编写一个Linux字符驱动demo

以下是一个简单的Linux字符驱动示例代码，实现了一个名为mychar的字符设备驱动：

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

#define DEVICE_NAME "mychar"
#define BUF_LEN 80

static int Major;
static char msg[BUF_LEN];
static int msg_len;
static struct cdev mychar_cdev;

static int mychar_open(struct inode *inode, struct file *file)
{
    printk(KERN_INFO "mychar: Device opened.\n");
    return 0;
}

static int mychar_release(struct inode *inode, struct file *file)
{
    printk(KERN_INFO "mychar: Device closed.\n");
    return 0;
}

static ssize_t mychar_read(struct file *filp, char *buf, size_t count, loff_t *f_pos)
{
    int len = msg_len;
    if (len > count)
        len = count;
    if (len == 0)
        return 0;
    if (copy_to_user(buf, msg, len))
        return -EFAULT;
    *f_pos += len;
    return len;
}

static ssize_t mychar_write(struct file *filp, const char *buf, size_t count, loff_t *f_pos)
{
    if (count > BUF_LEN)
        count = BUF_LEN;
    if (copy_from_user(msg, buf, count))
        return -EFAULT;
    msg_len = count;
    return count;
}

static struct file_operations mychar_fops = {
    .owner = THIS_MODULE,
    .open = mychar_open,
    .release = mychar_release,
    .read = mychar_read,
    .write = mychar_write,
};

static int __init mychar_init(void)
{
    int err;
    dev_t dev;

    printk(KERN_INFO "mychar: Initializing.\n");

    err = alloc_chrdev_region(&dev, 0, 1, DEVICE_NAME);
    if (err < 0) {
        printk(KERN_ERR "mychar: Failed to allocate chrdev region.\n");
        return err;
    }
    Major = MAJOR(dev);

    cdev_init(&mychar_cdev, &mychar_fops);
    mychar_cdev.owner = THIS_MODULE;

    err = cdev_add(&mychar_cdev, dev, 1);
    if (err < 0) {
        printk(KERN_ERR "mychar: Failed to add cdev.\n");
        unregister_chrdev_region(dev, 1);
        return err;
    }

    return 0;
}

static void __exit mychar_exit(void)
{
    dev_t dev = MKDEV(Major, 0);

    printk(KERN_INFO "mychar: Exiting.\n");

    cdev_del(&mychar_cdev);
    unregister_chrdev_region(dev, 1);
}

module_init(mychar_init);
module_exit(mychar_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple character device driver");

该驱动实现了一个字符设备，可以通过读写操作来对msg变量进行读写。在驱动初始化时，使用alloc_chrdev_region函数分配设备号，然后通过cdev_init和cdev_add函数将驱动添加到系统中。在read和write函数中，通过copy_to_user和copy_from_user函数来进行用户态数据与内核态数据之间的拷贝。在驱动退出时，使用cdev_del和unregister_chrdev_region函数来删除驱动和释放设备号。