GStreamer has a few pre-defined message types that can be passed over the bus. The messages are extensible, however. Plug-ins can define additional messages, and applications can decide to either have specific code for those or ignore them. All applications are strongly recommended to at least handle error messages by providing visual feedback to the user.
All messages have a message source, type and timestamp. The message source can be used to see which element emitted the message. For some messages, for example, only the ones emitted by the top-level pipeline will be interesting to most applications (e.g. for state-change notifications). Below is a list of all messages and a short explanation of what they do and how to parse message-specific content.
Error, warning and information notifications: those are used
by elements if a message should be shown to the user about the
state of the pipeline. Error messages are fatal and terminate
the data-passing. The error should be repaired to resume pipeline
activity. Warnings are not fatal, but imply a problem nevertheless.
Information messages are for non-problem notifications. All those
messages contain a
GError with the main
error type and message, and optionally a debug string. Both
can be extracted using
_parse_warning () and
_parse_info (). Both error and debug strings
should be freed after use.
End-of-stream notification: this is emitted when the stream has ended. The state of the pipeline will not change, but further media handling will stall. Applications can use this to skip to the next song in their playlist. After end-of-stream, it is also possible to seek back in the stream. Playback will then continue automatically. This message has no specific arguments.
Tags: emitted when metadata was found in the stream. This can be
emitted multiple times for a pipeline (e.g. once for descriptive
metadata such as artist name or song title, and another one for
stream-information, such as samplerate and bitrate). Applications
should cache metadata internally.
() should be used to parse the taglist, which should
gst_tag_list_unref ()'ed when no longer
State-changes: emitted after a successful state change.
gst_message_parse_state_changed () can be
used to parse the old and new state of this transition.
Buffering: emitted during caching of network-streams. One can
manually extract the progress (in percent) from the message by
extracting the “buffer-percent” property from the
structure returned by
(). See also Chapter 15, Buffering.
Element messages: these are special messages that are unique to certain elements and usually represent additional features. The element's documentation should mention in detail which element messages a particular element may send. As an example, the 'qtdemux' QuickTime demuxer element may send a 'redirect' element message on certain occasions if the stream contains a redirect instruction.
Application-specific messages: any information on those can be extracted by getting the message structure (see above) and reading its fields. Usually these messages can safely be ignored.
Application messages are primarily meant for internal use in applications in case the application needs to marshal information from some thread into the main thread. This is particularly useful when the application is making use of element signals (as those signals will be emitted in the context of the streaming thread).