Major and Minor Numbers

Char devices are accessed through names in the filesystem. Those names are called
special files or device files or simply nodes of the filesystem tree; they are conventionally
located in the /dev directory. Special files for char drivers are identified by a “c”
in the first column of the output of ls –l. Block devices appear in /dev as well, but
they are identified by a “b.” The focus of this chapter is on char devices, but much of
the following information applies to block devices as well.
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, /dev/null and /dev/zero are both managed by driver 1, whereas virtual consoles
and serial terminals are managed by driver 4; 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.