Stack size allocation in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a task stack size of 256 words.

FreeRTOS

const uint16_t taskStackSize = [1];

Drag options to blanks, or click blank then click option'

A256

B1024

C128

D512

Attempts:

3 left

💡 Hint

Common Mistakes

Using bytes instead of words for stack size

Choosing a stack size too small for the task

The stack size is set to 256 words, which is a common size for simple tasks.

2fill in blank

medium

Complete the code to create a task with the stack size defined by taskStackSize.

FreeRTOS

xTaskCreate(TaskFunction, "TaskName", [1], NULL, tskIDLE_PRIORITY, NULL);

Drag options to blanks, or click blank then click option'

A128

BtaskStackSize

C512

D1024

Attempts:

3 left

💡 Hint

Common Mistakes

Hardcoding the stack size instead of using the variable

Passing stack size in bytes instead of words

The stack size parameter should use the previously defined variable taskStackSize.

3fill in blank

hard

Fix the error in the stack size definition to use the correct FreeRTOS type.

FreeRTOS

StackType_t [1][256];

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AStackBuffer

BStackSize

CTaskHandle

DtaskStack

Attempts:

3 left

💡 Hint

Common Mistakes

Using type names as variable names

Using unclear or reserved names

The stack array should be named appropriately, here taskStack is a clear and common name.

4fill in blank

hard

Fill both blanks to declare and initialize a static task stack.

FreeRTOS

static [1] [2][configMINIMAL_STACK_SIZE];

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AStackType_t

BTaskHandle_t

CtaskStack

DtaskControlBlock

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong types for stack array

Confusing stack with task handle

The stack is declared as an array of StackType_t named taskStack.

5fill in blank

hard

Fill all three blanks to create a task with static allocation using stack and control block.

FreeRTOS

xTaskCreateStatic(TaskFunction, "Task", [1], NULL, tskIDLE_PRIORITY, [2], [3]);

Drag options to blanks, or click blank then click option'

AconfigMINIMAL_STACK_SIZE

BtaskStack

CtaskControlBlock

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up stack size and stack buffer

Passing NULL instead of control block

The stack size is configMINIMAL_STACK_SIZE, the stack array is taskStack, and the control block is taskControlBlock.

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