# Clocks

The `GstClock` returns a monotonically increasing time with the method `_get_time()`. Its accuracy and base time depends on the specific clock implementation but time is always expressed in nanoseconds. Since the baseline of the clock is undefined, the clock time returned is not meaningful in itself, what matters are the deltas between two clock times. The time reported by the clock is called the `absolute_time`.

## Clock Selection

To synchronize the different elements, the `GstPipeline` is responsible for selecting and distributing a global `GstClock` for all the elements in it.

This selection happens whenever the pipeline goes to PLAYING. Whenever an element is added/removed from the pipeline, this selection will be redone in the next state change to PLAYING. Adding an element that can provide a clock will post a `GST_MESSAGE_CLOCK_PROVIDE` message on the bus to inform parent bins of the fact that a clock recalculation is needed.

When a clock is selected, a `NEW_CLOCK` message is posted on the bus signaling the clock to the application.

When the element that provided the clock is removed from the pipeline, a `CLOCK_LOST` message is posted. The application must then set the pipeline to PAUSED and PLAYING again in order to let the pipeline select a new clock and distribute a new base time.

The clock selection is performed as part of the state change from PAUSED to PLAYING and is described in states.

## Clock features

The clock supports periodic and single shot clock notifications both synchronous and asynchronous.

One first needs to create a `GstClockID` for the periodic or single shot notification using `_clock_new_single_shot_id()` or `_clock_new_periodic_id()`.

To perform a blocking wait for the specific time of the `GstClockID` use the `gst_clock_id_wait()`. To receive a callback when the specific time is reached in the clock use `gstclock_id_wait_async()`. Both these calls can be interrupted with the `gst_clock_id_unschedule()` call. If the blocking wait is unscheduled a value of `GST_CLOCK_UNSCHEDULED` is returned.

The async callbacks can happen from any thread, either provided by the core or from a streaming thread. The application should be prepared for this.

A `GstClockID` that has been unscheduled cannot be used again for any wait operation.

It is possible to perform a blocking wait on the same ID from multiple threads. However, registering the same ID for multiple async notifications is not possible, the callback will only be called once.

None of the wait operations unref the `GstClockID`, the owner is responsible for unreffing the ids itself. This holds true for both periodic and single shot notifications. The reason being that the owner of the ClockID has to keep a handle to the ID to unblock the wait on FLUSHING events or state changes and if we unref it automatically, the handle might be invalid.

These clock operations do not operate on the stream time, so the callbacks will also occur when not in PLAYING state as if the clock just keeps on running. Some clocks however do not progress when the element that provided the clock is not PLAYING.

## Clock implementations

The GStreamer core provides a `GstSystemClock` based on the system time. Asynchronous callbacks are scheduled from an internal thread.

Clock implementers 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.

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