Like other installable drivers, client applications must open a video compression and decompression driver before using it, and close it when finished so the driver will be available to other applications. When a driver receives an open request, it returns a value that the system will use for dwDriverID sent with subsequent messages. When your driver receives other messages, it can use this value to identify instance data needed for operation. Your drivers can use this data to maintain information related to the client that opened the driver.
Compression and decompression drivers should support more than one client simultaneously. If you support more than one client simultaneously, though, remember to check the dwDriverID parameter to determine which client is being accessed.
If the driver is opened for configuration by the Drivers option of the Control Panel, lParam2 contains zero. When opened this way, your driver should respond to the DRV_CONFIGURE and DRV_QUERYCONFIGURE messages.
If opened for compression or decompression services, lParam2 contains a pointer to an ICOPEN data structure.