#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>

#define device "/dev/bar"

int main(void)
{
        int fd;
        char buf[] = "Late Lee";
        char buf2[30]={0};
        int len;
        fd = open(device, O_RDWR);
        if (fd < 0)
        {
                perror("Open device faile!");
                return -1;
        }
        len = write(fd, buf, sizeof(buf));
        printf("buf: %s %d/n", buf, len);
        len = read(fd, buf2, 25); // 由此指定读取数据，可大可小，但是驱动只读取这个指定的(大者读实际值)，并返回
        printf("buf2: %s %d/n", buf2, len);
        close(fd);
        return 0;
}

# ./a.out
buf: Late Lee 9
buf2: The voice from hell: Late 25

/*************************************************************************
        Late Lee from http://www.latelee.org
简单的字符型设备驱动
从应用层获取一数据，再复制到应用层(在前面添加字符串)。
注册设备号及设备号的几个宏，均系ldd3例子scull。

何处释放data更好？看ldd3，似乎exit中更好。
2011-04-29 & 2011-05-06
*************************************************************************/

#include <linux/module.h>
#include <linux/kernel.h>       /**< printk() */
#include <linux/init.h>

#include <linux/cdev.h>                /**< cdev_* */
#include <linux/fs.h>
#include <asm/uaccess.h>        /**< copy_*_user */

#include <linux/types.h>        /**< size_t */
#include <linux/errno.h>        /**< error codes */
#include <linux/string.h>

#ifdef DEBUG /* define it in Makefile or somewhere */
/* KERN_INFO */
#define debug(fmt, ...) printk(KERN_DEBUG fmt, ##__VA_ARGS__)
#else
#define debug(fmt, ...)
#endif

#define DEV_NAME "foo"

//#define HAVE_MAJOR
#ifdef HAVE_MAJOR
#define DEVICE_MAJOR 248
#else        /* auto alloc */
#define DEVICE_MAJOR 0
#endif /* HAVE_MAJOR */

#define FOO_NR_DEVS 1

struct cdev foo_cdev;
int foo_major = DEVICE_MAJOR;
int foo_minor = 0;
int foo_nr_devs = FOO_NR_DEVS;
dev_t devno;
char *data;

static int foo_open(struct inode *inode, struct file *filp)
{
        debug("in %s()/n", __func__);
        return 0;
}

static int foo_release(struct inode *inode, struct file *filp)
{
        debug("in %s()/n", __func__);
        //kfree(data);
        //data = NULL;
        return 0;
}

static ssize_t foo_read(struct file *filp, char *buf, size_t count, loff_t *f_ops)
{
        int len;
        char *tmp;
       
        if (count < 0)
                return -EINVAL;
        tmp = (char *)kmalloc(sizeof(char) * (count+1), GFP_KERNEL); // ?? here
        sprintf(tmp, "The voice from hell: %s", data);
        len = strlen(tmp);
        if (len < count)
                count = len;
        if ( copy_to_user(buf, tmp, count) )
                return -EFAULT;
        debug("in %s() tmp: %s/n", __func__, tmp);
        debug("in %s() buf: %s/n", __func__, buf);
        debug("in %s() data: %s/n", __func__, data);
        kfree(tmp);
        return count;
}

static ssize_t foo_write(struct file *filp, const char *buf, size_t count, loff_t *f_ops)
{
        if (count < 0)
                return -EINVAL;
        data = (char *)kmalloc(sizeof(char) * (count+1), GFP_KERNEL);
        if (data == NULL)
                return -ENOMEM;
        if (copy_from_user(data, buf, count+1))
                return -EFAULT;
        debug("in %s() buff: %s/n", __func__, buf);
        debug("in %s() data: %s/n", __func__, data);
        return count;
}

static int foo_ioctl(struct inode *inode, struct file *filp, unsigned int cmd, unsigned long arg)
{
        switch (cmd)
        {
       
        default:
                return -EINVAL;
        }
        return 0;
}

static struct file_operations foo_fops = {
        .owner   = THIS_MODULE,
        .open    = foo_open,
        .release = foo_release,
        .read    = foo_read,
        .write   = foo_write,
        .ioctl   = foo_ioctl,
};

static int __init foo_init(void)
{
        int ret = -1;

cdev_init(&foo_cdev, &foo_fops);
        foo_cdev.owner = THIS_MODULE;
        /* register to who? */
        if (foo_major)
        {
                devno = MKDEV(foo_major, foo_minor);
                ret = register_chrdev_region(devno, foo_nr_devs, DEV_NAME);
        }
        else
        {
                ret = alloc_chrdev_region(&devno, foo_minor, foo_nr_devs, DEV_NAME); /* get devno */
                foo_major = MAJOR(devno); /* get major */
               
        }
        if (ret < 0)
        {
                debug(" %s can't get major %d/n", DEV_NAME, foo_major);
                return -EINVAL;
        }
       
        ret = cdev_add(&foo_cdev, devno, 1);
        if (ret < 0)
        {
                debug(" %s cdev_add failure!/n", DEV_NAME);
                return -EINVAL;
        }
       
        debug("%s init ok!/n", DEV_NAME);
        return 0;
}

static void __exit foo_exit(void)
{
        if (data != NULL)
                kfree(data);
        unregister_chrdev_region(devno, 1);
        cdev_del(&foo_cdev);
        debug("%s exit ok!/n", DEV_NAME);
}

module_init(foo_init);
module_exit(foo_exit);
MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Chiangchin Li");

# insmod GotoHell.ko
# dmesg | tail
……
foo init ok!

# ./a.out
buf: Late Lee 9
buf2: The voice from hell: Late 25
# dmesg | tail
……
foo init ok!
in foo_open()
in foo_write() buff: Late Lee
in foo_write() data: Late Lee
in foo_read() tmp: The voice from hell: Late Lee
in foo_read() buf: The voice from hell: Late
in foo_read() data: Late Lee
in foo_release()

# rmmod GotoHell.ko
# dmesg | tail     
……
foo init ok!
in foo_open()
in foo_write() buff: Late Lee
in foo_write() data: Late Lee
in foo_read() tmp: The voice from hell: Late Lee
in foo_read() buf: The voice from hell: Late
in foo_read() data: Late Lee
in foo_release()
foo exit ok!

static struct file_operations foo_fops = {
 .owner   = THIS_MODULE,
 .open    = foo_open,
 .release = foo_release,
 .read    = foo_read,
 .write   = foo_write,
 .ioctl   = foo_ioctl,
};