标签：Linux

f_pos定义在file结构体（定义在<linux/fs.h>），表示文件当前的读写位置：

struct file {
    ......
    loff_t          f_pos;
    ......
}

LDD3中关于f_pos的描述：

loff_t f_pos;

The current reading or writing position. loff_t is a 64-bit value on all platforms ( long long in gcc terminology). The driver can read this value if it needs to know the current position in the file but should not normally change it; read and write should update a position using the pointer they receive as the last argument instead of acting on filp->f_pos directly. The one exception to this rule is in the llseek method, the purpose of which is to change the file position.

驱动的读写操作不需要直接更新filp->f_pos。关于其中原因，可参考这篇笔记。

Linux kernel 使用 cdev结构体代表字符设备（char device），定义在<linux/cdev.h>：

#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 *);

分配和初始化cdev结构体的两种方式：

（1）

struct cdev *my_cdev = cdev_alloc();
my_cdev->ops = &my_fops;
my_cdev->owner = THIS_MODULE;

（2）另外一种是cdev嵌入到代表设备的结构体中：

struct scull_dev {
    ......
    struct cdev cdev; /* Char device structure */
    ......
};

static void scull_setup_cdev(struct scull_dev *dev, int index)
{
    int err, devno = MKDEV(scull_major, scull_minor + index);
    cdev_init(&dev->cdev, &scull_fops);
    dev->cdev.owner = THIS_MODULE;
    dev->cdev.ops = &scull_fops;
    ......
}

两种方式都要注意把owner赋值为THIS_MODULE。

初始化cdev结构体以后，要使用cdev_add把设备加入系统：

/**
 * 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)
{
    int error;

    p->dev = dev;
    p->count = count;

    error = kobj_map(cdev_map, dev, count, NULL,
             exact_match, exact_lock, p);
    if (error)
        return error;

    kobject_get(p->kobj.parent);

    return 0;
}

要注意count指定的是连续的minor number数。

删除设备使用cdev_del函数：

/**
 * cdev_del() - remove a cdev from the system
 * @p: the cdev structure to be removed
 *
 * cdev_del() removes @p from the system, possibly freeing the structure
 * itself.
 */
void cdev_del(struct cdev *p)
{
    cdev_unmap(p->dev, p->count);
    kobject_put(&p->kobj);
}

2.6版本之前的注册和删除设备的register_chrdev和unregister_chrdev函数已经过时，不再使用。