The Stream Manager Helper (SMH) messages are exported for use by both device driver and DLL stream handlers through the SMHEntryPoint. The stream handlers use these helper messages to register with the Sync/Stream Manager, report events and synchronization cues, and request or return buffers. They are synchronous calls and are available to DLL stream handlers as a DLL call (device driver stream handlers as an IDC call).
In addition, device drivers stream handlers send SMH messages to the Sync/Stream Manager (SSM) through the SMHEntryPoint. For device driver stream handlers, this interface is created using the standard inter-device driver communication (IDC) approach, established by the AttachDD DevHelp function during initialization processing.