6.3.1. Nodes

ROS 2 nodes are the main participants on ROS 2 ecosystem. They will communicate between each other using publishers, subscriptions, services, etc. Further information about ROS 2 nodes can be found on Understanding nodes.

6.3.1.1. Initialization

Before a node is created, a rclc_support_t object needs to be created and initialized. The rclc_support_init method will handle micro-ROS initial configuration (memory initialization, transport configuration, …).

Note

The rclc_support_init function will try to establish communication with the Agent and will fail if its not reachable on a configurable time. A good practice is to the ping the Agent to check the connection before calling this method.

• Initialize micro-ROS and create a node with default configuration:

  // Get configured allocator
  rcl_allocator_t allocator = rcl_get_default_allocator();
  
  // Initialize support object
  rclc_support_t support;
  rclc_support_init(&support, argc, argv, &allocator);
  
  // Node name
  const char * node_name = "test_node";
  
  // Node namespace (Can remain empty: "")
  const char * namespace = "test_namespace";
  
  // Init default node
  rcl_node_t node;
  rclc_node_init_default(&node, node_name, namespace, &support);
  

• Configure the node Domain ID:

  ROS2 allows isolation of nodes through the ROS_DOMAIN_ID concept. Nodes can only discover and communicate nodes placed on the same domain. Further information about ROS 2 Domain ID can be found on About Domain ID.

  A node can be configured using the rclc_support_t internal rcl_init_options_t options structure. rcl provides the rcl_init_options_set_domain_id method to set a node domain. Example:

  // Get configured allocator
  rcl_allocator_t allocator = rcl_get_default_allocator();
  
  // Initialize and modify options (Set DOMAIN ID to 10)
  rcl_init_options_t init_options = rcl_get_zero_initialized_init_options();
  rcl_init_options_init(&init_options, allocator);
  rcl_init_options_set_domain_id(&init_options, 10);
  
  // Initialize rclc support object with custom options
  rclc_support_t support;
  rclc_support_init_with_options(&support, 0, NULL, &init_options, &allocator);
  
  // Node name
  const char * node_name = "test_node";
  
  // Node namespace (Can remain empty "")
  const char * namespace = "test_namespace";
  
  // Init node with configured support object
  rcl_node_t node;
  rclc_node_init_default(&node, node_name, namespace, &support);
  

  Note

  The configuration of the node will also be applied to its future elements (publishers, subscribers, services, …).

6.3.1.2. Cleaning Up

All entities owned by a node (publishers, subscribers, services, …) have to be destroyed before the node itself:

// Destroy created entities (Example)
rcl_publisher_fini(&publisher, &node);

// Destroy the node
rcl_node_fini(&node);

This will delete the node from ROS2 graph, including any generated infrastructure on the agent (if possible) and used memory on the client.