Today, when I visit tldr issue and I saw a talk about the command lsblk, although I used a lot before, I really don’t understand the MAJ:MIN in the result. Most time, I use it to check the harddrive disk and partitions.

lsblk

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

loop0 7:0 0 260.7M 1 loop /snap/kde-frameworks-5-core18/32

loop1 7:1 0 253.5M 1 loop /snap/electronic-wechat/7

loop2 7:2 0 69M 1 loop /snap/telegram-desktop/1634

loop3 7:3 0 21.3M 1 loop /snap/communitheme/1987

loop4 7:4 0 55M 1 loop /snap/core18/1754

loop5 7:5 0 93.9M 1 loop /snap/core/9066

loop6 7:6 0 54.8M 1 loop /snap/gtk-common-themes/1502

loop7 7:7 0 93.8M 1 loop /snap/core/8935

loop8 7:8 0 373.5M 1 loop /snap/anbox/158

loop10 7:10 0 397.1M 1 loop /snap/redis-desktop-manager/335

loop11 7:11 0 160.2M 1 loop /snap/gnome-3-28-1804/116

loop12 7:12 0 32M 1 loop /snap/git-fame/15

loop13 7:13 0 149.2M 1 loop /snap/postman/109

loop14 7:14 0 16M 1 loop /snap/communitheme/1768

loop15 7:15 0 55M 1 loop /snap/core18/1705

loop16 7:16 0 374.9M 1 loop /snap/redis-desktop-manager/400

loop17 7:17 0 69M 1 loop /snap/telegram-desktop/1627

loop18 7:18 0 62.1M 1 loop /snap/gtk-common-themes/1506

loop19 7:19 0 32.1M 1 loop /snap/git-fame/23

loop20 7:20 0 310.8M 1 loop

loop21 7:21 0 163.6M 1 loop /snap/postman/110

sda 8:0 0 931.5G 0 disk

├─sda1 8:1 0 214.9G 0 part

├─sda2 8:2 0 16.3G 0 part [SWAP]

└─sda3 8:3 0 700.4G 0 part /media/Backup

sdb 8:16 0 232.9G 0 disk

├─sdb1 8:17 0 232.9G 0 part /

└─sdb2 8:18 0 2M 0 part

However, when I take a close look at the output, I can see only the disk device output, but also see the snap package output. So I started to search informations about the MAJ:MIN.

Major and minor device number

We all know that under linux, all devices are managed under /dev folder. So lets check the special device first:

ls -al /dev/zero

crw-rw-rw- 1 root root 1, 5 May 29 19:40 /dev/zero

We can see that, ls output is a little bit different from normal output, /dev/zero device’s major number is 1 and minor is 5

Then let’s check /proc/devices:

cat /proc/devices

This file contains the list of device drivers configured into the current running kernal(block and character).

We can see that under /proc/devices file, there are a list of number and strings. For example:

Character devices:

1 mem

5 /dev/tty

5 /dev/console

7 vcs

Block devices:

8 sd

Each device node’s type (block or character) and numbers serve as identifiers for the kernel.

On Linux, the canonical list of devices, with a brief explanation of their function, is maintained in the kernel.

major number: identify the driver associated with the device. For example /dev/null and /dev/zero are both managerd by driver 1, whereas virtual consoles and serial terminals are managed by driver 4. Kernal uses the major number at open time to dispatch execution to the appropriate driver.

minor number: refers to an instance, which is used by the driver itself, specified by the major number. Minor number is used for driver to identify the difference between devices.

After version 2.4, the kernel introduced a new feature, the device file system or devfs. But for now most distributions do not add these feature. Read more from here.

When devfs is not being used, adding a new driver to the system means assigning a major number to it. The assignment should be made at driver (module) initialization by calling the following function, defined in :

int register_chrdev(unsigned int major, const char* name, struct file_operations* fops);

Once the driver has been registered in the kernel table, its operations are associated with the given major number.

And a name must be inserted into the /dev directory and associated with your driver’s major and minor numbers.

The command to create a device node on the filesystem is called mknod:

mknod /dev/scull0 c 254 0

Explain:

c means: create a char device

with major nubmer 254

and minor number 0, minor number should be in the range 0 to 255

reference

《Linux Device Drivers, Second Edition by Jonathan Corbet, Alessandro Rubini》