在Linux 2.6内核中的字符设备用cdev结构来描述，其定义如下：
struct cdev { struct kobject kobj; struct module *owner; //所属模块 const struct file_operations *ops; //文件操作结构 struct list_head list; dev_t dev; //设备号，int 类型，高12位为主设备号，低20位为次设备号 unsigned int count; };
下面一组函数用来对cdev结构进行操作：struct cdev *cdev_alloc(void);//分配一个cdev void cdev_init(struct cdev *, const struct file_operations *);//初始化cdev的file_operation void cdev_put(struct cdev *p);// //减少使用计数 //注册设备，通常发生在驱动模块的加载函数中 int cdev_add(struct cdev *, dev_t, unsigned); //注销设备，通常发生在驱动模块的卸载函数中 void cdev_del(struct cdev *);
使用cdev_add注册字符设备前应该先调用register_chrdev_region或alloc_chrdev_region分配设备号。register_chrdev_region函数用于指定设备号的情况，alloc_chrdev_region函数用于动态申请设备号，系统自动返回没有占用的设备号。
int register_chrdev_region(dev_t from, unsigned count, const char *name) ; int alloc_chrdev_region(dev_t *dev,unsigned baseminor,unsigned count,const char *name);
alloc_chrdev_region申请一个动态主设备号，并申请一系列次设备号。baseminor为起始次设备号，count为次设备号的数量。注销设备号(cdev_del)后使用unregister_chrdev_region：void unregister_chrdev_region(dev_t from,unsigned count) ;
cdev_add注册字符设备实例
struct file_operations simple_fops = {
.owner = THIS_MODULE, .read = simple_read, .write = simple_write, .open = simple_open, .release = simple_release, }; void simple_cleanup_module(void) { dev_t devno = MKDEV(simple_MAJOR, simple_MINOR); if (simple_devices) { cdev_del(&simple_devices->cdev); kfree(simple_devices); } unregister_chrdev_region(devno,1); } //模块初始化 int simple_init_module(void) { int result; dev_t dev = 0; dev = MKDEV(simple_MAJOR, simple_MINOR); result = register_chrdev_region(dev, 1, &quot;DEMO&quot;);//申请设备号 if (result <</SPAN> 0) { printk(KERN_WARNING &quot;DEMO: can't get major %d\n&quot;, simple_MAJOR); return result; } simple_devices = kmalloc(sizeof(struct simple_dev), GFP_KERNEL); if (!simple_devices) { result = -ENOMEM; goto fail; } memset(simple_devices, 0, sizeof(struct simple_dev)); //初始化设备结构 cdev_init(&simple_devices->cdev, &simple_fops); simple_devices->cdev.owner = THIS_MODULE; simple_devices->cdev.ops = &simple_fops; result = cdev_add (&simple_devices->cdev, dev, 1);//添加字符设备 if(result) { printk(KERN_NOTICE &quot;Error %d adding DEMO\n&quot;, result); goto fail; } return 0; fail: simple_cleanup_module(); return result; } module_init(simple_init_module); module_exit(simple_cleanup_module); |
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