Quality RTOS & Embedded Software

 Real time embedded FreeRTOS RSS feed 
Quick Start Supported MCUs PDF Books Trace Tools Ecosystem


pxHigherPriorityTaskWoken usage clarification

Posted by rsgrimes on August 11, 2016

Three questions, with a bit of background.

Many of the functions that may be safely called from an ISR have a pointer parameter often named pxHigherPriorityTaskWoken. My understanding is that if this is not NULL, and the associated call causes a higher priority task to unblock, then *pxHigherPriorityTaskWoken will be set true, and the calling ISR should call portYIELDFROMISR() prior to returning to activate the unblocked, higher priority task.

  1. Is this a correct understanding?
  2. If portYIELDFROMISR isn't called, then the task will eventually be unblocked, but not until some other event causes the scheduler to reevalute the ready tasks; is that correct?

Finally, I've noticed that the online API documentation (example: http://www.freertos.org/a00119.html) has changed to add this note to the pxHigherPriorityTaskWoken parameter: > "From FreeRTOS V7.3.0 pxHigherPriorityTaskWoken is an optional parameter and can be set to NULL." 3. If you don't supply this parameter, then you won't be notified of a higher-priority task being woken, so it won't get scheduled as soon as possible; is that the sole effect of not using this parameter? 4.

pxHigherPriorityTaskWoken usage clarification

Posted by richard_damon on August 12, 2016

1) Yes. 2) Yes, for example, the tick interrupt. 3) Yes, It isn't quite 'optional', as you have to provide something, but if you pass a NULL (or a 0) then you just don't get the indication that someone has woken. In effect, when/if it is time to set the flag true, that operation is skipped if you pass a null pointer.

[ Back to the top ]    [ About FreeRTOS ]    [ Privacy ]    [ Sitemap ]    [ ]

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.

NXP tweet showing LPC5500 (ARMv8-M Cortex-M33) running FreeRTOS.

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


FreeRTOS and other embedded software careers at AWS.

FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

Cadence Tensilica Cortes

Espressif ESP32

IAR Partner

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers





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