驱动程序学习（三）示例2 ：设备驱动程序与应用程序之间数据传输的另外一种方式

代码如下：

chardev.c

#include<linux/module.h>

#include<linux/init.h>

#include<linux/fs.h>

#include<asm/uaccess.h>

MODULE_LICENSE("GPL");

#define MAJOR_NUM 111

static ssize_t chardev_read(struct file *,char *,size_t,loff_t *);

static ssize_t chardev_write(struct file *,const char *,size_t, loff_t *);

struct file_operations chardev_fops=

{

read: chardev_read,

write:
chardev_write,

};

static int chardev_var = 0;

static int chardev_init(void)

{

int ret;

ret=register_chrdev(MAJOR_NUM,"chardev",&chardev_fops);

if(ret)

{

printk("chardev register failure");

}

else

{

printk("chardev register success");

}

return ret;

}

static void chardev_exit(void)

{

unregister_chrdev(MAJOR_NUM,"chardev");

}

static ssize_t chardev_read(struct file *filp,char *buf,size_t len,loff_t *off)

{

if(copy_to_user(buf,&chardev_var,sizeof(int)))

{

return -EFAULT;

}

return sizeof(int);

}

static ssize_t chardev_write(struct file *filp,const char *buf,size_t len,loff_t *off)

{

if(copy_from_user(&chardev_var,buf,sizeof(int)))

{

return -EFAULT;

}

return sizeof(int);

}

module_init(chardev_init);

module_exit(chardev_exit);

Makefile 代码如下：

KERNELDIR?= /lib/modules/2.6.25-14.fc9.i686/build/

PWD := $(shell pwd)

CC=$(CROSS_COMPILE)gcc

obj-m := chardev.o

modules:

$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:

$(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install

clean:

make编译

在/dev 目录下建立相应的设备节点，方法参照前两篇博客。

测试程序代码：

chardev_test.c

#include<sys/types.h>

#include<sys/stat.h>

#include<stdio.h>

#include<fcntl.h>

main()

{

int fd,num;

fd=open("/dev/chardev",O_RDWR,S_IRUSR|S_IWUSR);

if(fd!=-1)

{

read(fd,&num,sizeof(int));

printf("The chardev is %d\n",num);

printf("Please input the num written to chardev\n");

scanf("%d",&num);

write(fd,&num,sizeof(int));

read(fd,&num,sizeof(int));

printf("The chardev is %d\n",num);

close(fd);

}

else

{

printf("Device open failure\n");

}

}

运行测试程序，会在终端里让你输入一个字符，同时返回一个你输入的字符。

完成应用程序与驱动程序的数据交互。

但是这个不同于上一个是直接进行的数据传递。这个示例用到了两个关键的函数

copy_from_user

.函数原型在[arch/i386/lib/usercopy.c]中

这个是完成从用户空间拷贝数据到内核空间，完成应用程序到驱动程序的数据传递

unsigned long

copy_from_user(void *to, const void __user *from, unsigned long n)

{

might_sleep();

if (access_ok(VERIFY_READ, from, n))

n = __copy_from_user(to, from, n);

else

memset(to, 0, n);

return n;

}

详见参考：
http://linux.chinaunix.net/techdoc/develop/2007/10/28/970816.shtml
copy_to_user

这个函数是完成从内核空间到用户空间拷贝数据的作用，完成从驱动程序到应用程序的传递

函数原型为

835 /**

836 * copy_to_user: - Copy a block of data into user space.

837 * @to: Destination address, in user space.

838 * @from: Source address, in kernel space.

839 * @n: Number of bytes to copy.

840 *

841 * Context: User context only. This function may sleep.

842 *

843 * Copy data from kernel space to user space.

844 *

845 * Returns number of bytes that could not be copied.

846 * On success, this will be zero.

847 */

848 unsigned long

849 copy_to_user(void __user *to, const void *from, unsigned long n)

850 {

851 if (access_ok(VERIFY_WRITE, to, n))

852 n = __copy_to_user(to, from, n);

853 return n;

854 }

详见参考
http://blog.chinaunix.net/space.php?uid=21289517&do=blog&id=1828184