Stack high water mark monitoring in FreeRTOS

Introduction

Stack high water mark monitoring helps you see how much stack memory a task uses. It shows the smallest amount of free stack left, so you can avoid running out of memory.

When you want to check if a task's stack size is enough during development.

When debugging a task that crashes or behaves oddly due to stack overflow.

When optimizing memory usage in a system with many tasks.

When you want to monitor stack usage over time to detect leaks or growth.

When testing new code to ensure it does not use excessive stack.

Syntax

FreeRTOS

UBaseType_t uxTaskGetStackHighWaterMark(TaskHandle_t xTask);

This function returns the minimum amount of stack space that has remained for the task since it started.

The returned value is in words, not bytes. Multiply by sizeof(StackType_t) to get bytes.

Examples

Get the high water mark for the current task.

FreeRTOS

UBaseType_t highWaterMark = uxTaskGetStackHighWaterMark(NULL);

Get the high water mark for a specific task using its handle.

FreeRTOS

UBaseType_t highWaterMark = uxTaskGetStackHighWaterMark(taskHandle);

If the high water mark is zero, the task likely used all its stack space.

FreeRTOS

UBaseType_t highWaterMark = uxTaskGetStackHighWaterMark(NULL);
if(highWaterMark == 0) {
    // Stack overflow likely occurred
}

Sample Program

This program creates a task that uses some stack space. It then prints the smallest amount of free stack space left during the task's life.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

void vExampleTask(void *pvParameters) {
    volatile int dummyArray[50]; // Use some stack space
    for(int i = 0; i < 50; i++) {
        dummyArray[i] = i * i;
    }
    UBaseType_t highWaterMark = uxTaskGetStackHighWaterMark(NULL);
    printf("Stack high water mark (words) for ExampleTask: %u\n", (unsigned int)highWaterMark);
    vTaskDelete(NULL);
}

int main(void) {
    TaskHandle_t exampleTaskHandle = NULL;
    xTaskCreate(vExampleTask, "ExampleTask", 100, NULL, 1, &exampleTaskHandle);
    vTaskStartScheduler();
    return 0;
}

Important Notes

The time complexity of uxTaskGetStackHighWaterMark is O(1) because it returns a stored value.

Space complexity is O(1) as it only reads a variable.

A common mistake is confusing words with bytes; remember to multiply by sizeof(StackType_t) for bytes.

Use this function to check stack usage instead of guessing stack sizes.

Summary

Stack high water mark shows the minimum free stack space a task had.

It helps prevent stack overflow by monitoring stack usage.

Use uxTaskGetStackHighWaterMark with the task handle or NULL for current task.

Practice

1.

What does the stack high water mark indicate in FreeRTOS?

easy

A. The total stack size allocated to a task

B. The minimum amount of free stack space a task has had

C. The maximum CPU usage of a task

D. The number of tasks currently running

Solution

1. Step 1: Understand stack high water mark meaning

   The stack high water mark shows the smallest amount of unused stack space a task has had since it started.

2. Step 2: Compare with other options

   Other options describe CPU usage, total stack size, or task count, which are unrelated to stack high water mark.

3. Final Answer:

   The minimum amount of free stack space a task has had -> Option B

4. Quick Check:

   Stack high water mark = minimum free stack space [OK]

Hint: Remember: 'high water mark' means lowest free stack left [OK]

Common Mistakes:

• Confusing stack size with stack usage
• Thinking it measures CPU or task count
• Assuming it shows maximum stack used

2.

Which is the correct way to get the stack high water mark of the current task in FreeRTOS?

?

easy

A. uxTaskGetStackHighWaterMark(NULL);

B. uxTaskGetStackHighWaterMark(currentTask);

C. getStackHighWaterMark(NULL);

D. uxGetStackHighWaterMark();

Solution

1. Step 1: Recall function signature

   The FreeRTOS function to get stack high water mark is uxTaskGetStackHighWaterMark, which takes a task handle or NULL for current task.

2. Step 2: Identify correct usage

   Passing NULL gets the current task's high water mark. Other options use wrong function names or parameters.

3. Final Answer:

   uxTaskGetStackHighWaterMark(NULL); -> Option A

4. Quick Check:

   Use NULL for current task in uxTaskGetStackHighWaterMark [OK]

Hint: Use NULL to refer to current task handle [OK]

Common Mistakes:

• Using wrong function names
• Passing invalid parameters instead of NULL
• Assuming no parameter needed

3.

Given the following code snippet, what will be the output if the task's minimum free stack space was 50 words?

UBaseType_t highWaterMark = uxTaskGetStackHighWaterMark(NULL);
printf("High water mark: %u\n", highWaterMark);

medium

A. High water mark: 50

B. High water mark: 0

C. High water mark: Stack overflow

D. Compilation error

Solution

1. Step 1: Understand uxTaskGetStackHighWaterMark output

   This function returns the minimum free stack space (in words) the task had, which is 50 in this case.

2. Step 2: Analyze printf output

   The printf prints the unsigned integer value returned, so output will be "High water mark: 50".

3. Final Answer:

   High water mark: 50 -> Option A

4. Quick Check:

   Function returns minimum free stack, printed as number [OK]

Hint: Function returns minimum free stack words, print as unsigned int [OK]

Common Mistakes:

• Assuming it returns zero on no overflow
• Thinking it returns error string
• Confusing with stack size or usage

4.

Identify the error in this code snippet intended to get the stack high water mark of a task named myTaskHandle:

UBaseType_t mark = uxTaskGetStackHighWaterMark(myTaskHandle);

Assume myTaskHandle is a valid task handle.

medium

A. Return type should be int, not UBaseType_t

B. Function name is incorrect, should be uxGetStackHighWaterMark

C. Parameter should be NULL to get any task's high water mark

D. No error, code is correct

Solution

1. Step 1: Verify function and parameter usage

   uxTaskGetStackHighWaterMark takes a task handle to get that task's high water mark. Using myTaskHandle is valid.

2. Step 2: Check return type and function name

   Return type UBaseType_t is correct. Function name is correct as per FreeRTOS API.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   Correct function and parameter usage means no error [OK]

Hint: Use task handle parameter for specific task, NULL for current task [OK]

Common Mistakes:

• Thinking NULL is required for all tasks
• Using wrong function names
• Confusing return types

5.

You want to monitor stack usage of multiple tasks and log a warning if any task's stack high water mark falls below 20 words. Which approach is best?

hard

A. Only check the current task's stack high water mark using NULL parameter

B. Increase all task stack sizes to 20 words to avoid overflow

C. Periodically call uxTaskGetStackHighWaterMark for each task handle and check if value < 20

D. Use vTaskDelete to remove tasks with low stack

Solution

1. Step 1: Understand monitoring multiple tasks

   To monitor multiple tasks, you must check each task's stack high water mark individually using their task handles.

2. Step 2: Implement threshold check

   Calling uxTaskGetStackHighWaterMark for each task and comparing to 20 words allows detecting low stack and logging warnings.

3. Final Answer:

   Periodically call uxTaskGetStackHighWaterMark for each task handle and check if value < 20 -> Option C

4. Quick Check:

   Check each task's high water mark against threshold [OK]

Hint: Check each task's high water mark, compare to threshold [OK]

Common Mistakes:

• Checking only current task misses others
• Assuming increasing stack size fixes monitoring
• Deleting tasks instead of warning