Graceful shutdown sequence in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to notify the shutdown event.

FreeRTOS

xEventGroupSetBits(shutdownEventGroup, [1]);

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

ASHUTDOWN_BIT

BSTART_BIT

CERROR_BIT

DRUNNING_BIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using START_BIT instead of SHUTDOWN_BIT.

Forgetting to set any event bit.

Using ERROR_BIT which signals errors, not shutdown.

The shutdown event is signaled by setting the SHUTDOWN_BIT in the event group.

2fill in blank

medium

Complete the code to wait for the shutdown event.

FreeRTOS

xEventGroupWaitBits(shutdownEventGroup, [1], pdTRUE, pdFALSE, portMAX_DELAY);

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

ASHUTDOWN_BIT

BERROR_BIT

CSTART_BIT

DRUNNING_BIT

Attempts:

3 left

💡 Hint

Common Mistakes

Waiting on START_BIT which is unrelated to shutdown.

Using pdFALSE for clearOnExit incorrectly.

Not using portMAX_DELAY to wait indefinitely.

Waiting for SHUTDOWN_BIT ensures the task blocks until shutdown is requested.

3fill in blank

hard

Fix the error in the shutdown task loop condition.

FreeRTOS

while (![1](shutdownEventGroup, SHUTDOWN_BIT, pdTRUE, pdFALSE, 0)) {
    vTaskDelay(pdMS_TO_TICKS(100));
}

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

AxEventGroupSync

BxEventGroupWaitBits

CxEventGroupSetBits

DxEventGroupClearBits

Attempts:

3 left

💡 Hint

Common Mistakes

Using xEventGroupSetBits which sets bits instead of waiting.

Using xEventGroupClearBits which clears bits but does not wait.

Using xEventGroupSync which is for synchronization, not bit checking.

xEventGroupWaitBits is used to check event bits with timeout; others do not return bit status.

4fill in blank

hard

Fill both blanks to correctly stop peripherals and delete the task.

FreeRTOS

stopPeripheral([1]);
vTaskDelete([2]);

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

AperipheralHandle

BNULL

CtaskHandle

DperipheralID

Attempts:

3 left

💡 Hint

Common Mistakes

Passing taskHandle to stopPeripheral instead of peripheral ID.

Passing NULL to stopPeripheral which expects an ID.

Passing taskHandle to vTaskDelete instead of NULL.

stopPeripheral needs the peripheral ID; vTaskDelete(NULL) deletes the current task.

5fill in blank

hard

Fill all three blanks to create a graceful shutdown sequence with notification, wait, and cleanup.

FreeRTOS

xEventGroupSetBits(shutdownEventGroup, [1]);

xEventGroupWaitBits(shutdownEventGroup, [2], pdTRUE, pdFALSE, portMAX_DELAY);

cleanupResources([3]);

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

ASHUTDOWN_BIT

CresourceHandle

DtaskHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using different bits for set and wait causing deadlock.

Passing taskHandle instead of resourceHandle to cleanupResources.

Not waiting for shutdown event before cleanup.

Set and wait on SHUTDOWN_BIT to coordinate shutdown; cleanupResources needs the resource handle.

Practice

1. What is the main purpose of a graceful shutdown sequence in FreeRTOS?

easy

A. To immediately stop all tasks without waiting

B. To restart the scheduler automatically

C. To allow tasks to finish their work safely before stopping

D. To delete all tasks without cleanup

Solution

1. Step 1: Understand graceful shutdown concept

   Graceful shutdown means letting tasks complete their current work safely before stopping.

2. Step 2: Compare options

   Only To allow tasks to finish their work safely before stopping describes this safe stopping process; others describe abrupt or incorrect actions.

3. Final Answer:

   To allow tasks to finish their work safely before stopping -> Option C

4. Quick Check:

   Graceful shutdown = safe task completion [OK]

Hint: Graceful shutdown means safe stop, not immediate kill [OK]

Common Mistakes:

• Thinking shutdown means immediate task stop
• Confusing restart with shutdown
• Ignoring task cleanup before stopping

2. Which FreeRTOS function is commonly used to notify a task to start its shutdown process?

easy

A. vTaskDelete()

B. xTaskNotifyGive()

C. vTaskStartScheduler()

D. xQueueSend()

Solution

1. Step 1: Identify notification function

   xTaskNotifyGive() is used to send a notification to a task, often to signal events like shutdown.

2. Step 2: Eliminate other options

   vTaskDelete() deletes a task immediately, vTaskStartScheduler() starts the scheduler, xQueueSend() sends data to a queue, not a direct notification.

3. Final Answer:

   xTaskNotifyGive() -> Option B

4. Quick Check:

   Notify task to shutdown = xTaskNotifyGive() [OK]

Hint: Use xTaskNotifyGive() to signal tasks for shutdown [OK]

Common Mistakes:

• Using vTaskDelete() to notify instead of delete
• Confusing scheduler start with notification
• Sending shutdown signal via queue instead of notification

3. Given the code below, what will be the output if the shutdown flag is set to true?

bool shutdownFlag = false;
void TaskFunction(void *pvParameters) {
  while (!shutdownFlag) {
    // Do work
  }
  printf("Task stopped safely\n");
}

int main() {
  shutdownFlag = true;
  TaskFunction(NULL);
  return 0;
}

medium

A. Task stopped safely

B. No output, infinite loop

C. Compilation error

D. Task continues running

Solution

1. Step 1: Analyze the loop condition

   The loop runs while shutdownFlag is false. Since shutdownFlag is set true before calling TaskFunction, the loop is skipped.

2. Step 2: Check what happens after loop

   After the loop, the printf prints "Task stopped safely".

3. Final Answer:

   Task stopped safely -> Option A

4. Quick Check:

   shutdownFlag true skips loop, prints message [OK]

Hint: If shutdown flag true, loop exits and prints message [OK]

Common Mistakes:

• Assuming loop runs infinitely
• Confusing flag initial value
• Expecting compilation error due to bool usage

4. Identify the error in this shutdown sequence code snippet:

void ShutdownTask(void *pvParameters) {
  vTaskDelete(NULL);
  // Cleanup code here
}

medium

A. Task should call vTaskStartScheduler() before deletion

B. vTaskDelete(NULL) deletes all tasks, causing crash

C. Missing call to vTaskSuspendAll() before deletion

D. Cleanup code runs after task deletion, so it never executes

Solution

1. Step 1: Understand vTaskDelete behavior

   vTaskDelete(NULL) deletes the current task immediately, stopping execution at that point.

2. Step 2: Check code after deletion

   Cleanup code after vTaskDelete(NULL) will never run because the task is deleted before it.

3. Final Answer:

   Cleanup code runs after task deletion, so it never executes -> Option D

4. Quick Check:

   vTaskDelete stops task immediately, no code after runs [OK]

Hint: Code after vTaskDelete(NULL) won't run [OK]

Common Mistakes:

• Assuming cleanup runs after vTaskDelete
• Thinking vTaskDelete(NULL) deletes all tasks
• Confusing suspend with delete

5. You want to implement a graceful shutdown where multiple tasks stop safely before the scheduler stops. Which sequence is correct?

A) Set shutdown flag -> Wait for tasks to confirm stop -> Call vTaskEndScheduler()
B) Call vTaskEndScheduler() -> Set shutdown flag -> Tasks stop
C) Delete tasks immediately -> Set shutdown flag -> Stop scheduler
D) Stop scheduler -> Set shutdown flag -> Tasks stop

hard

A. Set shutdown flag -> Wait for tasks to confirm stop -> Call vTaskEndScheduler()

B. Call vTaskEndScheduler() -> Set shutdown flag -> Tasks stop

C. Delete tasks immediately -> Set shutdown flag -> Stop scheduler

D. Stop scheduler -> Set shutdown flag -> Tasks stop

Solution

1. Step 1: Understand graceful shutdown steps

   First, signal tasks to stop by setting a shutdown flag or notification.

2. Step 2: Wait for tasks to confirm they stopped

   Ensure tasks finish cleanup and confirm they stopped before stopping the scheduler.

3. Step 3: Stop the scheduler safely

   Call vTaskEndScheduler() only after tasks have stopped to avoid abrupt termination.

4. Final Answer:

   Set shutdown flag -> Wait for tasks to confirm stop -> Call vTaskEndScheduler() -> Option A

5. Quick Check:

   Signal tasks -> wait -> stop scheduler = correct shutdown [OK]

Hint: Signal tasks first, wait, then stop scheduler [OK]

Common Mistakes:

• Stopping scheduler before tasks finish
• Deleting tasks immediately without cleanup
• Setting flag after scheduler stops