.h>
         #include <linux/kernel.h>
         #include <linux/init.h>
         #include <linux/fs.h>
         #include <linux/cdev.h>
         #include <asm/uaccess.h>
         #include <linux/device.h>
         MODULE_LICENSE ("GPL");
         int hello_major = 252;
         int hello_minor = 0;
         int number_of_devices = 1;
         char data[50]="foobar not equal to barfoo";
         struct cdev cdev;
         dev_t dev = 0;
         static int hello_open (struct inode *inode, struct file *file)
         {
         printk (KERN_INFO "Hey! device opened\n");
         return 0;
         }
         static int hello_release (struct inode *inode, struct file *file)
         {
         printk (KERN_INFO "Hmmm... device closed\n");
         return 0;
         }
         ssize_t hello_read (struct file *filp, char *buff, size_t count, loff_t *offp)
         {
         ssize_t result = 0;
         if (copy_to_user (buff, data, sizeof(data)-1))
         result = -EFAULT;
         else
         printk (KERN_INFO "wrote %d bytes\n", count);
         return result;
         }
         ssize_t hello_write (struct file *filp, const char *buf, size_t count, loff_t *f_pos)
         {
         ssize_t ret = 0;
         printk (KERN_INFO "Writing %d bytes\n", count);
         if (count>127) return -ENOMEM;
         if (count<0) return -EINVAL;
         if (copy_from_user (data, buf, count)) {
         ret = -EFAULT;
         }
         else {
         data[127]='\0';
         printk (KERN_INFO"Received: %s\n", data);
         ret = count;
         }
         return ret;
         }
         struct file_operations hello_fops = {
.         owner = THIS_MODULE,
.         open = hello_open,
.         release = hello_release,
.         read = hello_read,
.         write = hello_write
         };
         struct class *my_class;
         static void char_reg_setup_cdev (void)
         {
         int error, devno = MKDEV (hello_major, hello_minor);
         cdev_init (&cdev, &hello_fops);
         cdev.owner = THIS_MODULE;
         cdev.ops = &hello_fops;
         error = cdev_add (&cdev, devno , 1);
         if (error)
         printk (KERN_NOTICE "Error %d adding char_reg_setup_cdev", error);
         /* creating your own class */
         my_class =class_create(THIS_MODULE, "farsight_class");//add by lht
         if(IS_ERR(my_class)) {
         printk("Err: failed in creating class.\n");
         return ;
         }
         /* register your own device in sysfs, and this will cause udevd to create corresponding device node */
         class_device_create(my_class,NULL, devno, NULL,"farsight_dev");
         // device_create(my_class,NULL, devno,"farsight_dev");
         }
         static int __init hello_2_init (void)
         {
         int result;
         dev = MKDEV (hello_major, hello_minor);
         result = register_chrdev_region (dev, number_of_devices, "test");
         if (result<0) {
         printk (KERN_WARNING "hello: can't get major number %d\n", hello_major);
         return result;
         }
         char_reg_setup_cdev ();
         printk (KERN_INFO "char device registered\n");
         return 0;
         }
         static void __exit hello_2_exit (void)
         {
         dev_t devno = MKDEV (hello_major, hello_minor);
         cdev_del (&cdev);
         unregister_chrdev_region (devno, number_of_devices);
         class_device_destroy(my_class, devno);
         class_destroy(my_class);
         }
         module_init (hello_2_init);
         module_exit (hello_2_exit);v

在编译了驱动后，可以查看 /dev/farsight_dev设备结点，和 /sys/class/farsight_class/farsight_dev/ 本代码的测试环境是Ubantu7.04，内核版本是2.6.20-15-generi。在不同版本的内核中，有些系统函数的参数可能不太一样。