Events: Seeking, Navigation and More

There are many different event types but only two ways they can travel in the pipeline: downstream or upstream. It is very important to understand how both of these methods work because if one element in the pipeline is not handling them correctly the whole event system of the pipeline is broken. We will try to explain here how these methods work and how elements are supposed to implement them.

Downstream events

Downstream events are received through the sink pad's event handler, as set using gst_pad_set_event_function () when the pad was created.

Downstream events can travel in two ways: they can be in-band (serialised with the buffer flow) or out-of-band (travelling through the pipeline instantly, possibly not in the same thread as the streaming thread that is processing the buffers, skipping ahead of buffers being processed or queued in the pipeline). The most common downstream events (SEGMENT, CAPS, TAG, EOS) are all serialised with the buffer flow.

Here is a typical event function:

static gboolean
gst_my_filter_sink_event (GstPad  *pad, GstObject * parent, GstEvent * event)
{
  GstMyFilter *filter;
  gboolean ret;

  filter = GST_MY_FILTER (parent);
  ...

  switch (GST_EVENT_TYPE (event)) {
    case GST_EVENT_SEGMENT:
      /* maybe save and/or update the current segment (e.g. for output
       * clipping) or convert the event into one in a different format
       * (e.g. BYTES to TIME) or drop it and set a flag to send a segment
       * event in a different format later */
      ret = gst_pad_push_event (filter->src_pad, event);
      break;
    case GST_EVENT_EOS:
      /* end-of-stream, we should close down all stream leftovers here */
      gst_my_filter_stop_processing (filter);
      ret = gst_pad_push_event (filter->src_pad, event);
      break;
    case GST_EVENT_FLUSH_STOP:
      gst_my_filter_clear_temporary_buffers (filter);
      ret = gst_pad_push_event (filter->src_pad, event);
      break;
    default:
      ret = gst_pad_event_default (pad, parent, event);
      break;
  }

  ...
  return ret;
}

If your element is chain-based, you will almost always have to implement a sink event function, since that is how you are notified about segments, caps and the end of the stream.

If your element is exclusively loop-based, you may or may not want a sink event function (since the element is driving the pipeline it will know the length of the stream in advance or be notified by the flow return value of gst_pad_pull_range(). In some cases even loop-based element may receive events from upstream though (for example audio decoders with an id3demux or apedemux element in front of them, or demuxers that are being fed input from sources that send additional information about the stream in custom events, as DVD sources do).

Upstream events

Upstream events are generated by an element somewhere downstream in the pipeline (example: a video sink may generate navigation events that informs upstream elements about the current position of the mouse pointer). This may also happen indirectly on request of the application, for example when the application executes a seek on a pipeline this seek request will be passed on to a sink element which will then in turn generate an upstream seek event.

The most common upstream events are seek events, Quality-of-Service (QoS) and reconfigure events.

An upstream event can be sent using the gst_pad_send_event function. This function simply call the default event handler of that pad. The default event handler of pads is gst_pad_event_default, and it basically sends the event to the peer of the internally linked pad. So upstream events always arrive on the src pad of your element and are handled by the default event handler except if you override that handler to handle it yourself. There are some specific cases where you have to do that :

  • If you have multiple sink pads in your element. In that case you will have to decide which one of the sink pads you will send the event to (if not all of them).

  • If you need to handle that event locally. For example a navigation event that you will want to convert before sending it upstream, or a QoS event that you want to handle.

The processing you will do in that event handler does not really matter but there are important rules you have to absolutely respect because one broken element event handler is breaking the whole pipeline event handling. Here they are :

  • Always handle events you won't handle using the default gst_pad_event_default method. This method will depending on the event, forward the event or drop it.

  • If you are generating some new event based on the one you received don't forget to gst_event_unref the event you received.

  • Event handler function are supposed to return TRUE or FALSE indicating if the event has been handled or not. Never simply return TRUE/FALSE in that handler except if you really know that you have handled that event.

  • Remember that the event handler might be called from a different thread than the streaming thread, so make sure you use appropriate locking everywhere.

All Events Together

In this chapter follows a list of all defined events that are currently being used, plus how they should be used/interpreted. You can check the what type a certain event is using the GST_EVENT_TYPE macro (or if you need a string for debugging purposes you can use GST_EVENT_TYPE_NAME).

In this chapter, we will discuss the following events:

For more comprehensive information about events and how they should be used correctly in various circumstances please consult the GStreamer design documentation. This section only gives a general overview.

Stream Start

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Caps

The CAPS event contains the format description of the following buffers. See Caps negotiation for more information about negotiation.

Segment

A segment event is sent downstream to announce the range of valid timestamps in the stream and how they should be transformed into running-time and stream-time. A segment event must always be sent before the first buffer of data and after a flush (see above).

The first segment event is created by the element driving the pipeline, like a source operating in push-mode or a demuxer/decoder operating pull-based. This segment event then travels down the pipeline and may be transformed on the way (a decoder, for example, might receive a segment event in BYTES format and might transform this into a segment event in TIMES format based on the average bitrate).

Depending on the element type, the event can simply be forwarded using gst_pad_event_default (), or it should be parsed and a modified event should be sent on. The last is true for demuxers, which generally have a byte-to-time conversion concept. Their input is usually byte-based, so the incoming event will have an offset in byte units (GST_FORMAT_BYTES), too. Elements downstream, however, expect segment events in time units, so that it can be used to synchronize against the pipeline clock. Therefore, demuxers and similar elements should not forward the event, but parse it, free it and send a segment event (in time units, GST_FORMAT_TIME) further downstream.

The segment event is created using the function gst_event_new_segment (). See the API reference and design document for details about its parameters.

Elements parsing this event can use gst_event_parse_segment() to extract the event details. Elements may find the GstSegment API useful to keep track of the current segment (if they want to use it for output clipping, for example).

Tag (metadata)

Tagging events are being sent downstream to indicate the tags as parsed from the stream data. This is currently used to preserve tags during stream transcoding from one format to the other. Tags are discussed extensively in Tagging (Metadata and Streaminfo). Most elements will simply forward the event by calling gst_pad_event_default ().

The tag event is created using the function gst_event_new_tag (), but more often elements will send a tag event downstream that will be converted into a message on the bus by sink elements. All of these functions require a filled-in taglist as argument, which they will take ownership of.

Elements parsing this event can use the function gst_event_parse_tag () to acquire the taglist that the event contains.

End of Stream (EOS)

End-of-stream events are sent if the stream that an element sends out is finished. An element receiving this event (from upstream, so it receives it on its sinkpad) will generally just process any buffered data (if there is any) and then forward the event further downstream. The gst_pad_event_default () takes care of all this, so most elements do not need to support this event. Exceptions are elements that explicitly need to close a resource down on EOS, and N-to-1 elements. Note that the stream itself is not a resource that should be closed down on EOS! Applications might seek back to a point before EOS and continue playing again.

The EOS event has no properties, which makes it one of the simplest events in GStreamer. It is created using the gst_event_new_eos() function.

It is important to note that only elements driving the pipeline should ever send an EOS event. If your element is chain-based, it is not driving the pipeline. Chain-based elements should just return GST_FLOW_EOS from their chain function at the end of the stream (or the configured segment), the upstream element that is driving the pipeline will then take care of sending the EOS event (or alternatively post a SEGMENT_DONE message on the bus depending on the mode of operation). If you are implementing your own source element, you also do not need to ever manually send an EOS event, you should also just return GST_FLOW_EOS in your create or fill function (assuming your element derives from GstBaseSrc or GstPushSrc).

Table Of Contents

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Gap

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Flush Start

The flush start event is sent downstream (in push mode) or upstream (in pull mode) if all buffers and caches in the pipeline should be emptied. “Queue” elements will empty their internal list of buffers when they receive this event, for example. File sink elements (e.g. “filesink”) will flush the kernel-to-disk cache (fdatasync () or fflush ()) when they receive this event. Normally, elements receiving this event will simply just forward it, since most filter or filter-like elements don't have an internal cache of data. gst_pad_event_default () does just that, so for most elements, it is enough to forward the event using the default event handler.

As a side-effect of flushing all data from the pipeline, this event unblocks the streaming thread by making all pads reject data until they receive a Flush Stop signal (elements trying to push data will get a FLUSHING flow return and stop processing data).

The flush-start event is created with the gst_event_new_flush_start (). Like the EOS event, it has no properties. This event is usually only created by elements driving the pipeline, like source elements operating in push-mode or pull-range based demuxers/decoders.

Flush Stop

The flush-stop event is sent by an element driving the pipeline after a flush-start and tells pads and elements downstream that they should accept events and buffers again (there will be at least a SEGMENT event before any buffers first though).

If your element keeps temporary caches of stream data, it should clear them when it receives a FLUSH-STOP event (and also whenever its chain function receives a buffer with the DISCONT flag set).

The flush-stop event is created with gst_event_new_flush_stop (). It has one parameter that controls if the running-time of the pipeline should be reset to 0 or not. Normally after a flushing seek, the running_time is set back to 0.

Quality Of Service (QOS)

The QOS event contains a report about the current real-time performance of the stream. See more info in Quality Of Service (QoS).

Seek Request

Seek events are meant to request a new stream position to elements. This new position can be set in several formats (time, bytes or “default units” [a term indicating frames for video, channel-independent samples for audio, etc.]). Seeking can be done with respect to the end-of-file or start-of-file, and usually happens in upstream direction (downstream seeking is done by sending a SEGMENT event with the appropriate offsets for elements that support that, like filesink).

Elements receiving seek events should, depending on the element type, either just forward it upstream (filters, decoders), change the format in which the event is given and then forward it (demuxers), or handle the event by changing the file pointer in their internal stream resource (file sources, demuxers/decoders driving the pipeline in pull-mode) or something else.

Seek events are built up using positions in specified formats (time, bytes, units). They are created using the function gst_event_new_seek (). Note that many plugins do not support seeking from the end of the stream. An element not driving the pipeline and forwarding a seek request should not assume that the seek succeeded or actually happened, it should operate based on the SEGMENT events it receives.

Elements parsing this event can do this using gst_event_parse_seek().

Navigation events are sent upstream by video sinks to inform upstream elements of where the mouse pointer is, if and where mouse pointer clicks have happened, or if keys have been pressed or released.

All this information is contained in the event structure which can be obtained with gst_event_get_structure ().

Check out the navigationtest element in gst-plugins-good for an idea how to extract navigation information from this event.

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