uORB Messaging


The uORB is an asynchronous publish() / 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 uorb_tests.

Adding a new topic

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 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:

#include <uORB/topics/topic_name.h>

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: ORB_ID(offboard_mission).


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.

Listing Topics and Listening in

The listener command is only available on Pixracer (FMUv4) and Linux / OS X.

To list all topics, list the file handles:

ls /obj

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 listener command 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 Command

The command 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 orb_advertise_multi. 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_multi (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_multi and orb_advertise for the same topic.

The full API is documented in src/modules/uORB/uORBManager.hpp.

Troubleshooting and common Pitfalls

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_subscribe and orb_unsubscribe are called from the same task as orb_check and orb_copy. This applies to px4_task_spawn_cmd(), but also when using work queues (work_queue()).
  • Make sure to clean up: use orb_unsubscribe and orb_unadvertise.
  • A successful orb_check() or px4_poll() call requires an orb_copy(), otherwise the next poll will return immediately.
  • It is perfectly ok to call orb_subscribe before anyone advertised the topic.
  • orb_check() and 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() right after orb_subscribe() (note that orb_copy() will fail if there is no advertiser yet).

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