Stack size allocation in FreeRTOS - Step-by-Step Execution

Concept Flow - Stack size allocation

Define Task

↓

Specify Stack Size

↓

Create Task with Stack Size

↓

Task Runs

↓

Stack Used by Task

↓

Stack Overflow Check

↓

Adjust Stack Size if Needed

↩Back to Create Task

This flow shows how a FreeRTOS task is created with a specified stack size, how the task uses the stack, and how stack overflow is checked and handled.

Execution Sample

FreeRTOS

xTaskCreate(TaskFunction, "Task1", 128, NULL, 1, NULL);

Creates a FreeRTOS task named 'Task1' with a stack size of 128 words.

Execution Table

Step	Action	Stack Size (words)	Stack Used (words)	Stack Overflow Check	Result
1	Define TaskFunction	-	-	-	Task function code ready
2	Call xTaskCreate with stack size 128	128	0	No	Task created, stack allocated
3	Task starts running	128	20	No	Task uses 20 words of stack
4	Task runs more code	128	50	No	Stack usage increased to 50 words
5	Task runs deep function calls	128	130	Yes	Stack overflow detected
6	Adjust stack size to 256	256	130	No	Stack overflow prevented
7	Task runs safely	256	130	No	Task continues normally

💡 Execution stops when task runs safely without stack overflow.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
Stack Size (words)	-	128	128	128	128	256	256
Stack Used (words)	0	0	20	50	130	130	130
Stack Overflow	No	No	No	No	Yes	No	No

Key Moments - 3 Insights

Why does the stack overflow even though we allocated 128 words?

Because the task used 130 words which is more than the allocated 128, as shown in execution_table row 5.

What happens if we don't adjust the stack size after overflow?

The task may crash or behave unpredictably due to stack overflow, so increasing stack size as in row 6 prevents this.

Is the stack size in bytes or words?

In FreeRTOS, stack size is specified in words, not bytes, so 128 means 128 words (usually 4 bytes each).

Visual Quiz - 3 Questions

Look at the execution_table, what is the stack usage at step 4?

A 50 words

B 130 words

C 20 words

D 128 words

💡 Hint

Check the 'Stack Used (words)' column at step 4 in the execution_table.

At which step does the stack overflow occur?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look for 'Stack Overflow Check' column showing 'Yes' in the execution_table.

If we change the initial stack size to 256, what would happen at step 5?

A Stack overflow occurs

B Task creation fails

C No stack overflow occurs

D Stack usage becomes 256

💡 Hint

Refer to execution_table row 6 and 7 where stack size is increased to 256 and no overflow occurs.

Concept Snapshot

FreeRTOS stack size allocation:
- Stack size is set in words when creating a task.
- Task uses stack during execution; usage varies.
- If usage exceeds allocation, stack overflow occurs.
- Adjust stack size to prevent overflow.
- Stack size must be enough for worst-case usage.

Full Transcript

In FreeRTOS, when you create a task, you specify how much stack memory it needs in words. The system allocates this stack for the task. As the task runs, it uses some of this stack for function calls and variables. If the task uses more stack than allocated, a stack overflow happens, which can cause errors. To avoid this, you monitor stack usage and increase the stack size if needed. This process ensures your task runs safely without crashing due to stack overflow.