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).1
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/nulland/dev/zeroare 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 <linux.fs.h>:
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:
cmeans: 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》