GStreamer Daemon - Receiving Messages from the Bus

A GStreamer bus takes care of forwarding messages from a pipeline. A bus is needed so applications do not need to be thread-aware, even though the underlying pipeline is heavily threaded. For GStreamer Daemon, the concept of a bus is not important, but we use the terminology to maintain consistency with GStreamer. What is important is a pipeline can generate messages and GStreamer Daemon allows those messages to flow back to the client application.

This wiki describes the basics of how to interact with GStreamer bus properties. Specifically, how to receive messages from the bus. You'll find that the family of commands used to interact with pipelines are prefixed with bus_<action>.

Read bus

In order to read a bus use the following command:

bus_read <name> 
       Read the bus of the pipeline.

Read and filter bus

Sometimes is important to read and filter some messages from the bus, Gstd has the capability to read and filter (ie:error+warning+eos), In order to read and filter use the following command:

bus_filter <name> <filter> 
       Read and filter the bus of the pipeline.

Bus timeout

This command will block while waiting for messages set with the bus_filter command. You can specify a maximum time with the timeout parameter, the time argument is set on nanoseconds.

bus_timeout <name> <time in nanoseconds> 
       bus timeout of the pipeline.

Messages

There are lots of bus messages that are defined. The documentation tends to lag the implementation, so if you want the latest, most complete list of support bus messages, look at the gstd_msg_type_get_type table in gstd_msg_type.c.

The following table lists the supported bus messages. You can use either a hyphen or underscore in the bus message name.

Bus Message	Version	Meaning
eos	all	After the pipeline sink element finishes processing EOS, the bus reports the event to the application.
error	all	An element in the pipeline is in the error state.
warning	all	An element in the pipeline encountered a recoverable error.
info	all	An element in the pipeline produced some information.
tag	all	An element in the pipeline decoded metadata about the stream.
buffering	all	An element in the pipeline is buffering would add a delay until the buffer is full.
clock_lost	all	The pipeline clock is unusable.
new_clock	all	A new clock was selected for the pipeline.
structure_change	all	The buffer flow through the pipeline changed.
stream_status	all	The pipeline started, stopped, or paused.
element	all	Element specific message.
segment_start	all	New stream segment being processed.
segment_done	all	Segment seek requested and segment playback completed.
duration_changed	all	Stream duration changed.
latency	all	Pipeline latency changed.
async_start	all	Pipeline sink element changed its asynchronous processing behavior.
async_done	all	Pipeline sink element received first stream buffer after changing its asynchronous processing behavior.
request_state	all	Element in pipeline wants to change its state.
step_start	all	Pipeline stepping operation started.
qos	all	A buffer in the pipeline was dropped or an element changed its processing strategy for Quality of Service reasons.
progress	all	A pipeline element has made progress processing an asynchronous operation.
toc	all	Pipeline encountered a new table of contents.
reset_time	all
stream_start	all	Pipeline processing new stream.
extended	minor > 4
device_added	minor > 4
device_removed	minor > 4
property_notify	minor > 10
stream_collection	minor > 10
streams_selected	minor > 10
redirect	minor > 10

Examples Application

A very useful application using a bus filter is when is needed to know if there is an error or EOS. These examples show how to use the bus filter and bus read.

Error bus filter

This example only filter errors with an infinite timeout.

⇒ Gstd Commands:

# Create the pipeline that generate an error  
pipeline_create p filesrc location=/tmp/test.avi ! identity error-after=2000 ! avidemux ! avdec_mpeg4 ! fpsdisplaysink 

# Filter only a message error
bus_filter p error

# Play the pipeline
pipeline_play p

# Waiting until bus read a message error 
bus_read p

Error bus filter and timeout

This example only filter errors with a 100s of timeout. If did get an error message it returns

⇒ Gstd Commands:

# Create the pipeline that generate an error  
pipeline_create p filesrc location=/tmp/test.avi ! identity error-after=2000 ! avidemux ! avdec_mpeg4 ! fpsdisplaysink 

# Filter only a message error
bus_filter p error

# wait 100s to read message error, if not, returns 
bus_timeout p 100000000000 

# Play the pipeline
pipeline_play p

# Waiting until bus read a message error 
bus_read p

Error+EOS bus filter

This example filter errors and eos messages with an infinite timeout.

⇒ Gstd Commands:

# Create the pipeline that generate an eos  
pipeline_create p filesrc location=/tmp/test.avi  ! avidemux ! avdec_mpeg4 ! fpsdisplaysink 

# Filter a message error and EOS message
bus_filter p error+eos

# Play the pipeline
pipeline_play p

# Waiting until bus read a message error or eos message
bus_read p

⇒ Example Gstd bus_read Response:


{
  "code" : 0,
  "description" : "Success",
  "response" : {
    "type" : "error",
    "source" : "avidemux2",
    "timestamp" : "99:99:99.999999999",
    "seqnum" : 104,
    "message" : "Internal data stream error.",
    "debug" : "gstavidemux.c(5713): gst_avi_demux_loop (): /GstPipeline:p/GstAviDemux:avidemux2:\nstreaming stopped, reason not-linked"
  }
}

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