GStreamer Daemon - Receiving Messages from the Bus

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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 on how to interact with GStreamer bus properties. Specifically, how to receiving 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 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
error	all
warning	all
info	all
tag	all
buffering	all
state_changed	all
state_dirty	all
step_done	all
clock_provide	all
clock_lost	all
new_clock	all
structure_change	all
stream_status	all
application	all
element	all
segment_start	all
segment_done	all
duration_changed	all
latency	all
async_start	all
async_done	all
request_state	all
step_start	all
qos	all
progress	all
toc	all
reset_time	all
stream_start	all
need_context	minor > 2
have_context	minor > 2
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 known 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:

 1 # Create the pipeline that generate an error  
 2 pipeline_create p filesrc location=/tmp/test.avi ! identity error-after=2000 ! avidemux ! avdec_mpeg4 ! fpsdisplaysink 
 3 
 4 # Filter only a message error
 5 bus_filter p error
 6 
 7 # Play the pipeline
 8 pipeline_play p
 9 
10 # Waiting until bus read a message error 
11 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:

 1 # Create the pipeline that generate an error  
 2 pipeline_create p filesrc location=/tmp/test.avi ! identity error-after=2000 ! avidemux ! avdec_mpeg4 ! fpsdisplaysink 
 3 
 4 # Filter only a message error
 5 bus_filter p error
 6 
 7 # wait 100s to read message error, if not, returns 
 8 bus_timeout p 100000000000 
 9 
10 # Play the pipeline
11 pipeline_play p
12 
13 # Waiting until bus read a message error 
14 bus_read p

Error+EOS bus filter

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

⇒ Gstd Commands:

 1 # Create the pipeline that generate an eos  
 2 pipeline_create p filesrc location=/tmp/test.avi  ! avidemux ! avdec_mpeg4 ! fpsdisplaysink 
 3 
 4 # Filter a message error and EOS message
 5 bus_filter p error+eos
 6 
 7 # Play the pipeline
 8 pipeline_play p
 9 
10 # Waiting until bus read a message error or eos message
11 bus_read p

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