Linux Device Drivers

Major and Minor Numbers

Char devices are accessed through names (or ``nodes'') in the filesystem, usually located in the /dev directory. Device files are special files and are identified by a ``c'' in the first column of the output of ls -l, indicating that they are char nodes. Block devices appear in /dev as well, but they are identified by a ``b''; even if some of the following information applies also to block devices, I am now focusing on char drivers.

If you issue the ls command, you’ll see two numbers (separated by a comma) on the device file entries before the date of last modification, where the file length normally appears. These numbers are the “major” and “minor” numbers for the particular device. The following listing shows a few devices as they appear on my system. Their major numbers are 10, 1, and 4, while the minors are 0, 3, 5, 64-65, and 128-129.

crw-rw-rw-   1 root     root      10,   3 Nov 30  1993 bmouseatixl
crw-rw-rw-   1 root     sys        1,   3 Nov 30  1993 null
crw-rw-rw-   1 root     root       4, 128 Apr 30 13:02 ptyp0
crw-rw-rw-   1 root     root       4, 129 Apr 30 13:02 ptyp1
crw-rw-rw-   1 rubini   staff      4,   0 Jan 30  1995 tty0
crw-rw-rw-   1 root     tty        4,  64 Jan 25  1995 ttyS0
crw-rw-rw-   1 root     root       4,  65 May  1 00:04 ttyS1
crw-rw-rw-   1 root     sys        1,   5 Nov 30  1993 zero

The major number identifies the driver associated with the device. For example, /dev/null and /dev/zero are both managed by driver 1, while all the tty’s and pty’s are managed by driver 4. The kernel uses the major number to associate the ...