字符设备驱动

一、file_operations:

每一个系统调用，在驱动中都有与之对应的一个驱动函数，在 Linux 内核文件 include/linux/fs.h 中
有个叫做 file_operations 的结构体，此结构体就是 Linux 内核驱动操作函数集合，内容如下所
示：

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 (*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 *);
	i