作者：刘洪涛，华清远见嵌入式培训中心讲师。
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 lhtif(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。
相关资源：敏捷开发V1.0.pptx