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Micro-ROS, RCL, & RCLC

Posted Feb 1, 2025
By Bhaswanth Ayapilla
6 min read

Micro-ROS Publisher

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#include <micro_ros_arduino.h> 
#include <stdio.h>
#include <rcl/rcl.h>
#include <rcl/error_handling.h>
#include <rclc/rclc.h>
#include <rclc/executor.h>

#include <std_msgs/msg/int32.h>

rcl_publisher_t publisher;
std_msgs__msg__Int32 msg;
rclc_executor_t executor;
rclc_support_t support;
rcl_allocator_t allocator;
rcl_node_t node;
rcl_timer_t timer;

#define LED_PIN 13

#define RCCHECK(fn) { rcl_ret_t temp_rc = fn; if((temp_rc != RCL_RET_OK)){error_loop();}}
#define RCSOFTCHECK(fn) { rcl_ret_t temp_rc = fn; if((temp_rc != RCL_RET_OK)){}}

void error_loop(){
  while(1){
    digitalWrite(LED_PIN, !digitalRead(LED_PIN));
    delay(100);
  }
}

void timer_callback(rcl_timer_t * timer, int64_t last_call_time)
{  
  RCLC_UNUSED(last_call_time);
  if (timer != NULL) {
    RCSOFTCHECK(rcl_publish(&publisher, &msg, NULL));
    msg.data++;
  }
}

void setup() {
  set_microros_transports();
  
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, HIGH);  
  
  delay(2000);

  allocator = rcl_get_default_allocator();

  //create init_options
  RCCHECK(rclc_support_init(&support, 0, NULL, &allocator));

  // create node
  RCCHECK(rclc_node_init_default(&node, "micro_ros_arduino_node", "", &support));

  // create publisher
  RCCHECK(rclc_publisher_init_default(
    &publisher,
    &node,
    ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Int32),
    "micro_ros_arduino_node_publisher"));

  // create timer,
  const unsigned int timer_timeout = 1000;
  RCCHECK(rclc_timer_init_default(
    &timer,
    &support,
    RCL_MS_TO_NS(timer_timeout),
    timer_callback));

  // create executor
  RCCHECK(rclc_executor_init(&executor, &support.context, 1, &allocator));
  RCCHECK(rclc_executor_add_timer(&executor, &timer));

  msg.data = 0;
}

void loop() {
  delay(100);
  RCSOFTCHECK(rclc_executor_spin_some(&executor, RCL_MS_TO_NS(100)));
}

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#include <micro_ros_arduino.h>
#include <rcl/rcl.h>
#include <rcl/error_handling.h>
#include <rclc/rclc.h>
#include <rclc/executor.h>
#include <std_msgs/msg/int32.h>
  • micro_ros_arduino.h: Includes Micro-ROS Arduino-specific functions.
  • rcl/rcl.h and rclc: Provides core ROS client library (RCL) and RCLC utilities.
  • std_msgs/msg/int32.h: Includes the ROS std_msgs::msg::Int32 message type for publishing integers.
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rcl_publisher_t publisher;
std_msgs__msg__Int32 msg;
rclc_executor_t executor;
rclc_support_t support;
rcl_allocator_t allocator;
rcl_node_t node;
rcl_timer_t timer;
  • executor: Manages callbacks (like timers).
  • support: Manages initialization options for Micro-ROS.
  • allocator: Allocates memory for the Micro-ROS framework.
  • node: A Micro-ROS node (basic ROS entity that performs tasks).
  • timer: Periodically triggers the timer_callback.
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#define RCCHECK(fn) { rcl_ret_t temp_rc = fn; if((temp_rc != RCL_RET_OK)){error_loop();}}
#define RCSOFTCHECK(fn) { rcl_ret_t temp_rc = fn; if((temp_rc != RCL_RET_OK)){}}
  • RCCHECK: Ensures critical operations succeed. If an error occurs, it enters the error_loop.
  • RCSOFTCHECK: Similar, but doesn’t halt execution on failure.
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void error_loop(){
  while(1){
    digitalWrite(LED_PIN, !digitalRead(LED_PIN));
    delay(100);
  }
}
  • Blinks the onboard LED continuously to indicate an error.
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void timer_callback(rcl_timer_t * timer, int64_t last_call_time)
{  
  RCLC_UNUSED(last_call_time);
  if (timer != NULL) {
    RCSOFTCHECK(rcl_publish(&publisher, &msg, NULL));
    msg.data++;
  }
}
  • This function is called each time the timer triggers.
  • Increments msg.data and publishes it to the topic.
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void setup() {
  set_microros_transports();  // Initialize communication (e.g., Serial, WiFi, etc.)

  pinMode(LED_PIN, OUTPUT);   // Set LED_PIN as output.
  digitalWrite(LED_PIN, HIGH);  

  delay(2000);  // Wait for 2 seconds for initialization.

  allocator = rcl_get_default_allocator();  // Set up the memory allocator.

  // Initialize Micro-ROS support and components
  RCCHECK(rclc_support_init(&support, 0, NULL, &allocator));

  RCCHECK(rclc_node_init_default(&node, "micro_ros_arduino_node", "", &support));
  RCCHECK(rclc_publisher_init_default(
    &publisher,
    &node,
    ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Int32),
    "micro_ros_arduino_node_publisher"));

  const unsigned int timer_timeout = 1000; // Timer interval in milliseconds.
  RCCHECK(rclc_timer_init_default(
    &timer,
    &support,
    RCL_MS_TO_NS(timer_timeout),
    timer_callback));

  RCCHECK(rclc_executor_init(&executor, &support.context, 1, &allocator));
  RCCHECK(rclc_executor_add_timer(&executor, &timer));

  msg.data = 0;  // Initialize the message value.
}

Key setup steps:

  1. Transport Setup: Configures the communication method (e.g., serial or WiFi).
  2. Micro-ROS Initialization:
    • Initializes the support structure.
    • Creates a node ("micro_ros_arduino_node").
    • Sets up the publisher for the topic "micro_ros_arduino_node_publisher".
  3. Timer Setup: Calls timer_callback every second (1000 ms).
  4. Executor Setup: Manages the timer callback execution.
  5. Message Initialization: Starts with msg.data = 0.
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void loop() {
  delay(100);  // Short delay.
  RCSOFTCHECK(rclc_executor_spin_some(&executor, RCL_MS_TO_NS(100)));
}
  • The executor handles the timer callback, ensuring timer_callback is executed as needed.

RCL

ROS Client Library

Common Types and Return Codes

Allocator

allocator.h

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rcl_allocator_t allocator = rcl_get_default_allocator();
if (!allocator.allocate) {
    printf("Allocator not initialized\n");
}

Timer

timer.h

  • rcl_timer_t: Structure which encapsulates a ROS Timer
  • Macros:
    • RCL_MS_TO_NS : Convenience macro to convert milliseconds to nanoseconds

  • rcl_timer_init: Initializes a timer with a callback

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      rcl_ret_t rcl_timer_init(
    rcl_timer_t *timer,
    rcl_clock_t *clock,
    rcl_context_t *context,
    int64_t period,
    const rcl_timer_callback_t callback,
    rcl_allocator_t allocator
  )

Publisher

publisher.h

  • rcl_publisher_t : Structure which encapsulates a ROS Publisher

  • Initialization:

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      rcl_ret_t rcl_publisher_init(
      rcl_publisher_t *publisher, 
      const rcl_node_t *node, 
      const rosidl_message_type_support_t *type_support, 
      const char *topic_name, 
      const rcl_publisher_options_t *options
  )

  • Publish messages:

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      rcl_ret_t rcl_publish(
      const rcl_publisher_t *publisher, 
      const void *ros_message, 
      rmw_publisher_allocation_t *allocation
  )

  • Stop publisher: After calling, the node will no longer be advertising that it is publishing on this topic (assuming this is the only publisher on this topic)

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      rcl_ret_t rcl_publisher_fini(
  		const rcl_publisher_t *publisher,
  		const rcl_node_t *node
  )

Node

node.h

  • rcl_node_t : Structure which encapsulates a ROS Node
  • rcl_node_impl_t
  • rcl_service_t

  • Initialization:

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      rcl_ret_t rcl_node_init(
  		rcl_node_t *node,
  		const char *name,
  		const char *namespace_,
  		rcl_context_t *context,
  		const rcl_node_options_t *options
  )
  • rcl_ret_t rcl_node_fini(rcl_node_t *node): Destroys any automatically created infrastructure and deallocates memory. After calling, the rcl_node_t can be safely deallocated

RCLC

RCLC

Support

  • RCLC_UNUSED
  • rclc_support_t : Initializes RCL and creates foundational data structures

  • rclc_support_init: Initializes rcl and creates some support data structures. Initializes clock as RCL_STEADY_TIME

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      rcl_ret_t rclc_support_init(
  		rclc_support_t *support,
  		int argc,
  		char const *const *argv,
  		rcl_allocator_t *allocator
  )

Node

  • rcl_node_t : Structure which encapsulates a ROS Node

  • rclc_node_init_default: Creates a default RCL node

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      rcl_ret_t rclc_node_init_default(
  		rcl_node_t *node,
  		const char *name,
  		const char *namespace_,
  		rclc_support_t *support
  )

Timer

  • rcl_timer_t : Structure which encapsulates a ROS Timer

  • rclc_timer_init_default: Creates an rcl timer

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      rcl_ret_t rclc_timer_init_default(
  		rcl_timer_t *timer,
  		rclc_support_t *support,
  		const uint64_t timeout_ns,
  		const rcl_timer_callback_t callback
  )

Executor

  • rclc_executor_t: Handles callback execution for nodes, timers, and other handles

  • Initialization:

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      rcl_ret_t rclc_executor_init(
  		rclc_executor_t *executor,
  		rcl_context_t *context,
  		const size_t number_of_handles,
  		const rcl_allocator_t *allocator
  )

  • Add Timer to Executor: Adds time to an executor

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      rcl_ret_t rclc_executor_add_timer(
  		rclc_executor_t *executor,
  	  rcl_timer_t *timer
  )

Publisher

  • rclc_publisher_init_default: Creates an rcl publisher

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      rcl_ret_t rclc_publisher_init_default(
  		rcl_publisher_t *publisher,
  		const rcl_node_t *node,
  		const rosidl_message_type_support_t *type_support,
  		const char *topic_name
  )

Quick Initialization Flow

  1. Support Initialization:

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     rclc_support_t support;
 rclc_support_init(&support, argc, argv, &allocator);

  2. Node Initialization:

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     rcl_node_t node;
 rclc_node_init_default(&node, "node_name", "namespace", &support);

  3. Publisher Creation:

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     rcl_publisher_t publisher;
 rclc_publisher_init_default(&publisher, &node, type_support, "topic_name");

  4. Timer and Executor:

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     rcl_timer_t timer;
 rclc_timer_init_default(&timer, &support, RCL_MS_TO_NS(1000), my_callback);
    
 rclc_executor_t executor;
 rclc_executor_init(&executor, &support.context, 1, &allocator);
 rclc_executor_add_timer(&executor, &timer);
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