rclc_examples

General information about this repository, including legal information, build instructions and known issues/limitations, are given in README.md in the repository root.

The rclc_examples package

The rclc_examples package provides examples for using the RCLC-Exector and convenience functions.

• example_executor.c provides the example for the RCLC-Executor. It creates one publisher and one subscriber and configures the RCLC-Executor accordingly. Then the spin_some() function is demonstrated.
• example_executor_convenience.c provides the example for the RCLC-Executor with the convenience functions from rclc. It creates one publisher and one subscriber and configures the RCLC-Executor accordingly. Then the spin_some() function is demonstrated.
• example_executor_trigger.c demonstrates the trigger condition of the RCLC-Executor.
• example_service_node.c implements a service node with the RCLC-Executor.
• example_client_node.c implements a client node with RCLC-Executor.

The reduction of code lines for configuring the necessary RCL objects for RCLC-Executor directly with RCL objects compared to using the convenience functions is about 24%:

• example_executor.c: 92 LoC (lines 56-148)
• example_executor_convenience.c: 70 LoC (line 17 + lines 57-126)

counting only the lines of code in which the RCL objects are defined).

Example RCLC-Executor using RCL objects directly

Step 1 Setup ROS 2 Workspace

Open a terminal with ROS 2 workspace. Assuming that the ROS 2 installation resides in /opt/ros/ROSDISTRO, setup the ROS2 environment by:

~$ source /opt/ros/$ROSDISTRO/setup.bash

Step 2 Build the package Download and build the the packages rclc and rclc_examples in a workspace (for example ros2_ws). Then source the workspace:

~/ros2_ws/$ colcon build --packages-up-to rclc_examples
~/ros2_ws/$ source ./install/local_setup.bash

It should build these packages:

• rcl_yaml_param_parser
• rcl
• rclc
• rclc_examples

Step 3 Run the example executor.

The binary of the example is example_executor.

~/ros2_ws/$ ros2 run rclc_examples  example_executor

The publisher publishes the message Hello World!in topic_0 at a rate of 1Hz and the subscriber prints out in the callback Callback: I heard: Hello World!.

You should see the following output:

Created timer with timeout 1000 ms.
Created subscriber topic_0:
Debug: number of DDS handles: 2
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!

Example RCLC-Executor with convenience functions

Step 1 Setup ROS 2 Workspace

Open a terminal with ROS 2 workspace. Assuming that the ROS 2 installation resides in /opt/ros/eloquent, setup the ROS2 environment by:

~$ source /opt/ros/eloquent/setup.bash

Step 2 Build the package Download and build the the packages rclc and rclc_examples in a workspace (for example ros2_ws). Then source the workspace:

~/ros2_ws/$ colcon build --packages-up-to rclc_examples
~/ros2_ws/$ source ./install/local_setup.bash

It should build these packages:

• rcl_yaml_param_parser
• rcl
• rclc
• rclc_examples

Step 3 Run the example executor with the convenience functions from the package rclc.

The binary of the example is example_executor_convenience.

~/ros2_ws/$ ros2 run rclc_examples  example_executor_convenience

The same setup as in the example_executor, just using the RCLC convenience functions. You should see the exact same output:

Created timer with timeout 1000 ms.
Created subscriber topic_0:
Debug: number of DDS handles: 2
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!
Published message Hello World!
Callback: I heard: Hello World!

Example RCLC-Executor with trigger function

Step 1, Step 2 To setup ROS2 workspace and build the package refer to Step 1 and Step 2 in the Example RCLC-Executor.

Step 3 This example implements two RCLC Executors, one for publishing executor_puband one for subscribing messages executor_sub.

The Executor executor_pub publishes string topic_0 every 100ms (using a timer with 100ms) and an integer topic_1 every 1000ms (using a timer with 1000ms).

With the trigger condition rclc_executor_trigger_any this executor publishes whenenver any timer is ready.

Executor executor_sub has two subscriptions, my_string_sub and my_int_sub subscribing to topic_0 and topic_1, respectivly.

With the trigger condition rclc_executor_trigger_all this executor starts evaluating the callbacks only when both messages have arrived. To make this clearly visible, we set the quality of service parameter of the length of the DDS-queue to 0 for subscription my_string_sub, which subscribes to topic_0 with the higher rate.

my_subscription_options.qos.depth = 0
rc = rcl_subscription_init(
  &my_string_sub,
  &my_node,
  my_type_support,
  topic_name,
  &my_subscription_options);

Consequently, the messages "in between" are lost.

The binary of the example is example_executor_trigger. You run the example with:

~/ros2_ws/$ ros2 run rclc_examples  example_executor_trigger

Then you should see the following output:

Created timer 'my_string_timer' with timeout 100 ms.
Created 'my_int_timer' with timeout 1000 ms.
Created subscriber topic_0:
Created subscriber topic_1:
Executor_pub: number of DDS handles: 2
Executor_sub: number of DDS handles: 2
Published: Hello World! 0
Published: Hello World! 1
Published: Hello World! 2
Published: Hello World! 3
Published: Hello World! 4
Published: Hello World! 5
Published: Hello World! 6
Published: Hello World! 7
Published: Hello World! 8
Published: Hello World! 9
Published: 0
Callback 1: Hello World! 9  <---
Callback 2: 0               <---
Published: Hello World! 10
Published: Hello World! 11
Published: Hello World! 12
Published: Hello World! 13
Published: Hello World! 14
Published: Hello World! 15
Published: Hello World! 16
Published: Hello World! 17
Published: Hello World! 18
Published: Hello World! 19
Published: 1
Callback 1: Hello World! 19 <---
Callback 2: 1               <---

The results show, that the callbacks are triggered together, only when the integer message topic_1 was published and received. At that moment the current string message of the topic_0 is processed as well.

Example Service/client with RCLC-Executor

Step 1, Step 2 To setup ROS2 workspace and build the package refer to Step 1 and Step 2 in the Example RCLC-Executor.

Step 3 Open two Terminal windows and source the ROS 2 distribution/install/setup.bash and rclc repository/install/local_setup.bash.

window 1: start service node

$ ros2 run rclc_examples example_service_node
INFO: rcl_wait timeout 10 ms
Service request value: 24 + 42. Seq 1
Received service response 24 + 42 = 66. Seq 1
```C

window 2: start client node
```C
~$ ros2 run rclc_examples example_client_node
Send service request 24 + 42. Seq 1
INFO: rcl_wait timeout 10 ms
```C

A request message is sent from the client node to the service node and answered.