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DEVICE(9)	       FreeBSD Kernel Developer's Manual	     DEVICE(9)

     device -- an abstract representation of a device

     typedef struct device *device_t;

     The device	object represents a piece of hardware attached to the system
     such as an	expansion card,	the bus	which that card	is plugged into, disk
     drives attached to	the expansion card etc.	 The system defines one
     device, root_bus and all other devices are	created	dynamically during
     autoconfiguration.	 Normally devices representing top-level busses	in the
     system (ISA, PCI etc.) will be attached directly to root_bus and other
     devices will be added as children of their	relevant bus.

     The devices in a system form a tree.  All devices except root_bus have a
     parent (see device_get_parent(9)).	 In addition, any device can have
     children attached to it (see device_add_child(9),
     device_add_child_ordered(9), device_find_child(9),
     device_get_children(9), and device_delete_child(9)).

     A device which has	been successfully probed and attached to the system
     will also have a driver (see device_get_driver(9) and driver(9)) and a
     devclass (see device_get_devclass(9) and devclass(9)).  Various other
     attributes	of the device include a	unit number (see device_get_unit(9)),
     verbose description (normally supplied by the driver, see
     device_set_desc(9)	and device_get_desc(9)), a set of bus-specific vari-
     ables (see	device_get_ivars(9)) and a set of driver-specific variables
     (see device_get_softc(9)).

     Devices can be in one of several states:

     DS_NOTPRESENT  the	device has not been probed for existence or the	probe

     DS_ALIVE	    the	device probe succeeded but not yet attached

     DS_ATTACHED    the	device has been	successfully attached

     DS_BUSY	    the	device is currently open

     The current state of the device can be determined by calling

     devclass(9), driver(9)

     This manual page was written by Doug Rabson.

FreeBSD	9.2			 June 16, 1998			   FreeBSD 9.2


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