字符设备驱动（内核态用户态内存交互）

前言

内核驱动：运行在内核态的动态模块，遵循内核模块框架接口，更倾向于插件。
应用程序：运行在用户态的进程。
应用程序与内核驱动交互通过既定接口，内核态和用户态访问依然遵循内核既定接口。

环境搭建

系统：openEuler-20.03-LTS-SP3

yum install gcc kernel-devel

编写源码

char_module.c

#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <asm/device.h> //下面这三个头文件是由于动态创建需要加的
#include <linux/device.h>
#include <linux/cdev.h>MODULE_LICENSE("GPL");#define DEVICE_NAME "char_module"
#define BUF_SIZE 32static struct class *cdev_class;
dev_t dev_num = 0; // 这里是动态分配设备号和动态创建设备结点需要用到的
struct cdev dev_c;static char context_buf[BUF_SIZE]={"this a test context buffer\0"};static ssize_t read(struct file *, char *, size_t, loff_t *);
static ssize_t write(struct file *, const char *, size_t, loff_t *);
static int open(struct inode *, struct file *);
static int release(struct inode *, struct file *);// 初始化字符设备驱动的 file_operations 结构体
struct file_operations fops = {.read = read,.write = write,.open = open,.release = release
};static int __init demo_init(void)
{int ret, err;printk(KERN_INFO "%s: %s", DEVICE_NAME , __func__);// 注册设备驱动ret = alloc_chrdev_region(&dev_num, 0, 1, DEVICE_NAME); // 动态分配设备号if (ret){printk("demo_init register failure\n");unregister_chrdev_region(dev_num, 1);return ret;}printk("demo_init register success\n");// 初始化设备操作cdev_init(&dev_c, &fops);err = cdev_add(&dev_c, dev_num, 1);if (err){printk(KERN_NOTICE "error %d adding cdev\n", err);unregister_chrdev_region(dev_num, 1);return err;}// 动态创建设备结点cdev_class = class_create(THIS_MODULE, DEVICE_NAME); if (IS_ERR(cdev_class)){printk("ERR:cannot create a cdev_class\n");unregister_chrdev_region(dev_num, 1);return -1;}device_create(cdev_class, NULL, dev_num, 0, DEVICE_NAME);return ret;
}static void __exit demo_exit(void)
{printk(KERN_INFO "%s: %s", DEVICE_NAME , __func__);// 注销设备驱动device_destroy(cdev_class, dev_num);class_destroy(cdev_class);unregister_chrdev_region(dev_num, 1);
}static ssize_t read(struct file *filp, char *buf, size_t len, loff_t *off)
{// 内核空间到用户空间copyprintk(KERN_INFO "%s: %s", DEVICE_NAME , __func__);if (raw_copy_to_user(buf, &context_buf, sizeof(context_buf))){return -EFAULT;}printk(KERN_INFO "user space: %pF", buf);printk(KERN_INFO "read: %pF; size: %ld; data: %s", &context_buf, sizeof(context_buf), context_buf);return BUF_SIZE;
}static ssize_t write (struct file *filp, const char __user *buf, size_t len, loff_t *off)
{// 用户空间到内核空间copyprintk(KERN_INFO "%s: %s", DEVICE_NAME , __func__);if (raw_copy_from_user(&context_buf, buf, sizeof(context_buf))){return -EFAULT;}printk(KERN_INFO "user space: %pF", buf);printk(KERN_INFO "write: %pF; size: %ld; data: %s", &context_buf, sizeof(context_buf), context_buf);return BUF_SIZE;
}static int open(struct inode *inodp, struct file *filp)
{printk(KERN_INFO "%s: %s", DEVICE_NAME , __func__);return 0;
}static int release(struct inode *inodp, struct file *filp)
{printk(KERN_INFO "%s: %s", DEVICE_NAME, __func__);return 0;
}module_init(demo_init);
module_exit(demo_exit);

Makefile

ifneq ($(KERNELRELEASE),)
obj-m := char_module.oelse
PWD  := $(shell pwd)
KVER := $(shell uname -r)
KDIR := /lib/modules/$(KVER)/build
all:$(MAKE) -C $(KDIR) M=$(PWD) modules modules_install
clean:rm -rf .*.cmd *.o *.mod.c *.ko .tmp_versions modules.*  Module.*
endif

app.c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>#define CHAR_DEV_NAME "/dev/char_module"int main()
{int ret;int fd;char buf[32];fd = open(CHAR_DEV_NAME, O_RDWR | O_NDELAY);if(fd < 0){printf("open failed!\n");return -1;}int size = read(fd, buf, 32);printf("read size: %d;\nbuffer:[%s]\n", size, buf);char *write_buf = "use a application wirte to driver buffer";int w_size = write(fd, write_buf, strlen(write_buf));printf("write size: %d;\nbuffer:[%s]\n", w_size, write_buf);close(fd);return 0;
}