States
-======
+------
Both elements and pads can be in different states. The states of the pads are
linked to the state of the element so the design of the states is mainly
State definitions
------------------
+~~~~~~~~~~~~~~~~~
- NULL: This is the initial state of an element.
- READY: The element should be prepared to go to PAUSED.
- PAUSED: The element should be ready to accept and process data. Sink
elements however only accept one buffer and then block.
- - PLAYING: The same as PAUSED except for sinks, who are now accepting
- and rendering data.
+ - PLAYING: The same as PAUSED except for live sources and sinks. Sinks accept
+ and rendering data. Live sources produce data.
We call the sequence NULL->PLAYING an upwards state change and PLAYING->NULL
a downwards state change.
State transitions
------------------
+~~~~~~~~~~~~~~~~~
the following state changes are possible:
NULL -> READY
- - The element must check if the resources it needs are available.
- Audiosinks typically try to probe the device.
+ - The element must check if the resources it needs are available.
+ Device sinks and -sources typically try to probe the device to constrain
+ their caps.
+ - The element opens the device, this is needed if the previous step requires
+ the device to be opened.
READY -> PAUSED
- - The element opens the device and prepares itself for PLAYING.
- - the element pads are activated in order to receive data in PAUSED.
- streaming threads are started.
- - some elements might need to return ASYNC and complete the state change
+ - The element pads are activated in order to receive data in PAUSED.
+ Streaming threads are started.
+ - Some elements might need to return ASYNC and complete the state change
when they have enough information. It is a requirement for sinks to
return ASYNC and complete the state change when they receive the first
- buffer or EOS event (prerol). Sinks also block the dataflow when in PAUSED.
- - a pipeline resets the stream time to 0.
- - live sources return NO_PREROLL and don't generate data.
+ buffer or EOS event (preroll). Sinks also block the dataflow when in PAUSED.
+ - A pipeline resets the running_time to 0.
+ - Live sources return NO_PREROLL and don't generate data.
PAUSED -> PLAYING
- - most elements ignore this state change.
+ - Most elements ignore this state change.
- The pipeline selects a clock and distributes this to all the children
- before setting them to PLAYING. This means that it is only alowed to
+ before setting them to PLAYING. This means that it is only allowed to
synchronize on the clock in the PLAYING state.
- - The pipeline uses the clock and the stream time to calculate the base time.
- The base time is distributed to all children when performing the state
+ - The pipeline uses the clock and the running_time to calculate the base_time.
+ The base_time is distributed to all children when performing the state
change.
- - sink elements stop blocking on the preroll buffer or event and start
- rendering the data.
- - sinks can post the EOS message in the PLAYING state. It is not allowed to
+ - Sink elements stop blocking on the preroll buffer or event and start
+ rendering the data.
+ - Sinks can post the EOS message in the PLAYING state. It is not allowed to
post EOS when not in the PLAYING state.
- - while streaming in PAUSED or PLAYING elements can create and remove
- dynamic pads.
- - live sources start generating data and return SUCCESS.
+ - While streaming in PAUSED or PLAYING elements can create and remove
+ sometimes pads.
+ - Live sources start generating data and return SUCCESS.
PLAYING -> PAUSED
- - most elements ignore this state change.
- - The pipeline calculates the stream time based on the last selected clock
- and the base time. It stores this information to continue playback when
+ - Most elements ignore this state change.
+ - The pipeline calculates the running_time based on the last selected clock
+ and the base_time. It stores this information to continue playback when
going back to the PLAYING state.
- - sinks unblock any clock wait calls.
- - sinks return ASYNC from this state change and complete the state change
- when they receive a buffer or an EOS event.
- - any queued EOS messages are removed since they will be reposted when going
- back to the PLAYING state.
- - live sources stop generating data and return NO_PREROLL.
+ - Sinks unblock any clock wait calls.
+ - When a sink does not have a pending buffer to play, it returns ASYNC from
+ this state change and completes the state change when it receives a new
+ buffer or an EOS event.
+ - Any queued EOS messages are removed since they will be reposted when going
+ back to the PLAYING state. The EOS messages are queued in GstBins.
+ - Live sources stop generating data and return NO_PREROLL.
PAUSED -> READY
- - sinks unblock any waits in the preroll.
- - elements unblock any waits on devices
- - the element pads are deactivated so that streaming becomes impossible and
- all streaming threads are stopped.
+ - Sinks unblock any waits in the preroll.
+ - Elements unblock any waits on devices
+ - Chain or get_range functions return WRONG_STATE.
+ - The element pads are deactivated so that streaming becomes impossible and
+ all streaming threads are stopped.
+ - The sink forgets all negotiated formats
+ - Elements remove all sometimes pads
READY -> NULL
- - element removes any dynamically created pads
+ - Elements close devices
+ - Elements reset any internal state.
State variables
----------------
+~~~~~~~~~~~~~~~
-An element has a special lock to manage the state changes. This lock is called
-the STATE_LOCK.
-
-The STATE_LOCK protects 3 element variables:
+An element has 4 state variables that are protected with the object LOCK:
- STATE
- STATE_NEXT
The STATE_NEXT and STATE_PENDING can be VOID_PENDING if the element is in
the right state.
+An element has a special lock to protect against concurrent invocations of
+_set_state(), called the STATE_LOCK.
+
+
Setting state on elements
--------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~
-The state of an element can be changed with _element_set_state(). When chaning
+The state of an element can be changed with _element_set_state(). When changing
the state of an element all intermediate states will also be set on the element
until the final desired state is set.
GST_STATE_SUCCESS: The state change is completed successfully.
GST_STATE_ASYNC: The state change will complete later on. This can happen
- When the element needs a long time to perform the state
+ when the element needs a long time to perform the state
change or for sinks that need to receive the first buffer
before they can complete the state change (preroll).
GST_STATE_NO_PREROLL: The state change is completed successfully but the element
will not be able to produce data in the PAUSED state.
-In the case of an async state change, it is possible to proceed to the next
+In the case of an ASYNC state change, it is possible to proceed to the next
state before the current state change completed, however, the element will only
get to this next state before completing the previous ASYNC state change.
After receiving an ASYNC return value, you can use _element_get_state() to poll
state. Then the state change function of the element is called and the result of
that function is used to update the STATE and STATE_RETURN fields, STATE_NEXT,
STATE_PENDING and STATE_RETURN fields. If the function returned ASYNC, this result
-is immediatly returned to the caller.
+is immediately returned to the caller.
Getting state of elements
--------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~
The _get_state() function takes 3 arguments, two pointers that will hold the
current and pending state and one GstClockTime that holds a timeout value. The
* If the element aborts the ASYNC state change due to an error within the
specified timeout, this function returns FAILURE with the state set to last
- successfull state and pending set to the last attempt. The element should
+ successful state and pending set to the last attempt. The element should
also post an error message on the bus with more information about the problem.
States in GstBin
-----------------
+~~~~~~~~~~~~~~~~
A GstBin manages the state of its children. It does this by propagating the state
changes performed on it to all of its children. The _set_state() function on a
-bin will call the _set_state() function on all of its children.
+bin will call the _set_state() function on all of its children, that are
+not already in the target state or in a change state to the target state.
The children are iterated from the sink elements to the source elements. This makes
sure that when changing the state of an element, the downstream elements are in
If all the children return SUCCESS, the function returns SUCCESS as well.
If one of the children returns FAILURE, the function returns FAILURE as well. In
-this state it is possible that some elements successfuly changed state. The
+this state it is possible that some elements successfully changed state. The
application can check which elements have a changed state, which were in error
and which were not affected by iterating the elements and calling _get_state()
on the elements.
If after calling the state function on all children, one of the children returned
-ASYNC, the function returns ASYNC as well.
+ASYNC, the function returns ASYNC as well.
If after calling the state function on all children, one of the children returned
NO_PREROLL, the function returns NO_PREROLL as well.
+If both NO_PREROLL and ASYNC children are present, NO_PREROLL is returned.
+
The current state of the bin can be retrieved with _get_state().
If the bin is performing an ASYNC state change, it will automatically update its
Implementing states in elements
--------------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
READY
------
+^^^^^
upward state change
--------------------
+~~~~~~~~~~~~~~~~~~~
Upward state changes always return ASYNC either if the STATE_PENDING is
reached or not.
A->B: some elements ASYNC
- no commit state
- listen for commit messages on bus
- - for each commit message, poll elements
+ - for each commit message, poll elements, this happens in another
+ thread.
- if no ASYNC elements, commit state, continue state change
to STATE_PENDING
downward state change
-----------------------
+~~~~~~~~~~~~~~~~~~~~~
Downward state changes only return ASYNC if the final state is ASYNC.
This is to make sure that it's not needed to wait for an element to
Locking overview (element)
---------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~
* Element commiting SUCCESS
- STATE_LOCK is taken in set_state
- change state is called and returns ASYNC
- ASYNC returned to the caller.
- - element takes STATE_LOCK in streaming thread.
+ - element takes LOCK in streaming thread.
- element calls commit_state in streaming thread.
- commit state calls change_state to next state change.
STREAM_LOCK
| ...
STREAM_UNLOCK
-
-
-
-
-
+
+Remarks
+~~~~~~~
+
+set_state cannot be called from multiple threads at the same time. The STATE_LOCK
+prevents this.
+
+state variables are protected with the LOCK.
+
+calling set_state while gst_state is called should unlock the get_state with
+an error. The cookie will do that.
+
+
+ set_state(element)
+
+ STATE_LOCK
+
+ LOCK
+ update current, next, pending state
+ cookie++
+ UNLOCK
+
+ change_state
+
+ STATE_UNLOCK
+
+