1. cdev是linux用来管理字符设备的结构体，其在内核中采用数组结构设计，这样系统中有多少个主设备号就约定了数组大小，此设备号采用链表管理，同一主设备号下可以有多个子设备。设备即文件，上层应用要访问设备，必须通过文件，cdev中包含file_operations结构体，该结构体就是驱动的文件操作集合。
（根据于说得）
2. cdev定义在include/linux/cdev.h中，如下。具体实现在fs/char_dev.c中。
#include <linux/kobject.h>
#include <linux/kdev_t.h>
#include <linux/list.h>
struct file_operations;
struct inode;
struct module;
struct cdev {
struct kobject kobj;
struct module *owner;
const struct file_operations *ops;
struct list_head list;
dev_t dev;
unsigned int count;
};
void cdev_init(struct cdev *, const struct file_operations *);
struct cdev *cdev_alloc(void);
void cdev_put(struct cdev *p);
int cdev_add(struct cdev *, dev_t, unsigned);
void cdev_del(struct cdev *);
void cd_forget(struct inode *);
extern struct backing_dev_info directly_mappable_cdev_bdi;
//fs/char_dev.c
/**
* cdev_add() - add a char device to the system
* @p: the cdev structure for the device
* @dev: the first device number for which this device is responsible
* @count: the number of consecutive minor numbers corresponding to this * device
*
* cdev_add() adds the device represented by @p to the system, making it
* live immediately. A negative error code is returned on failure. */
int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
p->dev = dev;
p->count = count;
return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);
}
3. cdev函数
>>cdev定义有两种方式，struct cdev cdev；另外一种struct cdev *cdev; cdev=cdev_alloc();
一种静态声明定义，另一种动态分配。
>>cdev通过函数cdev_init()初始化，主要工作就是将file_operations和cdev关联起来。file_operations是字符驱动需要实现的主要内容。
>>cdev通过cdev_add()实现cdev的注册，所谓注册就是将cdev根据设备号（dev_t）添加到cdev数组（cdev_map）中供系统管理。
>>cdev通过cdev_del()将cdev从cdev_map中移除。
4. 设备号
申请设备号用如下两个函数：
alloc_chrdev_region() --自动分配设备号
register_chrdev_region() --分配已设定的设备号
/**
* register_chrdev_region() - register a range of device numbers
* @from: the first in the desired range of device numbers; must include
* the major number.
* @count: the number of consecutive device numbers required
* @name: the name of the device or driver.
*
* Return value is zero on success, a negative error code on failure.
*/
int register_chrdev_region(dev_t from, unsigned count, const char *name);
/**
* alloc_chrdev_region() - register a range of char device numbers
* @dev: output parameter for first assigned number
* @baseminor: first of the requested range of minor numbers
* @count: the number of minor numbers required
* @name: the name of the associated device or driver
*
* Allocates a range of char device numbers. The major number will be
* chosen dynamically, and returned (along with the first minor number)
* in @dev. Returns zero or a negative error code.
*/
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
const char *name);
5. more
内核给出的操作struct cdev结构的接口主要有以下几个:
a -- void cdev_init(struct cdev *, const struct file_operations *);
其源代码如代码清单如下：
[cpp] view plain copy
void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
memset(cdev, 0, sizeof *cdev);
INIT_LIST_HEAD(&cdev->list);
kobject_init(&cdev->kobj, &ktype_cdev_default);
cdev->ops = fops;
}
该函数主要对struct cdev结构体做初始化，最重要的就是建立cdev 和 file_operations之间的连接：
(1) 将整个结构体清零；
(2) 初始化list成员使其指向自身；
(3) 初始化kobj成员；
(4) 初始化ops成员；
b --struct cdev *cdev_alloc(void);
该函数主要分配一个struct cdev结构，动态申请一个cdev内存，并做了cdev_init中所做的前面3步初始化工作(第四步初始化工作需要在调用cdev_alloc后，显式的做初始化即: .ops=xxx_ops).
其源代码清单如下：
[cpp] view plain copy
struct cdev *cdev_alloc(void)
{
struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);
if (p) {
INIT_LIST_HEAD(&p->list);
kobject_init(&p->kobj, &ktype_cdev_dynamic);
}
return p;
}
在上面的两个初始化的函数中，我们没有看到关于owner成员、dev成员、count成员的初始化；其实，owner成员的存在体现了驱动程序与内核模块间的亲密关系，struct module是内核对于一个模块的抽象，该成员在字符设备中可以体现该设备隶属于哪个模块，在驱动程序的编写中一般由用户显式的初始化 .owner = THIS_MODULE, 该成员可以防止设备的方法正在被使用时，设备所在模块被卸载。而dev成员和count成员则在cdev_add中才会赋上有效的值。