The DRV_FREE message is the third message sent by Windows to an installable driver after an application calls the CloseDriver function.
dwDriverIdentifier
Not used.
hDriver
Identifies an instance of the installable driver.
lParam1
Not used.
lParam2
Not used.
An installable driver returns zero if it processes this message.
When an installable driver's DriverProc function receives this message, it should free the memory that was allocated for all driver-specific structures.