Major and Minor Numbers

If you issue the ls -l command in /dev , you'll see two numbers (separated by a comma) in the device file entries before the date of the last modification, where the file length normally appears. These numbers are the major and minor device number for the particular device.

The following listing shows a few devices as they appear on a typical system. Their major numbers are 1, 4, 7, and 10, while the minors are 1, 3, 5, 64, 65, and 129.

crw-rw-rw- 1 root root 1, 3 Apr 11 2002 null

crw------- 1 root root 10, 1 Apr 11 2002 psaux

crw------- 1 root root 4, 1 Oct 28 03:04 tty1

crw-rw-rw- 1 root tty 4, 64 Apr 11 2002 ttys0

crw-rw---- 1 root uucp 4, 65 Apr 11 2002 ttyS1

crw--w---- 1 vcsa tty 7, 1 Apr 11 2002 vcs1

crw--w---- 1 vcsa tty 7, 129 Apr 11 2002 vcsa1

crw-rw-rw- 1 root root 1, 5 Apr 11 2002 zero

Traditionally, the major number identifies the driver associated with the device.

For example,
1) /dev/null and /dev/zero are both managed by driver 1, whereas virtual consoles and serial terminals are managed by driver 4;
2) similarly, both vcs1 and vcsa1 devices are managed by driver 7.

. Modern Linux kernels allow multiple drivers to share major numbers, but most devices that you will see are still organized on the one-major-one-driver principle.

The minor number is used by the kernel to determine exactly which device is being referred to.Depending on how your driver is written (as we will see below), you can either get a direct pointer to your device from the kernel, or you can use the minor number yourself as an index into a local array of devices. Either way, the kernel itself knows almost nothing about minor numbers beyond the fact that they refer to devices implemented by your driver.