# uORB Messaging

## Introduction

The uORB is an asynchronous `publish()` / `subscribe()` messaging API used for inter-thread/inter-process communication.

uORB is implemented in the [`uorb` module](../modules/modules_communication.md#uorb).
It is started automatically (with `uorb start`) early in the PX4 boot sequence, as many applications depend on it.
Unit tests can be started with `uorb_tests`.

This document explains how to add uORB message definitions and their corresponding topic(s), how to use reference a topic in code, and how to view topics as they change in PX4.
The [First Application Tutorial (Hello Sky)](../modules/hello_sky.md) provides more comprehensive instructions for how to use topics in C++.

## Adding a New Topic

New uORB topics can be added either within the main PX4/PX4-Autopilot repository, or can be added in an [out-of-tree message definition](../advanced/out_of_tree_modules.md#out-of-tree-uorb-message-definitions).

To add new topics, you need to create a new **.msg** "message definition file" named following the CamelCase convention.
The file should be added to the [msg/](https://github.com/PX4/PX4-Autopilot/tree/main/msg/) directory (or [msg/versioned](https://github.com/PX4/PX4-Autopilot/tree/main/msg/versioned) if it needs to be versioned) and then listed in the `msg/CMakeLists.txt` file.

::: tip
Messages need to be versioned if they are exposed to ROS 2 and needs to remain compatible across multiple ROS and PX4 versions.
See [Message Versioning](#message-versioning) for more information.
:::

A message definition file can define one or more _topics_, which all have the same fields and structure.
By default a definition maps to a single topic that is named using a snake_case version of the message definition file name (for example, `TopicName.msg` would define a topic `topic_name`).
You can also specify multiple topics to be created by the message definition, which is useful when you need several topics that have the same fields and structure (see [Multi-Topic Messages](#multi-topic-messages) below).

The section [Message Definitions](#message-definitions) below describes the message format.

From the message definitions, the needed C/C++ code is automatically generated.

To use the topic in the code, first include the generated header, which will be named using the snake_case version of the (CamelCase) message definition file name.
For example, for a message named `VelocityLimits` you would include `velocity_limits.h` as shown:

```cpp
#include <uORB/topics/velocity_limits.h>
```

In code you refer to the topic using its id, which in this example would be: `ORB_ID(velocity_limits)`.

## Message Definitions

The message definition should start with a descriptive _comment_ that outlines its purpose (a comment starts with the `#` symbol and goes to the end of the line).
The message will then define one or more fields, which are defined with a _type_, such as `bool`, `uint8`, and `float32`, followed by a _name_.
By convention, each field is followed by a descriptive _comment_, which is any text from the `#` symbol to the end of the line.

::: warning
All message definitions **must** include the `uint64_t timestamp` field, and this should be filled in when publishing the associated topic(s).
This field is needed in order for the logger to be able to record UORB topics.
:::

::: info
All _versioned_ messages definitions must include the `uint32 MESSAGE_VERSION` field.
For more information, refer to the [Message Versioning](#message-versioning) section.
:::

For example the [VelocityLimits](../msg_docs/VelocityLimits.md) message definition shown below has a descriptive comment, followed by a number of fields, which each have a comment.

```text
# Velocity and yaw rate limits for a multicopter position slow mode only

uint64 timestamp # [us] Time since system start.

# absolute speeds, NAN means use default limit
float32 horizontal_velocity # [m/s] Horizontal velocity.
float32 vertical_velocity # [m/s] Vertical velocity.
float32 yaw_rate # [rad/s] Yaw rate.
```

By default this message definition will be compiled to a single topic with an id `velocity_limits`, a direct conversion from the CamelCase name to a snake_case version.

This is the simplest form of a message.
See the existing [`msg`](../msg_docs/index.md) files for other examples of how messages are defined.

### Multi-Topic Messages

Sometimes it is useful to use the same message definition for multiple topics.
This can be specified at the end of the message using a line prefixed with `# TOPICS `, followed by space-separated topic ids.
For example, the [ActuatorOutputs](../msg_docs/ActuatorOutputs.md) message definition is used to define the topic ids as shown:

```text
# TOPICS actuator_outputs actuator_outputs_sim actuator_outputs_debug
```

### Nested Messages

Message definitions can be nested within other messages to create complex data structures.

To nest a message, simply include the nested message type in the parent message definition. For example, [`PositionSetpoint.msg`](../msg_docs/PositionSetpoint.md) is used as a nested message in the [`PositionSetpointTriplet.msg`](../msg_docs/PositionSetpointTriplet.md) topic message definition.

```text
# Global position setpoint triplet in WGS84 coordinates.
#
# This are the three next waypoints (or just the next two or one).

uint64 timestamp # [us] Time since system start.

PositionSetpoint previous
PositionSetpoint current
PositionSetpoint next
```

### uORB Buffer Length (ORB_QUEUE_LENGTH)

uORB messages have a single-message buffer by default, which may be overwritten if the message publication rate is too high.
In most cases this does not matter: either we are only interested in the latest sample of a topic, such as a sensor value or a setpoint, or losing a few samples is not a particular problem.
For relatively few cases, such as vehicle commands, it is important that we don't drop topics.

In order to reduce the chance that messages will be dropped we can use named constant `ORB_QUEUE_LENGTH` to create a buffer of the specified length.
For example, to create a four-message queue, add the following line to your message definition:

```sh
uint8 ORB_QUEUE_LENGTH = 4
```

As long as subscribers are able to read messages out of the buffer quickly enough than it isn't ever fully filled to the queue length (by publishers), they will be able to get all messages that are sent.
Messages will still be lost they are published when the queue is filled.

Note that the queue length value must be a power of 2 (so 2, 4, 8, ...).

### Message/Field Deprecation {#deprecation}

As there are external tools using uORB messages from log files, such as [Flight Review](https://github.com/PX4/flight_review), certain aspects need to be considered when updating existing messages:

- Changing existing fields or messages that external tools rely on is generally acceptable if there are good reasons for the update.
  In particular for breaking changes to _Flight Review_, _Flight Review_ must be updated before code is merged to `master`.
- In order for external tools to reliably distinguish between two message versions, the following steps must be followed:
  - Removed or renamed messages must be added to the `deprecated_msgs` list in [msg/CMakeLists.txt](https://github.com/PX4/PX4-Autopilot/blob/c5a6a60903455c3600f47e3c45ecaa48614559c8/msg/CMakeLists.txt#L189) and the **.msg** file needs to be deleted.
  - Removed or renamed fields must be commented and marked as deprecated.
    For example `uint8 quat_reset_counter` would become `# DEPRECATED: uint8 quat_reset_counter`.
    This is to ensure that removed fields (or messages) are not re-added in future.
  - In case of a semantic change (e.g. the unit changes from degrees to radians), the field must be renamed as well and the previous one marked as deprecated as above.

## Message Versioning

<Badge type="tip" text="PX4 v1.16" />

Optional message versioning was introduced PX4 v1.16 to make it easier to maintain compatibility between PX4 and ROS 2 versions compiled against different message definitions.
Versioned messages are designed to remain more stable over time compared to their non-versioned counterparts, as they are intended to be used across multiple releases of PX4 and external systems, ensuring greater compatibility over longer periods.

Versioned messages include an additional field `uint32 MESSAGE_VERSION = x`, where `x` corresponds to the current version of the message.

Versioned and non-versioned messages are separated in the file system:

- Non-versioned topic message files and [server service](../ros2/user_guide.md#px4-ros-2-service-servers) message files remain in the [`msg/`](https://github.com/PX4/PX4-Autopilot/tree/main/msg) and [`srv/`](https://github.com/PX4/PX4-Autopilot/tree/main/srv) directories, respectively.
- The current (highest) version of message files are located in the `versioned` subfolders ([`msg/versioned`](https://github.com/PX4/PX4-Autopilot/tree/main/msg/versioned) and [`srv/versioned`](https://github.com/PX4/PX4-Autopilot/tree/main/msg/versioned)).
- Older versions of messages are stored in nested `msg/px4_msgs_old/` subfolders ([`msg/px4_msgs_old/msg/`](https://github.com/PX4/PX4-Autopilot/tree/main/msg/px4_msgs_old/msg) and [`msg/px4_msgs_old/srv/`](https://github.com/PX4/PX4-Autopilot/tree/main/msg/px4_msgs_old)).
  The files are also renamed with a suffix to indicate their version number.

::: tip
The file structure is outlined in more detail in [File structure (ROS 2 Message Translation Node)](../ros2/px4_ros2_msg_translation_node.md#file-structure).
:::

The [ROS 2 Message Translation Node](../ros2/px4_ros2_msg_translation_node.md) uses the above message definitions to seamlessly convert messages sent between PX4 and ROS 2 applications that have been compiled against different message versions.

Updating a versioned message involves more steps compared to updating a non-versioned one.
For more information see [Updating a Versioned Message](../ros2/px4_ros2_msg_translation_node.md#updating-a-versioned-message).

For the full list of versioned and non-versioned messages see: [uORB Message Reference](../msg_docs/index.md).

For more on PX4 and ROS 2 communication, see [PX4-ROS 2 Bridge](../ros/ros2_comm.md).

::: info
ROS 2 plans to natively support message versioning in the future, but this is not implemented yet.
See the related ROS Enhancement Proposal ([REP 2011](https://github.com/ros-infrastructure/rep/pull/358)).
See also this [Foxglove post](https://foxglove.dev/blog/sending-ros2-message-types-over-the-wire) on message hashing and type fetching.
:::

## Publishing

Publishing a topic can be done from anywhere in the system, including interrupt context (functions called by the `hrt_call` API).
However, the topic needs to be advertised and published outside of an interrupt context (at least once) before it can be published in an interrupt context.

### Multi-instance

uORB provides a mechanism to publish multiple independent instances of the _same_ topic.
This is useful, for example, if the system has several sensors of the same type.

::: info
This differs from [Multi-Topic Messages](#multi-topic-messages), where we create different topics that happen to have the same structure.
:::

A publisher can call `orb_advertise_multi` to create a new topic instance and get its instance index.
A subscriber will then have to choose to which instance to subscribe to using `orb_subscribe_multi` (`orb_subscribe` subscribes to the first instance).

Make sure not to mix `orb_advertise_multi` and `orb_advertise` for the same topic!

The full API is documented in [platforms/common/uORB/uORBManager.hpp](https://github.com/PX4/PX4-Autopilot/blob/main/platforms/common/uORB/uORBManager.hpp).

## Listing Topics and Listening in

::: info
The `listener` command available on most boards after FMUv4.
You can check for a particular board by searching for the `CONFIG_SYSTEMCMDS_TOPIC_LISTENER` key in the [kconfig](../hardware/porting_guide_config.md) board configuration (for example, see the FMUv6 [default.px4board](https://github.com/PX4/PX4-Autopilot/blob/release/1.15/boards/px4/fmu-v6x/default.px4board#L100) file).
:::

To list all topics, list the file handles:

```sh
ls /obj
```

To listen to the content of one topic for 5 messages, run the listener:

```sh
listener sensor_accel 5
```

The output is n-times the content of the topic:

```sh
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
```

:::tip
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](../debug/sensor_uorb_topic_debugging.md).
:::

### uorb top Command

The command `uorb top` shows the publishing frequency of each topic in real-time:

```sh
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.

## Plotting Changes in Topics

Topic changes can be plotted in realtime using PlotJuggler and the PX4 ROS 2 integration (note that this actually plots ROS topics that correspond to uORB topics, but the effect is the same).

For more information see: [Plotting uORB Topic Data in Real Time using PlotJuggler](../debug/plotting_realtime_uorb_data.md).

<video src="../../assets/debug/realtime_debugging/realtime_debugging.mp4" width="720" controls></video>

## See Also

- [uORB Documentation Standard](../uorb/uorb_documentation.md)

- _PX4 uORB Explained_ Blog series
  - [Part 1](https://px4.io/px4-uorb-explained-part-1/)
  - [Part 2](https://px4.io/px4-uorb-explained-part-2/)
  - [Part 3 (The deep stuff)](https://px4.io/px4-uorb-explained-part-3-the-deep-stuff/)
