mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-11 19:06:33 +00:00
116 lines
5 KiB
Markdown
116 lines
5 KiB
Markdown
# Messages
|
|
|
|
Messages are refcounted lightweight objects to signal the application of
|
|
pipeline events.
|
|
|
|
Messages are implemented as a subclass of `GstMiniObject` with a generic
|
|
`GstStructure` as the content. This allows for writing custom messages
|
|
without requiring an API change while allowing a wide range of different
|
|
types of messages.
|
|
|
|
Messages are posted by objects in the pipeline and are passed to the
|
|
application using the `GstBus` (See also [gstbus](design/gstbus.md)
|
|
and [gstpipeline](design/gstpipeline.md)).
|
|
|
|
## Message types
|
|
|
|
**`GST_MESSAGE_EOS`**: Posted by sink elements. This message is posted to the
|
|
application when all the sinks in a pipeline have posted an EOS message. When
|
|
performing a flushing seek, the EOS state of the pipeline and sinks is reset.
|
|
|
|
**`GST_MESSAGE_ERROR`**: An element in the pipeline got into an error state.
|
|
The message carries a GError and a debug string describing the error. This
|
|
usually means that part of the pipeline is not streaming anymore.
|
|
|
|
**`GST_MESSAGE_WARNING`**: An element in the pipeline encountered a condition
|
|
that made it produce a warning. This could be a recoverable decoding error or
|
|
some other non fatal event. The pipeline continues streaming after a warning.
|
|
|
|
**`GST_MESSAGE_INFO`**: An element produced an informational message.
|
|
|
|
**`GST_MESSAGE_TAG`**: An element decoded metadata about the stream. The
|
|
message carries a `GstTagList` with the tag information.
|
|
|
|
**`GST_MESSAGE_BUFFERING`**: An element is buffering data and that could
|
|
potentially take some time. This message is typically emitted by elements that
|
|
perform some sort of network buffering. While the pipeline is buffering it
|
|
should remain in the `PAUSED` state. When the buffering is finished, it can
|
|
resume `PLAYING`.
|
|
|
|
**`GST_MESSAGE_STATE_CHANGED`**: An element changed state in the pipeline.
|
|
The message carries the old, new and pending state of the element.
|
|
|
|
**`GST_MESSAGE_STATE_DIRTY`**: An internal message used to instruct
|
|
a pipeline hierarchy that a state recalculation must be performed because of an
|
|
ASYNC state change completed. This message is not used anymore.
|
|
|
|
**`GST_MESSAGE_STEP_DONE`**: An element stepping frames has finished. This is
|
|
currently not used.
|
|
|
|
**`GST_MESSAGE_CLOCK_PROVIDE`**: An element notifies its capability of
|
|
providing a clock for the pipeline.
|
|
|
|
**`GST_MESSAGE_CLOCK_LOST`**: The current clock, as selected by the pipeline,
|
|
became unusable. The pipeline will select a new clock on the next PLAYING state
|
|
change.
|
|
|
|
**`GST_MESSAGE_NEW_CLOCK`**: A new clock was selected for the pipeline.
|
|
|
|
**`GST_MESSAGE_STRUCTURE_CHANGE`**: The pipeline changed its structure, This
|
|
means elements were added or removed or pads were linked or unlinked. This
|
|
message is not yet used.
|
|
|
|
**`GST_MESSAGE_STREAM_STATUS`**: Posted by an element when it
|
|
starts/stops/pauses a streaming task. It contains information about the reason
|
|
why the stream state changed along with the thread id. The application can use
|
|
this information to detect failures in streaming threads and/or to adjust
|
|
streaming thread priorities.
|
|
|
|
**`GST_MESSAGE_APPLICATION`**: The application posted a message. This message
|
|
must be used when the application posts a message on the bus.
|
|
|
|
**`GST_MESSAGE_ELEMENT`**: Element-specific message. See the specific
|
|
element's documentation
|
|
|
|
**`GST_MESSAGE_SEGMENT_START`**: An element started playback of a new
|
|
segment. This message is not forwarded to applications but is used internally
|
|
to schedule `SEGMENT_DONE` messages.
|
|
|
|
**`GST_MESSAGE_SEGMENT_DONE`**: An element or bin completed playback of
|
|
a segment. This message is only posted on the bus if a `SEGMENT` seek is
|
|
performed on a pipeline.
|
|
|
|
**`GST_MESSAGE_DURATION_CHANGED`**: An element posts this message when it has
|
|
detected or updated the stream duration.
|
|
|
|
**`GST_MESSAGE_ASYNC_START`**: Posted by sinks when they start an
|
|
asynchronous state change.
|
|
|
|
**`GST_MESSAGE_ASYNC_DONE`**: Posted by sinks when they receive the first
|
|
data buffer and complete the asynchronous state change.
|
|
|
|
**`GST_MESSAGE_LATENCY`**: Posted by elements when the latency in a pipeline
|
|
changed and a new global latency should be calculated by the pipeline or
|
|
application.
|
|
|
|
**`GST_MESSAGE_REQUEST_STATE`**: Posted by elements when they want to change
|
|
the state of the pipeline they are in. A typical use case would be an audio
|
|
sink that requests the pipeline to pause in order to play a higher priority
|
|
stream.
|
|
|
|
**`GST_MESSAGE_STEP_START`**: A stepping operation has started.
|
|
|
|
**`GST_MESSAGE_QOS`**: A buffer was dropped or an element changed its
|
|
processing strategy for Quality of Service reasons.
|
|
|
|
**`GST_MESSAGE_PROGRESS`**: A progress message was posted. Progress messages
|
|
inform the application about the state of asynchronous operations.
|
|
|
|
**`GST_MESSAGE_TOC`**: A new table of contents (TOC) was found or a previously
|
|
found one was updated.
|
|
|
|
**`GST_MESSAGE_STREAM_START`**: Signals the start of a new stream.
|
|
|
|
**`GST_MESSAGE_NEED_CONTEXT`**: Signals that an element wants a specific context (Since 1.2)
|
|
|
|
**`GST_MESSAGE_HAVE_CONTEXT`**: Signals that an element created a context (Since 1.2)
|