file_operations -- file -- inode

http://blog.chinaunix.net/space.php?uid=11412571&do=blog&cuid=343846

驱动中有几个重要的数据结构:
file_operations,file,inode
中定义了这三个结构.
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 (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t, loff_t);
int (*readdir) (struct file *, void *, filldir_t);
unsigned int (*poll) (struct file *, struct poll_table_struct *);
int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *);
int (*release) (struct inode *, struct file *);
int (*fsync) (struct file *, struct dentry *, 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 (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
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 (*dir_notify)(struct file *filp, unsigned long arg);
}

file_operations结构定义了一组函数指针,每一个打开的文件(用struct
file表示)和他自己的一组函数(包含在一个叫f_op的域中,它指向一个struct
file_operations结构)相联系.这些操作都是来实现系统调用的,所以才被命名为open,read,等等.对于那些不需要的功能(比如你的
设备不需要write功能,即不需要向设备写数据),可以给write指针付NULL.

struct file {
struct list_head f_list;
struct dentry *f_dentry;
struct vfsmount *f_vfsmnt;
struct file_operations *f_op;
atomic_t f_count;
unsigned int f_flags;
mode_t f_mode;
int f_error;
loff_t f_pos;
struct fown_struct f_owner;
unsigned int f_uid, f_gid;
struct file_ra_state f_ra;

unsigned long f_version;
void *f_security;

/* needed for tty driver, and maybe others */
void *private_data;

#ifdef CONFIG_EPOLL
/* Used by fs/eventpoll.c to link all the hooks to this file */
struct list_head f_ep_links;
spinlock_t f_ep_lock;
#endif /* #ifdef CONFIG_EPOLL */
struct address_space *f_mapping;
};
每个打开的文件对应一个struct file.它在被打开时由内核创建,并传给它所有可以操作该文件的函数,到文件被关闭时才被删除.

The inode Structure.
Inode结构是用来在内核内部表示文件的.同一个文件可以被打开好多次,所以可以对应很多struct file,但是只对应一个struct inode.该结构里面包含了很多信息,但是,驱动开发者只关心里面两个重要的域:
dev_t i_rdev;//含有真正的设备号
struct cdev *i_cdev;//struct cdev是内核内部表示字符设备的结构.

struct file_operations是struct file的一个域，我们在使用系统调用open()打开一个设备节点struct
inode时，我们会得到一个文件struct
file,同时返回一个文件描述符，该文件描述符是一个整数，我们称之为句柄，通过访问句柄我们能够访问设备文件struct
file,描述符是一个有着特殊含义的整数，特定位都有一定的意义或属性。