mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-10-04 09:42:19 +00:00
43 lines
2.1 KiB
Markdown
43 lines
2.1 KiB
Markdown
# push-pull
|
|
|
|
Normally a source element will push data to the downstream element using
|
|
the `gst_pad_push()` method. The downstream peer pad will receive the
|
|
buffer in the Chain function. In the push mode, the source element is
|
|
the driving force in the pipeline as it initiates data transport.
|
|
|
|
It is also possible for an element to pull data from an upstream
|
|
element. The downstream element does this by calling
|
|
`gst_pad_pull_range()` on one of its sinkpads. In this mode, the
|
|
downstream element is the driving force in the pipeline as it initiates
|
|
data transfer.
|
|
|
|
It is important that the elements are in the correct state to handle a
|
|
`push()` or a `pull_range()` from the peer element. For `push()` based
|
|
elements this means that all downstream elements should be in the
|
|
correct state and for `pull_range()` based elements this means the
|
|
upstream elements should be in the correct state.
|
|
|
|
Most sinkpads implement a chain function. This is the most common case.
|
|
sinkpads implementing a loop function will be the exception. Likewise,
|
|
srcpads implementing a getrange function will be the exception.
|
|
|
|
## State changes
|
|
|
|
The `GstBin` sets the state of all the sink elements. These are the
|
|
elements without source pads.
|
|
|
|
Setting the state on an element will first activate all the srcpads and
|
|
then the sinkpads. For each of the sinkpads,
|
|
`gst_pad_check_pull_range()` is performed. If the sinkpad supports a
|
|
loopfunction and the peer pad returns TRUE from the `GstPadCheckPullRange`
|
|
function, then the peer pad is activated first as it must be in the
|
|
right state to handle a `_pull_range()`. Note that the state change of
|
|
the element is not yet performed, just the activate function is called
|
|
on the source pad. This means that elements that implement a getrange
|
|
function must be prepared to get their activate function called before
|
|
their state change function.
|
|
|
|
Elements that have multiple sinkpads that require all of them to operate
|
|
in the same mode (push/pull) can use the `_check_pull_range()` on all
|
|
their pads and can then remove the loop functions if one of the pads
|
|
does not support pull based mode.
|