FreeRTOS is an open source RTOS (real-time operating system) kernel that is integrated into ESP-IDF as a component. Thus, all ESP-IDF applications and many ESP-IDF components are written based on FreeRTOS. The FreeRTOS kernel is ported to all architectures (i.e., Xtensa and RISC-V) available of ESP chips.
Furthermore, ESP-IDF provides different implementations of FreeRTOS in order to support SMP (Symmetric Multiprocessing) on multi-core ESP chips. This document provides an overview of the FreeRTOS component, the different FreeRTOS implementations offered by ESP-IDF, and the common aspects across all implementations.
The `official FreeRTOS <https://www.freertos.org/index.html>`_ (henceforth referred to as Vanilla FreeRTOS) is a single-core RTOS. In order to support the various multi-core ESP targets, ESP-IDF supports different FreeRTOS implementations as listed below:
ESP-IDF FreeRTOS is a FreeRTOS implementation based on Vanilla FreeRTOS v10.5.1, but contains significant modifications to support SMP. ESP-IDF FreeRTOS only supports two cores at most (i.e., dual core SMP), but is more optimized for this scenario by design. For more details regarding ESP-IDF FreeRTOS and its modifications, please refer to the :doc:`freertos_idf` document.
Amazon SMP FreeRTOS is an SMP implementation of FreeRTOS that is officially supported by Amazon. Amazon SMP FreeRTOS is able to support N-cores (i.e., more than two cores). Amazon SMP FreeRTOS can be enabled via the :ref:`CONFIG_FREERTOS_SMP` option. For more details regarding Amazon SMP FreeRTOS, please refer to the `official Amazon SMP FreeRTOS documentation <https://freertos.org/symmetric-multiprocessing-introduction.html>`_.
The Amazon SMP FreeRTOS implementation (and its port in ESP-IDF) are currently in experimental/beta state. Therefore, significant behavioral changes and breaking API changes can occur.
Vanilla FreeRTOS requires that ports and applications configure the kernel by adding various ``#define config...`` macro definitions to the ``FreeRTOSConfig.h`` header file. Vanilla FreeRTOS supports a list of kernel configuration options which allow various kernel behaviors and features to be enabled or disabled.
**However, for all FreeRTOS ports in ESP-IDF, the FreeRTOSConfig.h header file is considered private and must not be modified by users**. A large number of kernel configuration options in ``FreeRTOSConfig.h`` are hard-coded as they are either required/not supported by ESP-IDF. All kernel configuration options that are configurable by the user are exposed via menuconfig under ``Component Config/FreeRTOS/Kernel``.
For the full list of user configurable kernel options, see :doc:`/api-reference/kconfig`. The list below highlights some commonly used kernel configuration options:
-:ref:`CONFIG_FREERTOS_UNICORE` runs FreeRTOS only on Core 0. Note that this is **not equivalent to running Vanilla FreeRTOS**. Furthermore, this option may affect behavior of components other than :component:`freertos`. For more details regarding the effects of running FreeRTOS on a single core, refer to :ref:`freertos-idf-single-core` (if using ESP-IDF FreeRTOS) or the official Amazon SMP FreeRTOS documentation. Alternatively, users can also search for occurrences of ``CONFIG_FREERTOS_UNICORE`` in the ESP-IDF components.
-:ref:`CONFIG_FREERTOS_ENABLE_BACKWARD_COMPATIBILITY` enables backward compatibility with some FreeRTOS macros/types/functions that were deprecated from v8.0 onwards.
All other FreeRTOS related configuration options that are not part of the kernel configuration are exposed via menuconfig under ``Component Config/FreeRTOS/Port``. These options configure aspects such as:
Unlike Vanilla FreeRTOS, users of FreeRTOS in ESP-IDF **must never call**:cpp:func:`vTaskStartScheduler` and :cpp:func:`vTaskEndScheduler`. Instead, ESP-IDF starts FreeRTOS automatically. Users must define a ``void app_main(void)`` function which acts as the entry point for user's application and is automatically invoked on ESP-IDF startup.
- An idle task (``IDLEx``) is created for (and pinned to) each core, where ``x`` is the core's number. The ``x`` is dropped when single-core configuration is enabled.
- When :ref:`CONFIG_FREERTOS_UNICORE` is false, an IPC task (``ipcx``) is created for (and pinned to) each core. IPC tasks are used to implement the Inter-processor Call (IPC) feature.
Note that if an application uses other ESP-IDF features (e.g., Wi-Fi or Bluetooth), those features may create their own background tasks in addition to the tasks listed in the table above.
ESP-IDF provides some supplemental features to FreeRTOS such as Ring Buffers, ESP-IDF style Tick and Idle Hooks, and TLSP deletion callbacks. See :doc:`freertos_additions` for more details.
Vanilla FreeRTOS provides its own `selection of heap implementations <https://www.freertos.org/a00111.html>`_. However, ESP-IDF already implements its own heap (see :doc:`/api-reference/system/mem_alloc`), thus ESP-IDF does not make use of the heap implementations provided by Vanilla FreeRTOS. All FreeRTOS ports in ESP-IDF map FreeRTOS memory allocation or free calls (e.g., ``pvPortMalloc()`` and ``pvPortFree()``) to ESP-IDF heap API (i.e., :cpp:func:`heap_caps_malloc` and :cpp:func:`heap_caps_free`). However, the FreeRTOS ports ensure that all dynamic memory allocated by FreeRTOS is placed in internal memory.
- Allocate the task or object using one of the ``...CreateWithCaps()`` API, such as :cpp:func:`xTaskCreateWithCaps` and :cpp:func:`xQueueCreateWithCaps` (see :ref:`freertos-idf-additional-api` for more details).
- Manually allocate external memory for those objects using :cpp:func:`heap_caps_malloc`, then create the objects from the allocated memory using on of the ``...CreateStatic()`` FreeRTOS functions.
