Stack size allocation in FreeRTOS - Mini Project: Build & Apply

Stack Size Allocation in FreeRTOS

📖 Scenario: You are working on a small embedded system using FreeRTOS. You need to create tasks with proper stack sizes to ensure they run correctly without wasting memory.

🎯 Goal: Learn how to allocate stack sizes for FreeRTOS tasks by creating tasks with specific stack sizes and then printing their stack size configuration.

📋 What You'll Learn

Create a FreeRTOS task with a specific stack size

Define a stack size constant

Use the stack size constant when creating the task

Print the stack size used for the task

💡 Why This Matters

🌍 Real World

Embedded systems often have limited memory. Allocating the right stack size for tasks prevents crashes and saves memory.

💼 Career

Understanding stack size allocation is essential for embedded software developers working with real-time operating systems like FreeRTOS.

1

Define a stack size constant

Create a constant called STACK_SIZE and set it to 256 to represent the stack size in words.

FreeRTOS

# Define the stack size constant
# Your code here

Hint

Use #define STACK_SIZE 256 to create the constant.

2

Create a FreeRTOS task with the stack size

Create a task called vTaskFunction with the stack size STACK_SIZE using xTaskCreate. Use NULL for parameters you don't need, and priority 1.

FreeRTOS

#define STACK_SIZE 256

// Create the task with stack size STACK_SIZE
// Your code here

Hint

Use xTaskCreate(vTaskFunction, "Task1", STACK_SIZE, NULL, 1, NULL); inside main.

3

Add a variable to hold the stack size

Create a variable called taskStackSize and set it to STACK_SIZE to store the stack size value.

FreeRTOS

#define STACK_SIZE 256

void vTaskFunction(void *pvParameters) {
    for (;;) {}
}

int main(void) {
    xTaskCreate(vTaskFunction, "Task1", STACK_SIZE, NULL, 1, NULL);
    int taskStackSize = 0;  // Your code here
    vTaskStartScheduler();
    return 0;
}

Hint

Declare int taskStackSize = STACK_SIZE; inside main.

4

Print the stack size used for the task

Use printf to display the text "Task stack size: " followed by the value of taskStackSize.

FreeRTOS

#define STACK_SIZE 256

void vTaskFunction(void *pvParameters) {
    for (;;) {}
}

int main(void) {
    xTaskCreate(vTaskFunction, "Task1", STACK_SIZE, NULL, 1, NULL);
    int taskStackSize = STACK_SIZE;
    // Print the stack size
    // Your code here
    vTaskStartScheduler();
    return 0;
}

Hint

Use printf("Task stack size: %d\n", taskStackSize); to print the value.

Practice

1. What does the stack size parameter specify when creating a FreeRTOS task?

easy

A. The total heap size available to the system

B. The number of tasks that can run simultaneously

C. The amount of memory allocated for the task's temporary data in words

D. The priority level of the task

Solution

1. Step 1: Understand stack size meaning in FreeRTOS

   The stack size defines how much memory a task can use for its temporary data and function calls.

2. Step 2: Identify the unit of stack size

   In FreeRTOS, stack size is given in words, not bytes, when creating a task.

3. Final Answer:

   The amount of memory allocated for the task's temporary data in words -> Option C

4. Quick Check:

   Stack size = task memory in words [OK]

Hint: Stack size means task memory in words, not bytes [OK]

Common Mistakes:

• Confusing stack size with heap size
• Thinking stack size is in bytes
• Mixing stack size with task priority

2. Which of the following is the correct way to specify stack size when creating a FreeRTOS task in C?

easy

A. xTaskCreate(taskFunction, "Task", 128 / sizeof(int), NULL, 1, NULL);

B. xTaskCreate(taskFunction, "Task", 128 * sizeof(char), NULL, 1, NULL);

C. xTaskCreate(taskFunction, "Task", 128 * sizeof(int), NULL, 1, NULL);

D. xTaskCreate(taskFunction, "Task", 128, NULL, 1, NULL);

Solution

1. Step 1: Recall stack size unit in FreeRTOS

   Stack size is specified in words, so just giving 128 means 128 words.

2. Step 2: Analyze each option

   xTaskCreate(taskFunction, "Task", 128, NULL, 1, NULL); uses 128 words directly, which is correct. xTaskCreate(taskFunction, "Task", 128 * sizeof(char), NULL, 1, NULL); and C multiply by size, which is incorrect. xTaskCreate(taskFunction, "Task", 128 / sizeof(int), NULL, 1, NULL); divides, which is also wrong.

3. Final Answer:

   xTaskCreate(taskFunction, "Task", 128, NULL, 1, NULL); -> Option D

4. Quick Check:

   Stack size = number of words, no sizeof needed [OK]

Hint: Use stack size as word count, no sizeof multiplication [OK]

Common Mistakes:

• Multiplying stack size by sizeof(int)
• Dividing stack size by sizeof(int)
• Confusing bytes and words in stack size

3. Given the code snippet:

void vTaskCode(void * pvParameters) {
    int x = 0;
    for(int i = 0; i < 10; i++) {
        x += i;
    }
    vTaskDelete(NULL);
}

xTaskCreate(vTaskCode, "Task1", 50, NULL, 2, NULL);

What is the minimum stack size allocation (in words) to safely run this task without overflow?

medium

A. 50 words

B. 10 words

C. 100 words

D. 5 words

Solution

1. Step 1: Analyze task stack usage

   The task uses a few local variables (x, i) and a small loop. This requires some stack but not very large.

2. Step 2: Check stack size given

   The task is created with 50 words stack size, which is enough for simple local variables and loop.

3. Final Answer:

   50 words -> Option A

4. Quick Check:

   Simple task needs moderate stack, 50 words is safe [OK]

Hint: Simple tasks need tens of words, not just a few [OK]

Common Mistakes:

• Assuming 10 words is enough for loop variables
• Overestimating stack need as 100 words
• Underestimating stack causing overflow

4. You have a FreeRTOS task created with stack size 20 words, but it crashes with stack overflow. What is the best fix?

medium

A. Decrease the stack size to save memory

B. Increase the stack size to a larger value like 100 words

C. Change the task priority to higher

D. Remove local variables from the task

Solution

1. Step 1: Identify cause of crash

   Stack overflow means the task's stack is too small for its needs.

2. Step 2: Choose appropriate fix

   Increasing stack size to a larger value like 100 words prevents overflow safely.

3. Final Answer:

   Increase the stack size to a larger value like 100 words -> Option B

4. Quick Check:

   Stack overflow fix = increase stack size [OK]

Hint: Stack overflow? Increase stack size [OK]

Common Mistakes:

• Reducing stack size to save memory
• Changing priority instead of stack size
• Ignoring local variables impact

5. You want to create two FreeRTOS tasks: Task A needs 80 words stack, Task B needs 40 words stack. The total heap is 100 words. How should you allocate stack sizes to avoid crashes and minimize wasted memory?

hard

A. Allocate 60 words to Task A and 40 words to Task B

B. Allocate 80 words to Task A and 40 words to Task B

C. Allocate 80 words to Task A and 20 words to Task B

D. Allocate 50 words to each task

Solution

1. Step 1: Understand total heap limit

   Total heap is 100 words, so sum of stack sizes must not exceed 100.

2. Step 2: Allocate stack sizes within limit

   Task A needs 80 words ideally, Task B needs 40 words, total 120 words which is over heap.

3. Step 3: Adjust allocation to fit heap

   Reduce Task A to 60 words and keep Task B at 40 words to fit 100 words total.

4. Final Answer:

   Allocate 60 words to Task A and 40 words to Task B -> Option A

5. Quick Check:

   Sum stack ≤ heap, adjust sizes accordingly [OK]

Hint: Sum stack sizes ≤ heap size to avoid crashes [OK]

Common Mistakes:

• Allocating more stack than heap allows
• Ignoring total heap limit
• Splitting stack sizes evenly without need