How to remove dynamically assigned major number from /proc/devices? - linux-kernel

Question

In my kernel driver project I register with a dynamic major number by calling
register_chrdev(0, "xxxxx", &xxxxx);
and unregistered my module with
unregister_chrdev(0. "xxxxx");
When I load my driver with insmod, I received dynamic major number, for example 243, and, after rmmod, success removing module.
But, after removing the module /proc/devices still shows the major number (243).
How do I get removing my driver to also remove its major number from the list in /proc/devices?

Answer 1

When you call register_chrdev() with 0 as the first argument to request the assignment of a dynamic major number, the return value will be the assigned major number, which you should save.
Then when you call unregister_chrdev() you should pass the saved major number as an argument, rather than the 0 you were. Also make sure that the device name argument matches. And be aware that this function returns a result, which you can check for status/failure - in the latter case you definitely want to printk() a message so that you know that your code has not accomplished its goal.
You can see a complete example at http://www.tldp.org/LDP/lkmpg/2.6/html/x569.html with the key parts being:
static int Major; /* Major number assigned to our device driver */
int init_module(void)
{
Major = register_chrdev(0, DEVICE_NAME, &fops);
if (Major < 0) {
printk(KERN_ALERT "Registering char device failed with %d\n", Major);
return Major;
}
return SUCCESS;
}
void cleanup_module(void)
{
int ret = unregister_chrdev(Major, DEVICE_NAME);
if (ret < 0)
printk(KERN_ALERT "Error in unregister_chrdev: %d\n", ret);
}
Also be aware that this method of registering a device is considered outdated - you might want to research the newer method.