ssize_t read(struct file *filp, char *buff,
size_t count, loff_t *offp);
ssize_t write(struct file *filp, const char *buff,
size_t count, loff_t *offp);
unsigned long copy_to_user(void *to, const void *from,
unsigned long count);
unsigned long copy_from_user(void *to, const void *from,
unsigned long count);
ssize_t (*readv) (struct file *filp, const struct iovec *iov,
unsigned long count, loff_t *ppos);
ssize_t (*writev) (struct file *filp, const struct iovec *iov,
unsigned long count, loff_t *ppos);