Graceful shutdown sequence in FreeRTOS - Mini Project: Build & Apply

Graceful shutdown sequence

📖 Scenario: You are developing a FreeRTOS application for a smart home device. The device needs to shut down gracefully when a shutdown signal is received. This means it should stop accepting new tasks, finish current tasks, release resources, and then power off safely.

🎯 Goal: Build a graceful shutdown sequence in FreeRTOS that uses a flag to signal shutdown, stops task creation, waits for running tasks to finish, and then prints a shutdown complete message.

📋 What You'll Learn

Create a global flag variable shutdown_requested initialized to false.

Create a task called WorkerTask that runs a loop checking shutdown_requested and exits when it is true.

Create a task called ShutdownTask that sets shutdown_requested to true after a delay.

Print "Shutdown complete" after all tasks have stopped.

💡 Why This Matters

🌍 Real World

Smart devices and embedded systems often need to stop safely to avoid data loss or hardware damage.

💼 Career

Embedded software engineers must implement graceful shutdowns to ensure reliable and safe device operation.

1

Create the shutdown flag variable

Create a global variable called shutdown_requested of type bool and initialize it to false.

FreeRTOS

# Your code here

Hint

This flag will tell tasks when to stop running.

2

Create the WorkerTask that checks the shutdown flag

Create a FreeRTOS task function called WorkerTask that runs an infinite loop. Inside the loop, check if shutdown_requested is true. If it is, break the loop to exit the task. Use vTaskDelay(100 / portTICK_PERIOD_MS) inside the loop to simulate work.

FreeRTOS

#include 

volatile bool shutdown_requested = false;

// Your code here

Hint

The task should stop running when shutdown_requested becomes true.

3

Create the ShutdownTask to trigger shutdown

Create a FreeRTOS task function called ShutdownTask that waits for 500 milliseconds using vTaskDelay(500 / portTICK_PERIOD_MS) and then sets shutdown_requested to true. After setting the flag, delete itself with vTaskDelete(NULL).

FreeRTOS

#include 
#include "FreeRTOS.h"
#include "task.h"

volatile bool shutdown_requested = false;

void WorkerTask(void *pvParameters) {
    while (1) {
        if (shutdown_requested) {
            break;
        }
        vTaskDelay(100 / portTICK_PERIOD_MS);
    }
    vTaskDelete(NULL);
}

// Your code here

Hint

This task triggers the shutdown after a short delay.

4

Print shutdown complete after tasks stop

In the main function, create the WorkerTask and ShutdownTask using xTaskCreate. Then start the scheduler with vTaskStartScheduler(). After the scheduler ends, print "Shutdown complete" using printf.

FreeRTOS

#include 
#include "FreeRTOS.h"
#include "task.h"
#include 

volatile bool shutdown_requested = false;

void WorkerTask(void *pvParameters) {
    while (1) {
        if (shutdown_requested) {
            break;
        }
        vTaskDelay(100 / portTICK_PERIOD_MS);
    }
    vTaskDelete(NULL);
}

void ShutdownTask(void *pvParameters) {
    vTaskDelay(500 / portTICK_PERIOD_MS);
    shutdown_requested = true;
    vTaskDelete(NULL);
}

int main(void) {
    // Your code here
}

Hint

The scheduler runs tasks until shutdown is requested. After it ends, print the shutdown message.

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