file_operations中的 ioctrl

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);
	ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
	int (*iterate) (struct file *, struct dir_context *);
	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 (*mremap)(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 **, void **);
	long (*fallocate)(struct file *file, int mode, loff_t offset,
			  loff_t len);
	void (*show_fdinfo)(struct seq_file *m, struct file *f);
#ifndef CONFIG_MMU
	unsigned (*mmap_capabilities)(struct file *);
#endif
};

#include 
int ioctl(int d, int request, ...);

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

https://www.cnblogs.com/songfeixiang/p/3381300.html

ioctl(fd, IOCGREG, &my_msg);
        ret = copy_from_user(&msg, \
            (struct msg __user *)arg, sizeof(my_msg));
        if (ret) 
            return -EFAULT;
        msg->data = read_reg(msg->addr);
        ret = copy_to_user((struct msg __user *)arg, \
                &msg, sizeof(my_msg));

3. 函数功能
	1. 数据传输
		内核->用户 // 大小由cmd 中 的第三个数值决定, 地址 为 unsigned long 参数,应该传个指针.
		用户->内核
	2. 命令交互
		由cmd 的第二个数值和第一个数值决定
4. 魔数打包

_IO (魔数， 基数);
_IOR (魔数， 基数， 变量型)
_IOW  (魔数， 基数， 变量型)
_IOWR (魔数， 基数，变量型 )

#include ""
_IO(type,nr)
_IOR(type,nr,size)
_IOW(type,nr,size)
_IOWR(type,nr,size)
6.魔数拆包
#include ""
_IOC_DIR(nr) // 这个是解压 读写属性 的 , R W RW
_IOC_TYPE(nr)
_IOC_NR(nr)
_IOC_SIZE(nr)

6. 返回值
	返回0 正确

Usually, on success zero is returned.  A few ioctl() requests use the
       return value as an output parameter and return a nonnegative value on
       success.  On error, -1 is returned, and errno is set appropriately.
	errno 怎么设置??