1.首先来看下struct file_operations 的定义：

struct file_operations {

struct module *owner;

loff_t (*llseek) (struct file *, loff_t, int);

ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);

ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);

ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);

ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);

int (*readdir) (struct file *, void *, filldir_t);

unsigned int (*poll) (struct file *, struct poll_table_struct *);

long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);

long (*compat_ioctl) (struct file *, unsigned int, unsigned long);

int (*mmap) (struct file *, struct vm_area_struct *);

int (*open) (struct inode *, struct file *);

int (*flush) (struct file *, fl_owner_t id);

int (*release) (struct inode *, struct file *);

int (*fsync) (struct file *, loff_t, loff_t, int datasync);

int (*aio_fsync) (struct kiocb *, int datasync);

int (*fasync) (int, struct file *, int);

int (*lock) (struct file *, int, struct file_lock *);

ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);

unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);

int (*check_flags)(int);

int (*flock) (struct file *, int, struct file_lock *);

ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);

ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);

int (*setlease)(struct file *, long, struct file_lock **);

long (*fallocate)(struct file *file, int mode, loff_t offset,

loff_t len);

};

在这里面有很多函数，比如 open 、read、write函数，他们通过以下方式初始化：

static const struct file_operations fts_proc_fops = {

.owner = THIS_MODULE,

.open = fts_debug_open,

.write = fts_debug_write,

};

那问题就来了，fts_debug_open是如何实现调用的呢？

/************************************

来看一个在应用程序中的调用实例：

fd=open("/dev/hello",O_RDWR)

以上是通过系统调用open函数打开/dev/hello这个设备文件，此设备节点对应有一个设备号

打开/dev/hello通过主次设备号找到相对应的字符设备，

struct cdev {

struct kobject kobj;

struct module *owner;

const struct file_operations *ops;

struct list_head list;

dev_t dev;

unsigned int count;

};

cdev里面又包含了file_operations结构体，含有对设备的各种操作，打开时即调用里面的.open 函数指针指向的open函数。