2004/01/07 minor number

© January 2004 Tony Lawrence

Device files show up in long listings with two numbers where the size would ordinarily be:

crw-rw-rw-  1 root    wheel    1,   0 Jan  3 11:29 /dev/tty
crw--w----  1 pcunix  tty      5,   0 Jan  5 16:53 /dev/ttyp0
crw-rw-rw-  1 root    wheel    5,   1 Jan  3 12:48 /dev/ttyp1
crw-rw-rw-  1 root    wheel    5,   2 Jan  3 13:15 /dev/ttyp2
 

The first number is the major number for the device, and actually is simply an index into a table of memory addresses in your kernel. When you attempt to open /dev/tty, the kernel calls the code referenced by position one in its index. The second number (the minor number) is passed to that code as an argument.

The significance of the minor number is up to the code in the device driver. In the case of the ttyp* devices above, it is used to select which pseudo tty is being used. For a tape drive, it might indicate whether or not to rewind the tape before doing i/o.

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