源代码

alloc_chrdev_region与register_chrdev_region的区别在于，
前者不知道主设备号，由操作系统自动分配
后者由人工设置主设备号！！

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

#include <linux/slab.h>   // kfree,kmalloc
#include <linux/cdev.h>   // cdev_xxx
#include <linux/device.h> // device_xxx

static dev_t g_simple_cdev_dev_no;
static struct cdev *g_simple_cdev;
static struct class *g_simple_cdev_class;
static struct device *g_simple_cdev_device;

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

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

/* File operations struct for character device */
static const struct file_operations g_simple_cdev_fops = {
    .owner   = THIS_MODULE,
    .open    = simple_cdev_open,
    .release = simple_cdev_release,
};

static int __init simple_cdev_init(void)
{
    int ret;

    ret = alloc_chrdev_region(&g_simple_cdev_dev_no, 0, 1, "simple_cdev");
    if (ret < 0) {
        return ret;
    }

    g_simple_cdev = cdev_alloc();
    if (g_simple_cdev == NULL) {
        return -1;
    }
    g_simple_cdev->owner = THIS_MODULE;
    g_simple_cdev->ops   = &g_simple_cdev_fops;

    ret = cdev_add(g_simple_cdev, g_simple_cdev_dev_no, 1);
    if (ret < 0) {
        return ret;
    }

    g_simple_cdev_class  = class_create(THIS_MODULE, "simple_cdev");
    g_simple_cdev_device = device_create(g_simple_cdev_class, NULL, g_simple_cdev_dev_no, NULL, "simple_cdev");

    return 0;
}

static void __exit simple_cdev_exit(void)
{
    device_destroy(g_simple_cdev_class, g_simple_cdev_dev_no);
    class_destroy(g_simple_cdev_class);
    cdev_del(g_simple_cdev);
    unregister_chrdev_region(g_simple_cdev_dev_no, 1);
}

MODULE_LICENSE("GPL");
module_init(simple_cdev_init);
module_exit(simple_cdev_exit);