To assist device driver writers in allocating state structures, the Solaris DDI/DKI provides a set of memory management routines called software state management routines, which are also known as the soft-state routines. These routines dynamically allocate, retrieve, and destroy memory items of a specified size, and hide the details of list management. An instance number identifies the desired memory item. This number is typically the instance number assigned by the system.
Drivers typically allocate a soft-state structure for each device instance that attaches to the driver by calling ddi_soft_state_zalloc(9F), passing the instance number of the device. Because no two device nodes can have the same instance number, ddi_soft_state_zalloc(9F) fails if an allocation already exists for a given instance number.
A driver's character or block entry point (cb_ops(9S)) references a particular soft state structure by first decoding the device's instance number from the dev_t argument that is passed to the entry point function. The driver then calls ddi_get_soft_state(9F), passing the per-driver soft-state list and the instance number that was derived. A NULL return value indicates that effectively the device does not exist and the appropriate code should be returned by the driver.
See Creating Minor Device Nodes for additional information on how instance numbers and device numbers, or dev_t's, are related.