The uORB is an asynchronous
subscribe() messaging API used for inter-thread/inter-process communication.
Look at the tutorial to learn how to use it in C++.
uORB is automatically started early on bootup as many applications depend on it.
It is started with
uorb start. Unit tests can be started with
New uORB topics can be added either within the main PX4/Firmware repository, or can be added in an out-of-tree message definitions. For information on adding out-of-tree uORB message definitions, please see this section.
To add a new topic, you need to create a new .msg file in the
msg/ directory and add the file name to the
From this, the needed C/C++ code is automatically generated.
Have a look at the existing
msg files for supported types.
A message can also be used nested in other messages.
To each generated C/C++ struct, a field
uint64_t timestamp will be added.
This is used for the logger, so make sure to fill it in when publishing the message.
To use the topic in the code, include the header:
By adding a line like the following in the
.msg file, a single message definition can be used for multiple independent topics:
# TOPICS mission offboard_mission onboard_mission
Then in the code, use them as topic id:
Publishing a topic can be done from anywhere in the system, including interrupt context (functions called by the
However, advertising a topic is only possible outside of interrupt context.
A topic has to be advertised in the same process as it's later published.
listenercommand is only available on Pixracer (FMUv4) and Linux / OS X.
To list all topics, list the file handles:
To listen to the content of one topic for 5 messages, run the listener:
listener sensor_accel 5
The output is n-times the content of the topic:
TOPIC: sensor_accel #3 timestamp: 84978861 integral_dt: 4044 error_count: 0 x: -1 y: 2 z: 100 x_integral: -0 y_integral: 0 z_integral: 0 temperature: 46 range_m_s2: 78 scaling: 0 TOPIC: sensor_accel #4 timestamp: 85010833 integral_dt: 3980 error_count: 0 x: -1 y: 2 z: 100 x_integral: -0 y_integral: 0 z_integral: 0 temperature: 46 range_m_s2: 78 scaling: 0
On NuttX-based systems (Pixhawk, Pixracer, etc) the
listenercommand can be called from within the QGroundControl MAVLink Console to inspect the values of sensors and other topics. This is a powerful debugging tool because it can be used even when QGC is connected over a wireless link (e.g. when the vehicle is flying). For more information see: Sensor/Topic Debugging.
uorb top shows the publishing frequency of each topic in real-time:
update: 1s, num topics: 77 TOPIC NAME INST #SUB #MSG #LOST #QSIZE actuator_armed 0 6 4 0 1 actuator_controls_0 0 7 242 1044 1 battery_status 0 6 500 2694 1 commander_state 0 1 98 89 1 control_state 0 4 242 433 1 ekf2_innovations 0 1 242 223 1 ekf2_timestamps 0 1 242 23 1 estimator_status 0 3 242 488 1 mc_att_ctrl_status 0 0 242 0 1 sensor_accel 0 1 242 0 1 sensor_accel 1 1 249 43 1 sensor_baro 0 1 42 0 1 sensor_combined 0 6 242 636 1
The columns are: topic name, multi-instance index, number of subscribers, publishing frequency in Hz, number of lost messages per second (for all subscribers combined), and queue size.
uORB provides a mechanism to publish multiple independent instances of the same topic through
It will return an instance index to the publisher.
A subscriber will then have to choose to which instance to subscribe to using
orb_subscribe subscribes to the first instance).
Having multiple instances is useful for example if the system has several sensors of the same type.
Make sure not to mix
orb_advertise for the same topic.
The full API is documented in src/modules/uORB/uORBManager.hpp.