class_create(),device_create自动创建设备文件结点 .

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。(不太明白什么是devfs,udev,对linux中的文件系统没有什么概念呢。)

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。
class_create()

-------------------------------------------------

linux-2.6.22/include/linux/device.h

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

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.

在/sys/class/下创建类目录

class_device_create()

-------------------------------------------------

linux-2.6.22/include/linux/device.h

struct class_device *class_device_create(structclass *cls,

struct class_device *parent,

dev_t devt,

struct device *device,

const char *fmt, ...)

class_device_create - creates a class device and registersit with sysfs

@cls: pointer to the struct class that this device should beregistered to.

@parent: pointer to the parent struct class_device of thisnew device, if any.

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

@device: a pointer to a struct device that is assiociatedwith this class device.

@fmt: string for the class device's name

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;

structlist_head devices;

structlist_head interfaces;

structkset class_dirs;

struct semaphore sem; /* lockschildren, devices, interfaces */

struct class_attribute *class_attrs;

structdevice_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 canthen be used

* in calls to device_create().

*

* Note, the pointer created here is to be destroyed whenfinished 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 itwith sysfs

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

* @parent: pointer to the parent struct device of thisnew 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. Astruct device

* will be created in sysfs, registered to thespecified class.

*

* A "dev" file will be created, showing thedev_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 insysfs.

* The pointer to the struct device will be returnedfrom the call.

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

* pointer.

*

* Note: the struct class passed to this function musthave previously

* been created with a call to class_create().

*/

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

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

{

va_list vargs;

struct device *dev;

va_start(vargs, fmt);

dev =device_create_vargs(class, parent, devt, NULL, fmt, vargs);

va_end(vargs);

return dev;

}

第一个参数指定所要创建的设备所从属的类，第二个参数是这个设备的父设备，如果没有就指定为NULL，第三个参数是设备号，第四个参数是设备名称，第五个参数是从设备号。

下面以一个简单字符设备驱动来展示如何使用这几个函数

#include <linux/module.h>

#include <linux/types.h>

#include <linux/fs.h>

#include <linux/errno.h>

#include <linux/mm.h>

#include <linux/sched.h>

#include <linux/init.h>

#include <asm/io.h>

#include <asm/system.h>

#include <asm/uaccess.h>

#include <linux/slab.h>

#include <linux/device.h>

#include "memdev.h"
static int mem_major =MEMDEV_MAJOR;

struct mem_dev *mem_devp; /*设备结构体指针*/

bool have_data = false; /*表明设备有足够数据可供读*/

struct cdev cdev;

struct class *myclass=NULL;
/*文件打开函数*/

int mem_open(struct inode *inode, struct file *filp)

{

struct mem_dev *dev;

/*获取次设备号*/

int num = MINOR(inode->i_rdev);
if (num >=MEMDEV_NR_DEVS)

return-ENODEV;

dev = &mem_devp[num];

/*将设备描述结构指针赋值给文件私有数据指针*/

filp->private_data = dev;

return 0;

}
/*文件释放函数*/

int mem_release(struct inode *inode, struct file *filp)

{

return 0;

}
/*读函数*/

static ssize_t mem_read(struct file *filp, char __user *buf, size_t size,loff_t *ppos)

{

unsigned long p = *ppos;

unsigned int count = size;

int ret = 0;

struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/

/*判断读位置是否有效*/

if (p >= MEMDEV_SIZE)

return 0;

if (count > MEMDEV_SIZE - p)

count = MEMDEV_SIZE - p;
while (!have_data) /* 没有数据可读，考虑为什么不用if，而用while,中断信号唤醒
*/

{

if (filp->f_flags &O_NONBLOCK)

return-EAGAIN;

wait_event_interruptible(dev->inq,have_data);

}
/*读数据到用户空间*/

if (copy_to_user(buf, (void*)(dev->data + p), count))

{

ret = - EFAULT;

}

else

{

*ppos += count;

ret = count;

printk(KERN_INFO "read %d bytes(s) from %d\n",(int)count, (int)p);

}

have_data = false; /* 表明不再有数据可读 */

return ret;

}
/*写函数*/

static ssize_t mem_write(struct file *filp, const char __user *buf, size_tsize, loff_t *ppos)

{

unsigned long p = *ppos;

unsigned int count = size;

int ret = 0;

struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/

/*分析和获取有效的写长度*/

if (p >= MEMDEV_SIZE)

return 0;

if (count > MEMDEV_SIZE - p)

count = MEMDEV_SIZE - p;

/*从用户空间写入数据*/

if (copy_from_user(dev->data + p, buf, count))

ret = - EFAULT;

else

{

*ppos += count;

ret = count;

printk(KERN_INFO "written %d bytes(s) from %d\n",(int)count, (int)p);

}

have_data = true; /* 有新的数据可读 */

/* 唤醒读进程 */

wake_up(&(dev->inq));

return ret;

}
/* seek文件定位函数 */

static loff_t mem_llseek(struct file *filp, loff_t offset, int whence)

{

loff_t newpos;
switch(whence){

case 0: /* SEEK_SET */

newpos = offset;

break;
case 1: /* SEEK_CUR */

newpos = filp->f_pos + offset;

break;
case 2: /* SEEK_END */

newpos = MEMDEV_SIZE -1 + offset;

break;
default: /* can't happen */

return -EINVAL;

}

if ((newpos<0) || (newpos>MEMDEV_SIZE))

return -EINVAL;

filp->f_pos = newpos;

return newpos;
}
//static int mem_ioctl(struct inode * inode,struct file *flip,unsigned int cmd,

// unsignedlong arg)

static int mem_ioctl(struct file *flip,unsigned int cmd,

unsignedlong arg)

{

struct mem_dev *dev = flip->private_data; /*获得设备结构体指针*/

int err = 0;

int ret = 0;

int ioarg = 0;

/* 检测命令的有效性 */

if (_IOC_TYPE(cmd) != MEMDEV_IOC_MAGIC)

return -EINVAL;

if (_IOC_NR(cmd) > MEMDEV_IOC_MAXNR)

return -EINVAL;
/* 根据命令类型，检测参数空间是否可以访问 */

if (_IOC_DIR(cmd) & _IOC_READ)

err = !access_ok(VERIFY_WRITE, (void*)arg, _IOC_SIZE(cmd));

else if (_IOC_DIR(cmd) & _IOC_WRITE)

err = !access_ok(VERIFY_READ, (void*)arg, _IOC_SIZE(cmd));

if (err)

return -EFAULT;

/* 根据命令，执行相应的操作 */

switch(cmd) {
/*打印当前设备信息 */

case MEMDEV_IOCPRINT:

printk("<--- CMD MEMDEV_IOCPRINTDone--->\n\n");

break;
case MEMDEV_IOCMEMEST:

memset(dev->data, 0, MEMDEV_SIZE);

break;

/* 获取参数 */

case MEMDEV_IOCGETDATA:
ioarg = 1101;

ret = __put_user(ioarg, (int *)arg);

break;

/* 设置参数 */

case MEMDEV_IOCSETDATA:
ret = __get_user(ioarg, (int *)arg);

printk("<--- In Kernel MEMDEV_IOCSETDATAioarg = %d --->\n\n",ioarg);

break;
default:

return -EINVAL;

}

return ret;

}

/*文件操作结构体*/

static const struct file_operations mem_fops =

{

.owner = THIS_MODULE,

.llseek = mem_llseek,

.read = mem_read,

.write = mem_write,

.open = mem_open,

.release = mem_release,

.unlocked_ioctl = mem_ioctl,

};

/*设备驱动模块加载函数*/

static int memdev_init(void)

{

int result;

int i;
dev_t devno =MKDEV(mem_major, 0);

if (mem_major)

result = register_chrdev_region(devno,2,"ZXJ");

else

{

result =alloc_chrdev_region(&devno,0,2,"ZXJ");

mem_major = MAJOR(devno);

}

if (result < 0)

return result;

/*

cdev_init(&cdev,&mem_fops);

cdev.owner = THIS_MODULE;

cdev.ops = &mem_fops;

cdev_add(&cdev,MKDEV(mem_major, 0),MEMDEV_NR_DEVS);

*/

/* 为设备描述结构分配内存*/

mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);

if (!mem_devp) /*申请失败*/

{

result = - ENOMEM;

goto fail_malloc;

}

/* 清零存储器 */

memset(mem_devp, 0, sizeof(struct mem_dev) * MEMDEV_NR_DEVS);

/* 创建类/sys文件夹下面 */

myclass=class_create(THIS_MODULE,"test");

/*为设备分配内存*/

for (i=0; i < MEMDEV_NR_DEVS; i++)

{

mem_devp[i].size = MEMDEV_SIZE;

mem_devp[i].data = kmalloc(MEMDEV_SIZE,GFP_KERNEL);

if (!mem_devp[i].data)

printk("BadKmalloc\n");

memset(mem_devp[i].data, 0,MEMDEV_SIZE);

sprintf(mem_devp[i].name,"memdev%d",i);

cdev_init(&mem_devp[i].cdev,&mem_fops);

mem_devp[i].cdev.owner = THIS_MODULE;

result =cdev_add(&mem_devp[i].cdev,MKDEV(mem_major, i),1);

if (result)

{

printk("Badcdev\n");

return result;

}

/* 初始化信号量 */

init_waitqueue_head(&(mem_devp[i].inq));

device_create(myclass,NULL,MKDEV(mem_major,i),NULL,"memdev%d",i);

}

return 0;

fail_malloc:

unregister_chrdev_region(devno, 1);

return result;

}
/*模块卸载函数*/

static void memdev_exit(void)

{

int i;

for (i=0; i < MEMDEV_NR_DEVS; i++)

{

device_destroy(myclass,MKDEV(mem_major, i));//删除设备文件

cdev_del(&mem_devp[i].cdev); /*注销设备*/

kfree(mem_devp[i].data); /*释放内存*/

}

class_destroy(myclass);

kfree(mem_devp); /*释放设备结构体内存*/

unregister_chrdev_region(MKDEV(mem_major, 0), 2); /*释放设备号*/

}

MODULE_AUTHOR("Smart.zhao");

MODULE_LICENSE("GPL");
module_init(memdev_init);

module_exit(memdev_exit);

当加载模块的时候，会在/dev/memdev0,/dev/memdev1两个设备文件，如图所示
本人使用的linux-3.1的内核