The uORB is an asynchronous
subscribe() messaging API used for
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
directory and add the file name to the
msg/CMakeLists.txt list. 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
hrt_call API). 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 (all subscribers combined), and queue size.
uORB provides a mechanism to publish multiple independent instances of the same
orb_advertise_multi. It will return an instance index to the
publisher. A subscriber will then have to choose to which instance to subscribe
orb_subscribe subscribes to the first
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.
The following explains some common pitfalls and corner cases:
- The topic is not published: make sure the
ORB_ID()'s of each call match. It is also important that
orb_unsubscribeare called from the same task as
orb_copy. This applies to
px4_task_spawn_cmd(), but also when using work queues (
- Make sure to clean up: use
- A successful
px4_poll()call requires an
orb_copy(), otherwise the next poll will return immediately.
- It is perfectly ok to call
orb_subscribebefore anyone advertised the topic.
px4_poll()will only return true for publications that are done after
orb_subscribe(). This is important for topics that are not published regularly. If a subscriber needs the previous data, it should just do an unconditional
orb_copy()will fail if there is no advertiser yet).