Queues, Mutexes, Semaphores…
[Inter-task communication and synchronisation]

[See also Blocking on Multiple RTOS Objects]

The FreeRTOS tutorial book provides additional information on queues, binary semaphores, mutexes, counting semaphores and recursive semaphores, along with simple worked examples in a set of accompanying example projects.


FreeRTOS Recursive Mutexes

A mutex used recursively can be ‘taken’ repeatedly by the owner. The mutex doesn’t become available again until the owner has called xSemaphoreGiveRecursive() for each successful xSemaphoreTakeRecursive() request. For example, if a task successfully ‘takes’ the same mutex 5 times then the mutex will not be available to any other task until it has also ‘given’ the mutex back exactly five times.

This type of semaphore uses a priority inheritance mechanism so a task ‘taking’ a semaphore MUST ALWAYS ‘give’ the semaphore back once the semaphore it is no longer required.

Mutex type semaphores cannot be used from within interrupt service routines.

Mutexes should not be used from an interrupt because:

  • They include a priority inheritance mechanism which only makes sense if the mutex is given and taken from a task, not an interrupt.

  • An interrupt cannot block to wait for a resource that is guarded by a mutex to become available.





Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.

Latest News

FreeRTOS v10.2.1 is available for immediate download. MIT licensed, includes 64-bit RISC-V, NXP Cortex-M33 demo & Nuvoton Cortex-M23 demo & STM32H745 dual core (AMP) demo.

New Updates to MIT licensed IoT MQTT and POSIX libraries are now available. Includes demo projects.

View a recording of the “OTA Update Security and Reliability” webinar, presented by TI and AWS.

Careers

FreeRTOS and other embedded software careers at AWS.

FreeRTOS Partners
ARM Connected RTOS partner for all ARM microcontroller cores
Espressif ESP32
IAR Partner
Microchip Premier RTOS Partner
Renesas
STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0
Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers
OpenRTOS and SafeRTOS
Xilinx Microblaze and Zynq partner