1、前言
众所周知，Linux内核主要包括三种驱动模型，字符设备驱动，块设备驱动以及网络设备驱动。
其中，Linux字符设备驱动，可以说是Linux驱动开发中最常见的一种驱动模型。
我们该系列文章，主要为了帮助大家快速入门Linux驱动开发，该篇主要来了解一些字符设备驱动的框架和机制。
系列文章基于
Kernel 4.19
2、关键数据结构
2.1 cdev
struct cdev {
struct kobject kobj;
struct module *owner;
const struct file_operations *ops;
struct list_head list;
dev_t dev;
unsigned int count;
} __randomize_layout;
结构体名称：cdev
文件位置：include/linux/cdev.h
主要作用：cdev可以理解为char device，用来抽象一个字符设备。
核心成员及含义：
-
kobj：表示一个内核对象。 -
owner：指向该模块的指针 -
ops：指向文件操作的指针，包括open、read、write等操作接口 -
list：用于将该设备加入到内核模块链表中 -
dev：设备号，由主设备号和次设备号构成 -
count：表示有多少个同类型设备，也间接表示设备号的范围 -
__randomize_layout：一个编译器指令，用于随机化结构体的布局，以增加安全性。
2.2 file_operations
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 *);
int (*iterate_shared) (struct file *, struct dir_context *);
__poll_t (*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 *);
unsigned long mmap_supported_flags;
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 (*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 *,