What types of devices fall under the major device number category and what types of devices fall under the minor device number category. What is the real difference between the two categories?
Linux : Major and Minor device numbers
deviceslinux
Related Solutions
Devices on Unix have a type (e.g. character or block), a major number (which typically refers to a driver), and a minor number (which typically refers to an instance).
So, for example:
% ls -l /dev/vda
brw-rw---- 1 root disk 253, 0 Feb 3 09:09 /dev/vda
This is a block device, major 253, minor 0.
If we look at /proc/devices we see it ends with something similar to
Block devices:
2 fd
259 blkext
9 md
253 virtblk
254 mdp
So we can see that 253 is "virtblk". Which makes sense, since this is a virtual machine with virtual disks!
The minor number, for this driver, refers to the block device and partition in the device
% ls -l /dev/vd*
brw-rw---- 1 root disk 253, 0 Feb 3 09:09 /dev/vda
brw-rw---- 1 root disk 253, 1 Feb 3 09:09 /dev/vda1
brw-rw---- 1 root disk 253, 2 Feb 3 09:09 /dev/vda2
brw-rw---- 1 root disk 253, 3 Feb 3 09:09 /dev/vda3
brw-rw---- 1 root disk 253, 16 Feb 3 09:09 /dev/vdb
brw-rw---- 1 root disk 253, 32 Feb 3 09:09 /dev/vdc
brw-rw---- 1 root disk 253, 33 Feb 3 09:09 /dev/vdc1
There are some special drivers which don't refer to "real" hardware. eg
% ls -l /dev/null
crw-rw-rw- 1 root root 1, 3 Feb 3 09:09 /dev/null
This is a character device, major 1, minor 3. /proc/devices tells us driver 1
1 mem
We can see this "mem" driver handles a few other devices as well
% ls -l /dev | grep ' 1, '
crw-rw-rw- 1 root root 1, 7 Feb 3 09:09 full
crw-r--r-- 1 root root 1, 11 Feb 3 09:09 kmsg
crw-r----- 1 root kmem 1, 1 Feb 3 09:09 mem
crw-rw-rw- 1 root root 1, 3 Feb 3 09:09 null
crw------- 1 root root 1, 12 Feb 3 09:09 oldmem
crw-r----- 1 root kmem 1, 4 Feb 3 09:09 port
crw-rw-rw- 1 root root 1, 8 Feb 3 09:09 random
crw-rw-rw- 1 root root 1, 9 Feb 3 09:09 urandom
crw-rw-rw- 1 root root 1, 5 Feb 3 09:09 zero
Best Answer
All devices have a
major, minornumber pair. Themajornumber is a larger, more generic category (e.g. hard disks, input/output devices etc...) while theminornumber is more specific (i.e. tells what bus the device is connected to).Check the kernel documentation for more examples.