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Squeeze all memory into FreeRtos heap

Posted by joehinkle on October 22, 2016

I'm using heap4 which requires me to specify the amount of ram to allocate to the FreeRtos heap.

I would like to allocate all unallocated ram to the heap without playing with the configuration size.

Is there a way to use the linker to figure out what amount of ram is unallocated and assign it to my heap allocation size?


Squeeze all memory into FreeRtos heap

Posted by edwards3 on October 22, 2016

There are as many answers to that question as there are compilers.

If you are using GCC then check out the gnu linker documentation. That describes how to set linker variables, then access those variables from C code. If the linker fills your RAM from low to high, then set a linker variable to the end of the section filled by the linker, and another to the top of the RAM, then you have the limits available to the heap. Here is an example that is more complicated than you need, it has lots of linker variables defined. Look for

__data_end__ = . ;


The linker variables are used in this C file. Search "extern uint32t" to see how to bring them in https://sourceforge.net/p/freertos/code/HEAD/tree/trunk/FreeRTOS/Demo/CORTEXMPUSimulatorKeil_GCC/main.c

Then best to use heap5.c because even with configAPPLICATIONALLOCATEDHEAP (http://www.freertos.org/a00110.html#configAPPLICATIONALLOCATEDHEAP) GCC makes it hard to place the ucHeap array at the address you want. heap5.c makes that easy.

Squeeze all memory into FreeRtos heap

Posted by joehinkle on October 22, 2016


I defined my heap this way using heap4 and GCC compiler.

byte ucHeap[configTOTALHEAPSIZE] attribute ((section (".heap")));

Would I gain anything by using heap5 if this is working fine?

Squeeze all memory into FreeRtos heap

Posted by rtel on October 22, 2016

The problem with this is the definition of configTOTALHEAPSIZE has to be a fixed constant at compile time. If you use heap_5 then you can set the heap region size using a linker variable, so it automatically adjusts as the memory map changes.

Make sure you understand your linker file. Some cruder linker files will set the stack at the top and the heap at the bottom of any RAM that is left over, but not dimension either of them, so they will eventually crash in the middle if too much RAM is used.

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