Major and Minor Device Numbers

Each device has a major and minor device number assigned to it. These numbers identify the proper device location and device driver to the kernel. This number is used by the operating system to key into the proper device driver whenever a physical device file corresponding to one of the devices it manages is opened. The major device number maps to a device driver such as sd, st, or hme. The minor device number indicates the specific member within that class of devices. All devices managed by a given device driver contain a unique minor number. Some drivers of pseudo devices (software entities set up to look like devices) create new minor devices on demand. Together, the major and minor numbers uniquely define a device and its device driver.

Physical device files have a unique output when listed with the ls -l command, as shown in the following example:

cd /devices/pci@1f,0/pci@1,1/ide@3

ls -l

The system responds with this:

total 4
drwxr-xr-x  2 root     sys          512 Mar 24 13:25 dad@0,0
brw-r-----  1 root     sys      136,  8 Aug  5 11:31 dad@0,0:a
crw-r-----  1 root     sys      136,  8 Aug  5 11:57 dad@0,0:a,raw
brw-r-----  1 root     sys      136,  9 Aug  5 11:32 dad@0,0:b
crw-r-----  1 root     sys      136,  9 Aug  5 11:57 dad@0,0:b,raw
brw-r-----  1 root     sys      136, 10 Aug  5 11:57 dad@0,0:c
crw-r-----  1 root     sys      136, 10 Aug  5 11:57 dad@0,0:c,raw
brw-r-----  1 root     sys      136, 11 Aug  5 11:55 dad@0,0:d
crw-r-----  1 root     sys      136, 11 Aug  5 11:57 dad@0,0:d,raw
brw-r-----  1 root     sys      136, 12 Aug  5 11:32 dad@0,0:e
crw-r-----  1 root     sys      136, 12 Aug  5 11:57 dad@0,0:e,raw
brw-r-----  1 root     sys      136, 13 Aug  5 11:32 dad@0,0:f
crw-r-----  1 root     sys      136, 13 Aug  5 11:57 dad@0,0:f,raw
brw-r-----  1 root     sys      136, 14 Aug  5 11:32 dad@0,0:g
crw-r-----  1 root     sys      136, 14 Aug  5 11:57 dad@0,0:g,raw
brw-r-----  1 root     sys      136, 15 Aug  5 11:32 dad@0,0:h
crw-r-----  1 root     sys      136, 15 Aug  5 11:57 dad@0,0:h,raw

This long listing includes columns showing major and minor numbers for each device. The dad driver manages all the devices listed in the previous example, which have a major number of 136. Minor numbers are listed after the comma.

During the process of building the /devices directory, major numbers are assigned based on the kernel module attached to the device. Each device is assigned a major device number by using the name-to-number mappings held in the /etc/name_to_major file. This file is maintained by the system and is undocumented. The following is a sample of the /etc/name_to_major file:

more /etc/name_to_major
cn 0
rootnex 1
pseudo 2
ip 3
logindmux 4
icmp 5
fas 6
hme 7
p9000 8
p9100 9
sp 10
clone 11
sad 12
mm 13
iwscn 14
wc 15
conskbd 16
consms 17
ipdcm 18
dump 19
se 20
log 21
sy 22
ptm 23
pts 24
ptc 25
ptsl 26
bwtwo 27
audio 28
zs 29
cgthree 30
cgtwo 31
sd 32
st 33
...
...
envctrl 131
cvc 132
cvcredir 133
eide 134
hd 135
tadbat 136
ts102 137
simba 138
uata 139
dad 140
atapicd 141

To create the minor device entries, the devfsadmd daemon uses the information placed in the dev_info node by the device driver. Permissions and ownership information are kept in the /etc/minor_perm file.