Device number

Learn device numbers and registration.

dev_t: device number

On Linux every character or block device has a device number (dev_t). The number has two parts: major and minor.

The major number identifies the driver; the minor identifies a device under that driver. The dev_t type is defined in <linux/types.h>. MKDEV(major, minor) combines them; MAJOR(dev) and MINOR(dev) split them. ls -l /dev shows major and minor numbers. Driver authors request numbers with alloc_chrdev_region.

Number allocation

Driver authors get device numbers two ways: static register_chrdev_region or dynamic alloc_chrdev_region. The dynamic path lets the kernel pick a suitable major.

Dynamic allocation is preferred to avoid clashes. On success, the dev_t variable holds the major and minor. When the driver is removed, unregister_chrdev_region gives the number back. On the init error path the number must be released, or it leaks. The device cannot be registered before the number is allocated.

Exercise Print device numbers

Define void print_dev(int major, int minor) with body printf("dev=%d:%d\n", major, minor);. In main call it for three devices: (10, 0), (1, 5), (254, 1).

Show example solution

#include <stdio.h>

void print_dev(int major, int minor) {
    printf("dev=%d:%d\n", major, minor);
}

int main(void) {
    print_dev(10, 0);
    print_dev(1, 5);
    print_dev(254, 1);
    return 0;
}

Test yourself

What does the dev_t type hold?
- Only major
- Major and minor
- The device name
- Only minor
Show answer

Major and minor — dev_t stores both major and minor together.
Match device number macros

dev_t

Device number
MKDEV

Combine major and minor
MAJOR

Major number
MINOR

Minor number

Complete the MKDEV call

dev_t dev = MKDEV(____, minor);

Show answer

dev_t dev = MKDEV(major, minor);

Next lesson: fops

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