Commit fc0bf09d authored by Wim Taymans's avatar Wim Taymans

Docs updates

Original commit message from CVS:
* docs/design/part-TODO.txt:
* docs/design/part-events.txt:
* docs/design/part-gstbin.txt:
* docs/design/part-gstbus.txt:
* docs/design/part-gstpipeline.txt:
* docs/design/part-messages.txt:
* gst/gstbus.c:
* gst/gstmessage.c:
Docs updates
parent a032f72b
2005-03-22 Wim Taymans <>
* docs/design/part-TODO.txt:
* docs/design/part-events.txt:
* docs/design/part-gstbin.txt:
* docs/design/part-gstbus.txt:
* docs/design/part-gstpipeline.txt:
* docs/design/part-messages.txt:
* gst/gstbus.c:
* gst/gstmessage.c:
Docs updates
2005-03-21 Wim Taymans <>
* gst/gstbus.c: (gst_bus_post):
- changing an object's name after construction is not allowed. Checks are performed
when adding objects to parents that no duplicate names are used, changing the name
to a duplicate name after adding it is therefore allowed and voids internal
- check for race in _task_pause()/_stop() since the TASK_LOCK is not held by the
calling thread and the STREAM_LOCK not by the task code.
- only emit EOS in PLAYING. Make sure elements reemit the EOS message when going to
PLAYING again.
- implement return values from events in addition to the gboolean.
- implement query ala events as opposed to the current return value of only a
guint64. I don't have a use case where this a problem yet, though.
- implement state change order on get<->loop-get<->loop elements. This probably
requires a backtracking algorithm.
- implement iterators for traversing elements upstream or dowstream.
- unlinking pads in the PAUSED state needs to make sure the stream thread is not
executing code. Can this be done with a flush to unlock all downstream chain
- implement seek in GstPipeline.
- make events use GstStructure like GstMessage instead of the current union.
- make the seek event return the time where the seek will happen so that GstPipeline
can update the stream time.
- implement clock selection as explained in part-gstpipeline.txt
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_EOS: no more data is to be expected on a pad.
GST_EVENT_FLUSH: data is to be discarded or allowed again
GST_EVENT_DISCONTINUOUS: A new group of buffers with common start time
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_SIZE: Notification of suggested buffer size.
GST_EVENT_RATE: Notification to change the processing speed of a stream
GST_EVENT_NAVIGATION: A navigation event.
GST_EVENT_TAG: Stream metadata.
The EOS event can only be sent on a sinkpad. It is typically emited 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 renderers which should
then post an EOS message on the bus.
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
STREAM_LOCK should be taken.
Sometimes the EOS event is generated by another element than the source, for
example a demuxer filter 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_UNEXPECTED, 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, the pipeline collects all EOS events from all the sinks before
passing the EOS message to the application.
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 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 filter sends the flush 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 filter sends the flush event with the done flag set 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 endflush 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 pullregion function, they send both flush events to
the upstream pads in the same way top make sure that the pullregion function
unlocks and any pending buffers are cleared in the upstream elements.
A discont event is sent downstream by an element to indicate that the following
group of buffers start and end at the specified time.
After a seek event for example, a discont event is sent.
A seek event is issued by the application to start playback of a new
position in the stream. It is called form the application thread and
travels upstream.
The seek event contains the new start end end position of playback
after the seek is performed. Optionally the end position can be left
at -1 to continue playback to the end of the stream.
A stream usually flushes the graph to minimize latency after the seek.
The seek event is passed along 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.
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 gets serialized.
The general flow of executing the seek is as follows:
1) unblock the streaming threads, they could be blocked in a chain
function. This is done by sending a flush on all srcpads.
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) lock the STREAM_LOCK. This will work since the chain/loop function
was unlocked in step 1).
3) perform the seek.
4) send a flush event with the done flag set to allow streaming again.
5) start stopped tasks and unlock the STREAM_LOCK, dataflow will continue
now from the new position.
Some demuxers know an optimal size for any downstream buffers. They can
use this event to signal this fact. Similary an element can signal an
upstream element of a prefered buffer size.
When the application wants to change the playback rate of the stream, it
issues a rate event. A rate of 1.0 is the normal playback rate, 2.0 plays
at twice the speed and negative values play backwards.
Note that the clock speed does not change.
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 downstream.
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
GstBin is a container element for other GstElements. This makes it possible
to group elements together so that they can be treated as one single
Add/removing elements
The basic functionality of a bin is to add and remove GstElements to/from it.
gst_bin_add() and gst_bin_remove() perform these operations respectively.
The bin maintains a parent-child relationship with its elements (see part-
Retrieving elements
GstBin provides a number of functions to retrieve one or more children from
itself. A few examples of the provided functions:
gst_bin_get_by_name() retrieves an element by name.
gst_bin_iterate_elements() returns an iterator to all the children.
element management
The most important function of the GstBin is to distribute all GstElement
operations on itself to all of its children. This includes:
- state changes
- index get/set
- clock gst/set
- bus set/get
- scheduler set/get
The state change distribution is the most complex and is explained in
Subclasses of GstBin are free to implement their own add/remove implementations.
It is a good idea to update the GList of children so that the _iterate() functions
can still be used.
The GstBus is an object responsible for delivering GstMessages in
a first-in first-out way from the streaming threads to the application.
Since the application typically only wants to deal with delivery of these
messages from one thread, the GstBus will marshall the messages between
different threads.
The GstBus provides support for GSource based notifications. This makes it
possible to handle the delivery in the glib mainloop.
A message is posted on the bus with the gst_bus_post() method. With the
gst_bus_peek() and _pop() methods one can look at or retrieve a previously
posted message.
The bus can be polled with the gst_bus_poll() method. This methods blocks
up to the specified timeout value until one of the specified messages types
is posted on the bus. The application can then _pop() the messages from the
bus to handle them.
It is also possible to get messages from the bus without any thread
marshalling with the gst_bus_set_sync_handler() method. This makes it
possible to react to a message in the same thread that posted the
message on the bus. This should only be used if the application is able
to deal with messages from different threads.
A GstPipeline is usually a toplevel bin an provides all of its
children with a clock and a bus.
The GstPipeline will also collect EOS messages from its children and
will forward the EOS message to the application when all of the
sinks are in EOS.
The pipeline also calculates the stream time based on the selected
clock (see part-clocks.txt).
The pipeline manages the seek operation for the application.
The pipeline creates a GstBus and attaches a sync handler to receive
the EOS events.
Since the pipeline subclasses GstBin, all of its children will receive
the same bus when added to the Gstbin.
The application can retrieve the GstBus and integrate it in the
mainloop or it can just _pop() messages off in its own thread.
Clock selection
Since all of the children of a GstPipeline must use the same clock, the
pipeline must select a clock.
The default clock selection algorithm works as follows:
- If the application selected a clock, use that clock. (see below)
- use clock of source elements (*)
- use clock of other element, starting from the sinks going upstream.
- use GstSystemClock.
(*) currently not implemented.
(+) traversing the graph upstream to find the best clock is not implemented,
currently the first element found that provides a clock is used.
The application can influence this clock selection with two methods:
gst_pipeline_use_clock() and gst_pipeline_auto_clock().
The _use_clock() method forces the use of a specific clock on the pipeline
regardless of what clock providers are children of the pipeline. Setting
NULL disables the clock completely and makes the pipeline run as fast as
The _auto_clock() method removes the fixed clock and reactivates the auto-
matic clock selection algorithm described above.
The sink elements will post an EOS event on the bus when they reach EOS. The
EOS message is only posted to the bus when the element is in PLAYING.
The pipeline collects all EOS messages and forwards it to the application as
soon as all the sinks have posted an EOS.
The list of queued EOS messages is cleared when the pipeline goes to PAUSED
again. This means that all elements should repost the EOS message when going
to PLAYING again.
When performing a seek on the pipeline element using gst_element_send_event(),
the pipeline performs the following actions:
- record the current state of the pipeline.
- set the pipeline to paused
- send the seek event to all sinks
- update the stream time with the time of the seek
- restore old state of the pipeline.
Messages are refcounted lightweight objects to signal the application
of pipeline events.
......@@ -482,8 +482,11 @@ bus_watch_destroy (GstBusWatch * watch)
* gst_bus_add_watch_full:
* @bus: a #GstBus to create the watch for
* @bus: a #GstBus to create the watch for.
* @priority: The priority of the watch.
* @handler: A function to call when a message is received.
* @user_data: user data passed to @handler.
* @notify: the function to call when the source is removed.
* Adds the bus to the mainloop with the given priority. If the handler returns
* TRUE, the message will then be popped off the queue. When the handler is
......@@ -528,6 +531,8 @@ gst_bus_add_watch_full (GstBus * bus, gint priority,
* gst_bus_add_watch:
* @bus: a #GstBus to create the watch for
* @handler: A function to call when a message is received.
* @user_data: user data passed to @handler.
* Adds the bus to the mainloop with the default priority.
......@@ -356,7 +356,7 @@ gst_message_parse_state_changed (GstMessage * message, GstElementState * old,
* gst_message_parse_error:
* @message: A valid #GstMessage of type GST_MESSAGE_ERROR.
* Extracts the GError and debug strung from the GstMessage. The values returned
* Extracts the GError and debug string from the GstMessage. The values returned
* in the output arguments are copies; the caller must free them when done.
* MT safe.
......@@ -381,7 +381,7 @@ gst_message_parse_error (GstMessage * message, GError ** gerror, gchar ** debug)
* gst_message_parse_warning:
* @message: A valid #GstMessage of type GST_MESSAGE_WARNING.
* Extracts the GError and debug strung from the GstMessage. The values returned
* Extracts the GError and debug string from the GstMessage. The values returned
* in the output arguments are copies; the caller must free them when done.
* MT safe.
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