一、前言
在调用了alloc_chrdev_region函数或register_chrdev_region函数之后可以在/proc/devices中看到该设备的主设备号，比如我注册的hello模块的主设备号为1024，如下图：
现在使用lsmod能看到驱动名，使用cat /proc/devices能看到设备名，那怎么来使用这个设备呢，这个时候我们还需要一个设备文件，这个设备文件就是在应用程序中用open函数打开的文件。
二、创建设备文件
方法一：手动创建
使用mknod指令，mknod用法：mknod <filename> <type> <major> <minor>
filename：设备文件名
type：设备类型
major：主设备号
minor：次设备号
如：mknod /dev/haha c 1024 0
这就是说创建了一个/dev/haha的文件，类型是字符设备，主设备号为1024，次设备号为0
方法二：自动创建
三、struct file
有了设备文件之后，便可以用open打开这个设备文件，然后用read，write等函数来操作这个文件，但是在用户态中怎么调用到内核的东西呢。
系统每打开一个文件在内核空间都有一个相关联的struct file，它由内核在打开文件时创建，在关闭文件后释放。
struct file的定义如下：
1 struct file {
2 union {
3 struct llist_node fu_llist;
4 struct rcu_head fu_rcuhead;
5 } f_u;
6 struct path f_path;
7 struct inode *f_inode; /* cached value */
8 const struct file_operations *f_op;
9
10 /*
11 * Protects f_ep_links, f_flags.
12 * Must not be taken from IRQ context.
13 */
14 spinlock_t f_lock;
15 atomic_long_t f_count;
16 unsigned int f_flags;
17 fmode_t f_mode;
18 struct mutex f_pos_lock;
19 loff_t f_pos;
20 struct fown_struct f_owner;
21 const struct cred *f_cred;
22 struct file_ra_state f_ra;
23
24 u64 f_version;
25 #ifdef CONFIG_SECURITY
26 void *f_security;
27 #endif
28 /* needed for tty driver, and maybe others */
29 void *private_data;
30
31 #ifdef CONFIG_EPOLL
32 /* Used by fs/eventpoll.c to link all the hooks to this file */
33 struct list_head f_ep_links;
34 struct list_head f_tfile_llink;
35 #endif /* #ifdef CONFIG_EPOLL */
36 struct address_space *f_mapping;
37 } __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
其中有一个重要成员就是 const struct file_operations *f_op;
struct file_operations的定义如下：
1 struct file_operations {
2 struct module *owner;
3 loff_t (*llseek) (struct file *, loff_t, int);
4 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
5 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
6 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
7 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
8 int (*iterate) (struct file *, struct dir_context *);
9 unsigned int (*poll) (struct file *, struct poll_table_struct *);
10 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
11 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
12 int (*mmap) (struct file *, struct vm_area_struct *);
13 int (*open) (struct inode *, struct file *);
14 int (*flush) (struct file *, fl_owner_t id);
15 int (*release) (struct inode *, struct file *);
16 int (*fsync) (struct file *, loff_t, loff_t, int datasync);
17 int (*aio_fsync) (struct kiocb *, int datasync);
18 int (*fasync) (int, struct file *, int);
19 int (*lock) (struct file *, int, struct file_lock *);
20 ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
21 unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
22 int (*check_flags)(int);
23 int (*flock) (struct file *, int, struct file_lock *);
24 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
25 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
26 int (*setlease)(struct file *, long, struct file_lock **, void **);
27 long (*fallocate)(struct file *file, int mode, loff_t offset,
28 loff_t len);
29 void (*show_fdinfo)(struct seq_file *m, struct file *f);
30 #ifndef CONFIG_MMU
31 unsigned (*mmap_capabilities)(struct file *);
32 #endif
33 };
这里面就包含了打开文件之后可以对文件的操作，在用open打开一个文件之后获得一个文件描述符fd，比如要对该文件进行写操作，则调用write函数，实际上会调用到 file_operations中的read函数，而在内核驱动中是需要自己编写read函数的，这样就能实现应用和内核之间的交互。
原文地址：https://www.cnblogs.com/Suzkfly/p/11768444.html