gstreamer/docs/design/part-messages.txt

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Messages
--------
Messages are refcounted lightweight objects to signal the application
of pipeline events.
Messages are implemented as a subclass of GstData 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.
Message types
-------------
GST_MESSAGE_EOS:
The pipeline went to EOS. This means that all the sink elements in the
pipeline posted the EOS message to the bus.
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.
GST_MESSAGE_STATE_CHANGED:
An element changed state in the pipeline. The message carries the old an new
state of the element.
GST_MESSAGE_STEP_DONE:
An element stepping frames has finished.
GST_MESSAGE_NEW_CLOCK:
A new clock was selected for the pipeline.
GST_MESSAGE_STRUCTURE_CHANGE:
The pipeline changed of structure, This means elements were added or removed or
pads were linked or unlinked.
GST_MESSAGE_STREAM_STATUS:
An element posted information about the stream it is handling. This could include
information about the length of the stream.
GST_MESSAGE_APPLICATION:
An element posted an application specific message.