As the first step, we propose the rclc Executor for the rcl-layer in C programming language with several new features to support real-time and deterministic execution: It supports 1.) user-defined static sequential execution, 2) conditional execution semantics, 3) multi-threaded execution with scheduling configuration, and 4) logical execution semantics (LET). Sequential execution refers to the runtime behavior, that all callbacks are executed in a pre-defined order independent of the arrival time of messages. Conditional execution is available with a trigger condition which enables typical processing patterns in robotics (which are analyzed in detail in section [Analysis of processing patterns](#analysis-of-processing-patterns). Configuration of scheduling parameters for multi-threaded application accomplishes prioritized execution. The logical execution time concept (LET) provides data synchronization for fixed periodic task scheduling of embedded applications.

- [Sequence types in micro-ROS](#sequence-types-in-micro-ros)

- [Compound types in micro-ROS](#compound-types-in-micro-ros)

- [Sequences of compound types](#sequences-of-compound-types)

}

Due to the inclusion of [`rosidl_typesupport_introspection_c`](https://github.com/ros2/rosidl/tree/master/rosidl_typesupport_introspection_c) in micro-ROS Galactic distribution, an automated memory handling for micro-ROS types is available. The tools related to this feature are available in the package [`micro_ros_utilities`](https://github.com/micro-ROS/micro_ros_utilities).

permalink: /docs/tutorials/core/first_application_linux/

In this tutorial, you’ll learn the use of micro-ROS with Linux by testing a Ping Pong application.

In the follow-up tutorial [*First micro-ROS application on an RTOS*](/docs/tutorials/core/first_application_rtos/),

- /docs/tutorials/advanced/freertos/freertos_getting_started/

In this tutorial, you'll learn the use of micro-ROS with FreeRTOS by testing a Ping Pong application.

{% include first_application_common/target_hardware.md %}

- /docs/tutorials/advanced/zephyr/zephyr_getting_started/

In this tutorial, you'll learn the use of micro-ROS with Zephyr by testing a Ping Pong application.

{% include first_application_common/target_hardware.md %}

permalink: /docs/tutorials/programming_rcl_rclc/executor/

- [Timers](#timers)

- [Initialization](#initialization)

- [Callback](#callback)

They are usually used to handle periodic publications or events.

const unsigned int timer_period = RCL_MS_TO_NS(1000);

This will deallocate used memory and make the timer invalid

- [Allocators](#allocators)

- [Custom allocator](#custom-allocator)

- [Time sync](#time-sync)

Custom methods prototypes and examples:

Allocates memory given a size, an error should be indicated by returning `NULL`:

```c

- reallocate:

Reallocate memory if possible, otherwise it deallocates and allocates:

```c

// Function prototype:

void * (*reallocate)(void * pointer, size_t size, void * state);

*Note: the `state` input argument is espected to be unused*

This utility is based on the NTP protocol, taking into account delays caused by the transport layer. An usage example can be found on [`micro-ROS-demos/rclc/epoch_synchronization`](https://github.com/micro-ROS/micro-ROS-demos/blob/galactic/rclc/epoch_synchronization/main.c).

rmw_uros_sync_session(timeout_ms);

{

// Get time in milliseconds or nanoseconds

int64_t time_ms = rmw_uros_epoch_millis();

int64_t time_ns = rmw_uros_epoch_nanos();

}

micro_ros_fragment_t fragment;

// Serialize array size

while(len < IMAGE_BYTES){

// Wait for new image "fragment"

permalink: /docs/tutorials/programming_rcl_rclc/node/

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 [here](https://docs.ros.org/en/galactic/Tutorials/Understanding-ROS2-Nodes.html)

```

```c

// Initialize micro-ROS allocator

rcl_lifecycle_get_zero_initialized_state_machine();

permalink: /docs/tutorials/programming_rcl_rclc/parameters/

ROS 2 parameters allow the user to create variables on a node and manipulate/read them with different ROS 2 commands. Further information about ROS 2 parameters can be found [here](https://docs.ros.org/en/rolling/Tutorials/Parameters/Understanding-ROS2-Parameters.html)

Ready to use code examples related to this tutorial can be found in [`rclc/rclc_examples/src/example_parameter_server.c`](https://github.com/ros2/rclc/blob/master/rclc_examples/src/example_parameter_server.c).

- notify_changed_over_dds: Publish parameter events to other ROS 2 nodes as well.

- max_params: Maximum number of parameters allowed on the `rclc_parameter_server_t` object.

- allow_undeclared_parameters: Allows creation of parameters from external parameter clients. A new parameter will be created if a `set` operation is requested on a non-existing parameter.

```c

// Parameter server object

- Parameter value change: Internal and external parameter set on existing parameters.

- Parameter creation: External parameter set on unexisting parameter if `allow_undeclared_parameters` is set.

- Parameter delete: External parameter delete on existing parameter.