task stack calculation

nobody wrote on Tuesday, April 19, 2005:

I wonder how to calculate the needed stack size for a task.
The demos use portMinimalStackSize as parameter for sTaskCreate(). Is this always correct, or do I have to count all local variables ?
Or what else is the correct procedure?

rapurvey wrote on Tuesday, April 19, 2005:

The usual method to calculate task stack is to look at the worst case scenario (local variables, etc) and double it.

Then reduce it after lots of testing, but this usually needs the stack to be initialised to some value so you can see the ‘tide mark’ for the maximum amount of stack being used.

Not sure if the stack in FreeRTOS is initialised or not.

rapurvey wrote on Tuesday, April 19, 2005:

Looks like the task stack is initialised with the value 0xA5.

rtel wrote on Tuesday, April 19, 2005:


Im on a business trip so am a bit slow replying at the moment.

The minimal stack is not always the correct size to use.  It is safe to use in the demo app and the idle task  but if your function call depth, or local variable usage, or number of parameters being passed to a function are large then it will have to be increased.

Stack problems are a common source of errors.

Take a look in the file:


at the function:


This walks the stack and checks for the number of 0xa5s that remain.  This lets you know the high water mark as described by rapurvey.  You can copy this function into one of your application files.

Incidentally this function has been moved into the tasks.c file in V3.0.0 so is more easily accessable.