Quality RTOS & Embedded Software

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


Questions on size_t data type usage and data alignment in heap_2.c in MemMang folder

Posted by diptopal on August 14, 2015

I was going through heap2.c in the MemMang folder in FreeRTOS V7.4.2 source to understand it. I have compared heap2.c with the latest vesrion too, this file remains more of less the same, so I did not move over to the newest version of FreeRTOS to get a newer version of heap_2.c. After going through the code, I have a few doubts.

  1. Why is configTOTALHEAPSIZE defined with a** sizet** type e.g. "**#define configTOTALHEAPSIZE ( ( sizet ) ( 19 * 1024 ) )**" and sizet is used in a few other places as well in heap2.c? What is the benefit of it ?

  2. Why does size of** xBlockLink, **blocks and** xWantedSize** need to be aligned or adjusted to the nearest word (4 bytes in case of Cortex M4), is it because some controllers are not capable of unaligned access ?

  3. To save stack depth prvInsertBlockIntoFreeList macro is defined in heap_2.c, whereas** prvHeapInit** is an independent function in the same file even though prvHeapInit is a static function as well. What are the considerations for using macros or functions here?

Questions on size_t data type usage and data alignment in heap_2.c in MemMang folder

Posted by rtel on August 14, 2015

In recent versios of FreeRTOS, heap4.c is normally used in favour of heap2.c.

1) size_t is used for its portability - FreeRTOS runs on 8-bit up to 64-bit hardware.

2) Alignment is a requirement of many processors, especially the alignment of the stack. Each port has the required alignment defined - breaching the alignment can result in mis-calculations, or complete crashes.

3) prvHeapInit is only called once, whereas prvInsertBlockIntoFreeList is called on each allocation, hence the latter is written as a macro to make it faster and use less stack. More important on small processors than large processors.


Questions on size_t data type usage and data alignment in heap_2.c in MemMang folder

Posted by diptopal on August 15, 2015

Thank you very much. I am planning to make a Youtube video on memory allocation for deeply embedded systems. I want to explain it using the heap_x.c files from FreeRTOS. I will maintain the copyrights as is. I will be using the evaluation version of the IAR compiler for this.

Please let me know if there is anything specific that I need to abide by if I use your code for the training.

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

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

Latest News

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

Meet Richard Barry and learn about running FreeRTOS on RISC-V at FOSDEM 2019

Version 10.1.1 of the FreeRTOS kernel is available for immediate download. MIT licensed.

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

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