在驱动模块初始化函数中实现设备节点的自动创建

我们在刚开始写Linux设备驱动程序的时候，很多时候都是利用mknod命令手动创建设备节点，实际上Linux内核为我们提供了一组函数，可以用来在模块加载的时候自动在/dev目录下创建相应设备节点，并在卸载模块时删除该节点，当然前提条件是用户空间移植了udev。

内核中定义了STruct class结构体，顾名思义，一个struct class结构体类型变量对应一个类，内核同时提供了class_create(…)函数，可以用它来创建一个类，这个类存放于sysfs下面，一旦创建好了这个类，再调用device_create(…)函数来在/dev目录下创建相应的设备节点。这样，加载模块的时候，用户空间中的udev会自动响应device_create(…)函数，去/sysfs下寻找对应的类从而创建设备节点。

注意，在2.6较早的内核版本中，device_create(…)函数名称不同，是class_device_create(…)，所以在新的内核中编译以前的模块程序有时会报错，就是因为函数名称不同，而且里面的参数设置也有一些变化。

struct class和device_create(…) 以及device_create(…)都定义在/include/linux/device.h中，使用的时候一定要包含这个头文件，否则编译器会报错。

在2.6.26.6内核版本中，struct class定义在头文件include/linux/device.h中：

/*

* device classes

*/

struct class {

coNSt char *name;

struct module *owner;

nbsp;struct kset subsys;

struct list_head devices;

struct list_head interfaces;

struct kset class_dirs;

struct semaphore sem; /* locks children, devices, interfaces */

struct class_attribute *class_attrs;

struct device_attribute *dev_attrs;

int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);

void (*class_release)(struct class *class);

void (*dev_release)(struct device *dev);

int (*suspend)(struct device *dev, pm_message_t state);

int (*resume)(struct device *dev);

};

class_create(…)在/drivers/base/class.c中实现：

/**

* class_create - create a struct class structure

* @owner: pointer to the module that is to own this struct class

* @nAME: pointer to a string for the name of this class.

*

* This is used to create a struct class pointer that can then be used

* in calls to device_create().

*

* Note, the pointer created here is to be destroyed when finished by

* making a call to class_destroy().

*/

struct class *class_create(struct module *owner, cONst char *name)

{

struct class *cls;

int retval;

cls = kzalloc(sizeof(*cls), GFP_KERNEL);

if (!cls) {

retval = -ENOMEM;

goto error;

}

cls->name = name;

cls->owner = owner;

cls->class_release = class_create_release;

retval = class_register(cls);

if (retval)

goto Error;

return cls;

error:

kfree(cls);

return ERR_PTR(retval);

}

第一个参数指定类的所有者是哪个模块，第二个参数指定类名。

在class.c中，还定义了class_destroy(…)函数，用于在模块卸载时删除类。

device_create(…)函数在/drivers/base/core.c中实现：

/**

* device_create - creates a device and registers it with sysfs

* @class: pointer to the struct class that this device should be registered to

* @parent: pointer to the parent struct device of this new device, if any

* @devt: the dev_t for the char device to be added

* @fmt: string for the device's name

*

* This funcTIon can be used by char device classes. A struct device

* will be created in sysfs, registered to the specified class.

*

* A dev file will be created, showing the dev_t for the device, if

* the dev_t is not 0,0.

* If a pointer to a parent struct device is passed in, the newly created

* struct device will be a child of that device in sysfs.

* The pointer to the struct device will be returned from the call.

* Any further sysfs files that might be required can be created using this

* pointer.

*

* Note: the struct class passed to this function must have previously

* been created with a call to class_create().

*/

struct device *device_create(struct class *class, struct device *parent,

dev_t devt, const char *fmt, ...)

{

va_li