Linux Device Drivers, Second Edition

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.

If you issue the ls -l command, you’ll see two numbers (separated by a comma) in the device file entries before the date of last modification, where the file length normally appears. These numbers are the major device number 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   Feb 23 1999  null
 crw------- 1 root   root   10, 1   Feb 23 1999  psaux
 crw------- 1 rubini tty     4, 1   Aug 16 22:22 tty1
 crw-rw-rw- 1 root   dialout 4, 64  Jun 30 11:19 ttyS0
 crw-rw-rw- 1 root   dialout 4, 65  Aug 16 00:00 ttyS1
 crw------- 1 root   sys     7, 1   Feb 23 1999  vcs1
 crw------- 1 root   sys     7, 129 Feb 23 1999  vcsa1
 crw-rw-rw- 1 root   root    1, 5   Feb 23 1999  zero

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 ...