Configuring for a Dynamic Major Number

Configuring for a Dynamic Major Number

You can place a hyphen ("-") in the third field of the descriptive line; that is, the field for the major device number (see "Descriptive Line"). When the line also contains the b or c flag, indicating a device driver, a hyphen in the third field means that lboot is to assign some unused major device number dynamically when the module is registered.

Since a device driver can only be called by opening a device special file, and since a device special file has to be defined based on a major device number, how can the device special files be created?

The assigned major number can be discovered using the ml command. The command

ml list -r

writes a list of all registered modules, including their major numbers. The following procedure can be used to make the assignment of the major device number completely dynamic.

Make the device driver loadable, specifying the d, R, and D flags.
Specify a hyphen for the major number.
In the driver, get the major number dynamically, if indeed the driver needs to know its major number at all.
In a script executed from rc2.d (just as the /dev/MAKEDEV script is executed), call ml to list registered drivers.
Parse the output of ml using UNIX utilities to extract the major device number from the line describing your driver.
Execute mknod or install commands to create special device files using the discovered major number.