4. 字符设备驱动核心

最小驱动骨架：

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

static dev_t devno;
static struct cdev my_cdev;
static char kbuf[128] = "hello from kernel";

static int my_open(struct inode *inode, struct file *filp) {
    return 0;
}

static ssize_t my_read(struct file *filp, char __user *buf,
                       size_t len, loff_t *off) {
    if (*off >= sizeof(kbuf)) return 0;
    if (len > sizeof(kbuf) - *off) len = sizeof(kbuf) - *off;
    if (copy_to_user(buf, kbuf + *off, len)) return -EFAULT;
    *off += len;
    return len;
}

static ssize_t my_write(struct file *filp, const char __user *buf,
                        size_t len, loff_t *off) {
    if (len > sizeof(kbuf)) len = sizeof(kbuf);
    if (copy_from_user(kbuf, buf, len)) return -EFAULT;
    return len;
}

static int my_release(struct inode *inode, struct file *filp) {
    return 0;
}

static struct file_operations my_fops = {
    .owner   = THIS_MODULE,
    .open    = my_open,
    .read    = my_read,
    .write   = my_write,
    .release = my_release,
};

static int __init my_init(void) {
    alloc_chrdev_region(&devno, 0, 1, "mychar");
    cdev_init(&my_cdev, &my_fops);
    my_cdev.owner = THIS_MODULE;
    cdev_add(&my_cdev, devno, 1);
    printk(KERN_INFO "mychar major=%d minor=%d\n",
           MAJOR(devno), MINOR(devno));
    return 0;
}

static void __exit my_exit(void) {
    cdev_del(&my_cdev);
    unregister_chrdev_region(devno, 1);
}

module_init(my_init);
module_exit(my_exit);
MODULE_LICENSE("GPL");