一个简单的字符设备驱动

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/init.h>

static struct cdev chr_dev;       //cdev是设备在内核中的内部结构
static dev_t ndev;               //设备编号

static int chr_open(struct inode *nd, struct file *filp)
{
    int major = MAJOR(nd -> i_rdev);              //表示设备的inode的字段中的i_rdev保存了真正的设备编号
    int minor = MINOR(nd -> i_rdev);
    printk("chr_open, major = %d , minor = %d\n.", major, minor);
    return 0;
}

static ssize_t chr_read(struct file *f, char __user *u, size_t sz, loff_t *off)
{
    printk("In the chr_read() function!\n");

    return 0;
}

struct file_operations chr_ops =
{
    .owner = THIS_MODULE,         //避免模块的操作正在被使用时卸载该模块
    .open = chr_open,
    .read = chr_read,
};

static int demo_init(void)
{
    int ret;
    cdev_init(&chr_dev, &chr_ops);                //字符设备cdev初始化，建立cdev和file_operations之间的连接
    ret = alloc_chrdev_region(&ndev, 0 , 1 ,"chr_dev");  //第一个参数类型是dev_t *是输出参数动态分配的设备编号，最后为设备名

    if (ret < 0)
        return ret;

    printk("demo_init(): major = %d ,minot = %d\n", MAJOR(ndev), MINOR(ndev));        //两个宏分别获得设备的主次编号
    ret = cdev_add(&chr_dev, ndev, 1);                                              //将设备添加到系统中，需要失败检查
    if (ret < 0)
        return ret;

    return 0;
}

static void demo_exit(void)
{
    printk("Removing chr_dev module...\n");
    cdev_del(&chr_dev);                                                           //移除字符设备
    unregister_chrdev_region(ndev, 1);                                   //释放设备编号
}

module_init(demo_init);
module_exit(demo_exit);

MODULE_LICENSE("GPL");

ifneq ($(KERNELRELEASE),)
    obj-m := demo_chr_dev.o
else
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

default:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules
endif

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

#define  CHR_DEV_NAME "/dev/chr_dev"

int main()
{
    int ret;
    char buf[32];
    int fd = open(CHR_DEV_NAME, O_RDONLY | O_NDELAY);
    if (fd < 0)
        {
            printf("open file faild!\n");
            return -1;
        }

    read(fd ,buf, 32);
    close(fd);

    return 0;
}

#insmod demo_chr_dev.ko
   demo_init(): major = 248, minor = 0
#mknod  /dev/chr_dev  c  248  0        //动态分配设备号无法预先创建设备节点
#./main