gstreamer/subprojects/gst-docs/markdown/additional/design/events.md

Events

Events are objects passed around in parallel to the buffer dataflow to notify elements of various events.

Events are received on pads using the event function. Some events should be interleaved with the data stream so they require taking the STREAM_LOCK, others don’t.

Different types of events exist to implement various functionalities.

• GST_EVENT_FLUSH_START: data is to be discarded
• GST_EVENT_FLUSH_STOP: data is allowed again
• GST_EVENT_CAPS: Format information about the following buffers
• GST_EVENT_SEGMENT: Timing information for the following buffers
• GST_EVENT_TAG: Stream metadata.
• GST_EVENT_BUFFERSIZE: Buffer size requirements. Currently not used yet.
• GST_EVENT_SINK_MESSAGE: An event turned into a message by sinks
• GST_EVENT_EOS: no more data is to be expected on a pad.
• GST_EVENT_QOS: A notification of the quality of service of the stream
• GST_EVENT_SEEK: A seek should be performed to a new position in the stream
• GST_EVENT_NAVIGATION: A navigation event.
• GST_EVENT_LATENCY: Configure the latency in a pipeline
• GST_EVENT_STEP: Stepping event
• GST_EVENT_RECONFIGURE: stream reconfigure event

src pads

A gst_pad_push_event() on a srcpad will first store the sticky event in the sticky array before sending the event to the peer pad. If there is no peer pad and the event was not stored in the sticky array, FALSE is returned.

Flushing pads will refuse the events and will not store the sticky events.

sink pads

A gst_pad_send_event() on a sinkpad will call the event function on the pad. If the event function returns success, the sticky event is stored in the sticky event array and the event is marked for update.

When the pad is flushing, the gst_pad_send_event() function returns FALSE immediately.

When the next data item is pushed, the pending events are pushed first.

This ensures that the event function is never called for flushing pads and that the sticky array only contains events for which the event function returned success.

pad link

When linking pads, the srcpad sticky events are marked for update when they are different from the sinkpad events. The next buffer push will push the events to the sinkpad.

FLUSH_START/STOP

A flush event is sent both downstream and upstream to clear any pending data from the pipeline. This might be needed to make the graph more responsive when the normal dataflow gets interrupted by for example a seek event.

Flushing happens in two stages.

1. a source element sends the FLUSH_START event to the downstream peer element. The downstream element starts rejecting buffers from the upstream elements. It sends the flush event further downstream and discards any buffers it is holding as well as return from the chain function as soon as possible. This makes sure that all upstream elements get unblocked. This event is not synchronized with the STREAM_LOCK and can be done in the application thread.

2. a source element sends the FLUSH_STOP event to indicate that the downstream element can accept buffers again. The downstream element sends the flush event to its peer elements. After this step dataflow continues. The FLUSH_STOP call is synchronized with the STREAM_LOCK so any data used by the chain function can safely freed here if needed. Any pending EOS events should be discarded too.

After the flush completes the second stage, data is flowing again in the pipeline and all buffers are more recent than those before the flush.

For elements that use the pullrange function, they send both flush events to the upstream pads in the same way to make sure that the pullrange function unlocks and any pending buffers are cleared in the upstream elements.

A FLUSH_START may instruct the pipeline to distribute a new base_time to elements so that the running_time is reset to 0. (see clocks and synchronisation).

EOS

The EOS event can only be sent on a sinkpad. It is typically emitted by the source element when it has finished sending data. This event is mainly sent in the streaming thread but can also be sent from the application thread.

The downstream element should forward the EOS event to its downstream peer elements. This way the event will eventually reach the sinks which should then post an EOS message on the bus when in PLAYING.

An element might want to flush its internally queued data before forwarding the EOS event downstream. This flushing can be done in the same thread as the one handling the EOS event.

For elements with multiple sink pads it might be possible to wait for EOS on all the pads before forwarding the event.

The EOS event should always be interleaved with the data flow, therefore the GStreamer core will take the STREAM_LOCK.

Sometimes the EOS event is generated by another element than the source, for example a demuxer element can generate an EOS event before the source element. This is not a problem, the demuxer does not send an EOS event to the upstream element but returns GST_FLOW_EOS, causing the source element to stop sending data.

An element that sends EOS on a pad should stop sending data on that pad. Source elements typically pause() their task for that purpose.

By default, a GstBin collects all EOS messages from all its sinks before posting the EOS message to its parent.

The EOS is only posted on the bus by the sink elements in the PLAYING state. If the EOS event is received in the PAUSED state, it is queued until the element goes to PLAYING.

A FLUSH_STOP event on an element flushes the EOS state and all pending EOS messages.

SEGMENT

A segment event is sent downstream by an element to indicate that the following group of buffers start and end at the specified positions. The newsegment event also contains the playback speed and the applied rate of the stream.

Since the stream time is always set to 0 at start and after a seek, a 0 point for all next buffer’s timestamps has to be propagated through the pipeline using the SEGMENT event.

Before sending buffers, an element must send a SEGMENT event. An element is free to refuse buffers if they were not preceded by a SEGMENT event.

Elements that sync to the clock should store the SEGMENT start and end values and subtract the start value from the buffer timestamp before comparing it against the stream time (see clocks).

An element is allowed to send out buffers with the SEGMENT start time already subtracted from the timestamp. If it does so, it needs to send a corrected SEGMENT downstream, ie, one with start time 0.

A SEGMENT event should be generated as soon as possible in the pipeline and is usually generated by a demuxer or source. The event is generated before pushing the first buffer and after a seek, right before pushing the new buffer.

The SEGMENT event should be sent from the streaming thread and should be serialized with the buffers.

Buffers should be clipped within the range indicated by the newsegment event start and stop values. Sinks must drop buffers with timestamps out of the indicated segment range.

TAG

The tag event is sent downstream when an element has discovered metadata tags in a media file. Encoders can use this event to adjust their tagging system. A tag is serialized with buffers.

BUFFERSIZE

Note

This event is not yet implemented.

An element can suggest a buffersize for downstream elements. This is typically done by elements that produce data on multiple source pads such as demuxers.

QOS

A QOS, or quality of service message, is generated in an element to report to the upstream elements about the current quality of real-time performance of the stream. This is typically done by the sinks that measure the amount of framedrops they have. (see qos)

SEEK

A seek event is issued by the application to configure the playback range of a stream. It is called from the application thread and travels upstream.

The seek event contains the new start and stop position of playback after the seek is performed. Optionally the stop position can be left at -1 to continue playback to the end of the stream. The seek event also contains the new playback rate of the stream, 1.0 is normal playback, 2.0 double speed and negative values mean backwards playback.

A seek usually flushes the graph to minimize latency after the seek. This behaviour is triggered by using the SEEK_FLUSH flag on the seek event.

The seek event usually starts from the sink elements and travels upstream from element to element until it reaches an element that can perform the seek. No intermediate element is allowed to assume that a seek to this location will happen. It is allowed to modify the start and stop times if it needs to do so. this is typically the case if a seek is requested for a non-time position.

The actual seek is performed in the application thread so that success or failure can be reported as a return value of the seek event. It is therefore important that before executing the seek, the element acquires the STREAM_LOCK so that the streaming thread and the seek get serialized.

The general flow of executing the seek with FLUSH is as follows:

1. unblock the streaming threads, they could be blocked in a chain function. This is done by sending a FLUSH_START on all srcpads or by pausing the streaming task, depending on the seek FLUSH flag. The flush will make sure that all downstream elements unlock and that control will return to this element chain/loop function. We cannot lock the STREAM_LOCK before doing this since it might cause a deadlock.

2. acquire the STREAM_LOCK. This will work since the chain/loop function was unlocked/paused in step 1).

3. perform the seek. since the STREAM_LOCK is held, the streaming thread will wait for the seek to complete. Most likely, the stream thread will pause because the peer elements are flushing.

4. send a FLUSH_STOP event to all peer elements to allow streaming again.

5. create a SEGMENT event to signal the new buffer timestamp base time. This event must be queued to be sent by the streaming thread.

6. start stopped tasks and unlock the STREAM_LOCK, dataflow will continue now from the new position.

More information about the different seek types can be found in seeking.

NAVIGATION

A navigation event is generated by a sink element to signal the elements of a navigation event such as a mouse movement or button click. Navigation events travel upstream.

LATENCY

A latency event is used to configure a certain latency in the pipeline. It contains a single GstClockTime with the required latency. The latency value is calculated by the pipeline and distributed to all sink elements before they are set to PLAYING. The sinks will add the configured latency value to the timestamps of the buffer in order to delay their presentation. (See also latency).