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# 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 focused around the element states.
An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an
element is initially instantiated, it is in the NULL state.
## 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 live sources and sinks. Sinks
accept and render 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.
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 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 (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.
- The pipeline selects a clock and distributes this to all the children
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 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
post EOS when not in the PLAYING state.
- 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 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.
- 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
- Chain or get_range functions return FLUSHING.
- 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*:
- Elements close devices
- Elements reset any internal state.
## State variables
An element has 4 state variables that are protected with the object LOCK:
- *STATE*
- *STATE_NEXT*
- *STATE_PENDING*
- *STATE_RETURN*
The STATE always reflects the current state of the element. The
STATE\_NEXT reflects the next state the element will go to. The
STATE\_PENDING always reflects the required state of the element. The
STATE\_RETURN reflects the last return value of a state change.
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 changing the state of an element all intermediate states will also
be set on the element until the final desired state is set.
The `set\_state()` function can return 3 possible values:
* *GST_STATE_FAILURE*: The state change failed for some reason. The plugin should
have posted an error message on the bus with information.
* *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 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 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 the status of the element. If the
polling returns `SUCCESS`, the element completed the state change to the
last requested state with `set\_state()`.
When setting the state of an element, the STATE\_PENDING is set to the
required 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 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 function returns a GstElementStateReturn.
- If the element returned `SUCCESS` to the previous \_set\_state()
function, this function will return the last state set on the
element and VOID\_PENDING in the pending state value. The function
returns GST\_STATE\_SUCCESS.
- If the element returned NO\_PREROLL to the previous \_set\_state()
function, this function will return the last state set on the
element and VOID\_PENDING in the pending state value. The function
returns GST\_STATE\_NO\_PREROLL.
- If the element returned FAILURE to the previous \_set\_state() call,
this function will return FAILURE with the state set to the current
state of the element and the pending state set to the value used in
the last call of \_set\_state().
- If the element returned `ASYNC` to the previous \_set\_state() call,
this function will wait for the element to complete its state change
up to the amount of time specified in the GstClockTime.
- If the element does not complete the state change in the
specified amount of time, this function will return `ASYNC` with
the state set to the current state and the pending state set to
the pending state.
- If the element completes the state change within the specified
timeout, this function returns the updated state and
VOID\_PENDING as the pending state.
- 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 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, 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 the correct state to process the eventual
buffers. In the case of a downwards state change, the sink elements will
shut down first which makes the upstream elements shut down as well
since the \_push() function returns a GST\_FLOW\_FLUSHING error.
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 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.
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 current state fields when it receives state messages from the
children.
## Implementing states in elements
### READY
## upward state change
Upward state changes always return `ASYNC` either if the STATE\_PENDING is
reached or not.
Element:
* A -> B => `SUCCESS`
- commit state
* A -> B => `ASYNC`
- no commit state
- element commits state `ASYNC`
* A -> B while `ASYNC`
- update STATE_PENDING state
- no commit state
- no change_state called on element
Bin:
* A->B: all elements `SUCCESS`
- commit state
* A->B: some elements `ASYNC`
- no commit state
- listen for commit messages on bus
- 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
complete the preroll or other `ASYNC` state changes when one only wants to
shut down an element.
Element:
A -> B => `SUCCESS`
- commit state
A -> B => `ASYNC` not final state
- commit state on behalf of element
A -> B => `ASYNC` final state
- element will commit `ASYNC`
Bin:
A -> B -> `SUCCESS`
- commit state
A -> B -> `ASYNC` not final state
- commit state on behalf of element, continue state change
A -> B => `ASYNC` final state
- no commit state
- listen for commit messages on bus
- for each commit message, poll elements
- if no `ASYNC` elements, commit state
## Locking overview (element)
- Element committing `SUCCESS`
- STATE\_LOCK is taken in set\_state
- change state is called if `SUCCESS`, commit state is called
- commit state calls change\_state to next state change.
- if final state is reached, stack unwinds and result is returned
to set\_state and
caller.
```
set_state(element) change_state (element) commit_state
| | |
| | |
STATE_LOCK | |
| | |
|------------------------>| |
| | |
| | |
| | (do state change) |
| | |
| | |
| | if `SUCCESS` |
| |---------------------->|
| | | post message
| | |
| |<----------------------| if (!final) change_state (next)
| | | else SIGNAL
| | |
| | |
| | |
|<------------------------| |
| `SUCCESS`
|
STATE_UNLOCK
|
`SUCCESS`
```
- Element committing `ASYNC`
- STATE\_LOCK is taken in set\_state
- change state is called and returns `ASYNC`
- `ASYNC` returned to the caller.
- element takes LOCK in streaming thread.
- element calls commit\_state in streaming thread.
- commit state calls change\_state to next state
change.
```
set_state(element) change_state (element) stream_thread commit_state (element)
| | | |
| | | |
STATE_LOCK | | |
| | | |
|------------------------>| | |
| | | |
| | | |
| | (start_task) | |
| | | |
| | STREAM_LOCK |
| | |... |
|<------------------------| | |
| ASYNC STREAM_UNLOCK |
STATE_UNLOCK | |
| .....sync........ STATE_LOCK |
ASYNC |----------------->|
| |
| |---> post_message()
| |---> if (!final) change_state (next)
| | else SIGNAL
|<-----------------|
STATE_UNLOCK
|
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.
``` c
set_state(element)
STATE_LOCK
LOCK
update current, next, pending state
cookie++
UNLOCK
change_state
STATE_UNLOCK
```
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