The GStreamer core provides a GstSystemClock based on the system time.
Asynchronous callbacks are scheduled from an internal thread.
Clock implementors are encouraged to subclass this systemclock as it implements the async notification.
Subclasses can however override all of the important methods for sync and async notifications to implement their own callback methods or
blocking wait operations.
Last reviewed on 2006-03-08 (0.10.4)