uORB Messaging

Introduction

The uORB is an asynchronous publish() / subcribe() 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 test.

Adding a new topic

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, there will automatically be C/C++ code 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 logging 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 topic instances:

# TOPICS mission offboard_mission onboard_mission

Then in the code, use them as topic id: ORB_ID(offboard_mission).

Publishing

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 its later published.

Listing Topics and Listening in

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