conventions among ioctl commands

[Kris A. Kugel]

In the document "UNIX PC Version 3.0 Device Driver Development Guide",
R. D. Niehaus says [about ioctl parameters]:

	One particularly good idea is to have a common parameter
	among all drivers; one that would permit feedback from
	the driver about its type.  In this way, a process could
	verify just what device was attached to a file descriptor.

Does anybody know if such a convention exists?  I couldn't find
any common element in the /usr/include/sys/*ioctl*.h files I looked at.

                               Kris A. Kugel
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