xTaskNotifyGive() as lightweight semaphore in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to notify a task using xTaskNotifyGive().

FreeRTOS

xTaskNotifyGive([1]);

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

AxTaskHandle

BxQueue

CxSemaphore

DxTaskToNotify

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a semaphore or queue handle instead of a task handle.

Using an undefined variable as the task handle.

The function xTaskNotifyGive() requires the handle of the task to notify. Here, xTaskToNotify is the correct task handle.

2fill in blank

medium

Complete the code to wait for a notification as a lightweight semaphore using ulTaskNotifyTake().

FreeRTOS

ulTaskNotifyTake([1], portMAX_DELAY);

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

A0

BpdTRUE

CpdFALSE

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Passing pdTRUE which clears the count immediately.

Passing numeric values like 0 or 1 instead of pdFALSE or pdTRUE.

The first parameter to ulTaskNotifyTake() is ClearCountOnExit. Passing pdFALSE means the count is not cleared on exit, which is typical for semaphore behavior.

3fill in blank

hard

Fix the error in the code to correctly use xTaskNotifyGive() as a semaphore signal.

FreeRTOS

void signalTask() {
    xTaskNotifyGive([1]);
}

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

AxTaskToNotify

BxTaskHandle

CNULL

DxSemaphore

Attempts:

3 left

💡 Hint

Common Mistakes

Passing NULL which causes no task to be notified.

Passing a semaphore handle instead of a task handle.

The function xTaskNotifyGive() must be called with a valid task handle. Using xTaskToNotify ensures the correct task is notified.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps task names to their notification counts if count is greater than zero.

FreeRTOS

taskNotifyCounts = {taskName: [1] for taskName, count in taskCounts.items() if count [2] 0}

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

Acount

B==

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using equality (==) instead of greater than (>) in the condition.

Mapping to taskName instead of count.

The dictionary comprehension maps taskName to count only if count > 0.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase task names to their counts if count is less than 5.

FreeRTOS

filteredCounts = [1]: [2] for task, count in counts.items() if count [3] 5}

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

Atask.upper()

Bcount

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using greater than (>) instead of less than (<) in the condition.

Mapping task names without converting to uppercase.

The comprehension maps uppercase task names to counts where count is less than 5.

Practice

1. What is the primary purpose of xTaskNotifyGive() in FreeRTOS?

easy

A. To send a simple signal from one task to another as a lightweight semaphore

B. To create a new task in the system

C. To delete a task permanently

D. To allocate memory dynamically

Solution

1. Step 1: Understand the function role

   xTaskNotifyGive() is designed to send a notification signal from one task to another.

2. Step 2: Compare with other options

   Creating or deleting tasks and memory allocation are unrelated to xTaskNotifyGive().

3. Final Answer:

   To send a simple signal from one task to another as a lightweight semaphore -> Option A

4. Quick Check:

   Signal sending = To send a simple signal from one task to another as a lightweight semaphore [OK]

Hint: Remember: xTaskNotifyGive() signals tasks like a lightweight semaphore [OK]

Common Mistakes:

• Confusing it with task creation or deletion functions
• Thinking it manages memory
• Assuming it blocks tasks directly

2. Which of the following is the correct syntax to send a notification using xTaskNotifyGive() to a task handle named xTaskHandle?

easy

A. xTaskNotifyGive(xTaskHandle, 1);

B. xTaskNotifyGive(&xTaskHandle);

C. xTaskNotifyGive(xTaskHandle);

D. xTaskNotifyGive();

Solution

1. Step 1: Recall function signature

   xTaskNotifyGive() takes a task handle as a parameter, not a pointer or extra arguments.

2. Step 2: Check each option

   xTaskNotifyGive(xTaskHandle); correctly passes the task handle. xTaskNotifyGive(&xTaskHandle); passes address (wrong), C adds an extra argument (wrong), D has no argument (wrong).

3. Final Answer:

   xTaskNotifyGive(xTaskHandle); -> Option C

4. Quick Check:

   Correct syntax = xTaskNotifyGive(xTaskHandle); [OK]

Hint: Pass the task handle directly without extra arguments [OK]

Common Mistakes:

• Passing pointer to task handle instead of handle itself
• Adding extra parameters
• Calling without any argument

3. Consider the following code snippet:

void Task1(void *pvParameters) {
    // Wait for notification
    ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
    // Continue after notification
    printf("Task1 resumed\n");
}

void Task2(void *pvParameters) {
    // Notify Task1
    xTaskNotifyGive(Task1Handle);
}

What will be printed when Task2 calls xTaskNotifyGive()?

medium

A. Task2 resumes instead of Task1

B. Task1 resumed

C. Syntax error at compile time

D. No output, Task1 blocks forever

Solution

1. Step 1: Understand notification flow

   Task1 waits for a notification with ulTaskNotifyTake(). When Task2 calls xTaskNotifyGive(), it sends the notification.

2. Step 2: Effect of notification

   The notification unblocks Task1, so it prints "Task1 resumed".

3. Final Answer:

   Task1 resumed -> Option B

4. Quick Check:

   Notification received = Task1 prints [OK]

Hint: xTaskNotifyGive() wakes waiting task to continue [OK]

Common Mistakes:

• Assuming Task1 never resumes
• Confusing which task prints output
• Thinking code causes compile error

4. Identify the error in this code snippet using xTaskNotifyGive():

void TaskA(void *pvParameters) {
    ulTaskNotifyTake(pdFALSE, portMAX_DELAY);
    printf("TaskA running\n");
}

void TaskB(void *pvParameters) {
    xTaskNotifyGive(&TaskAHandle);
}

medium

A. Passing address of task handle instead of task handle to xTaskNotifyGive()

B. Using pdFALSE in ulTaskNotifyTake() causes immediate return

C. Missing delay in TaskB before notify

D. printf statement is incorrect syntax

Solution

1. Step 1: Check parameter passed to xTaskNotifyGive()

   The function expects a task handle, but &TaskAHandle passes its address, which is incorrect.

2. Step 2: Validate other parts

   Using pdFALSE is valid to clear notification count, no syntax error in printf, and delay is optional.

3. Final Answer:

   Passing address of task handle instead of task handle to xTaskNotifyGive() -> Option A

4. Quick Check:

   Correct parameter type needed = Passing address of task handle instead of task handle to xTaskNotifyGive() [OK]

Hint: Pass task handle directly, not its address [OK]

Common Mistakes:

• Passing pointer to handle instead of handle
• Misunderstanding pdFALSE effect
• Assuming printf syntax error

5. You want to use xTaskNotifyGive() as a lightweight semaphore to synchronize two tasks: Producer and Consumer. The Consumer waits for the notification before processing data. Which sequence correctly implements this synchronization?

hard

A. Producer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY);, Consumer calls xTaskNotifyGive(ProducerHandle);

B. Consumer calls xTaskNotifyGive() and Producer calls ulTaskNotifyTake()

C. Both tasks call xTaskNotifyGive() simultaneously without waiting

D. Consumer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY); to wait, Producer calls xTaskNotifyGive(ConsumerHandle); to signal

Solution

1. Step 1: Identify roles of tasks

   The Consumer should wait for a signal before processing, so it uses ulTaskNotifyTake() to block until notified.

2. Step 2: Producer signals Consumer

   The Producer sends the notification using xTaskNotifyGive() with the Consumer's task handle.

3. Final Answer:

   Consumer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY); to wait, Producer calls xTaskNotifyGive(ConsumerHandle); to signal -> Option D

4. Quick Check:

   Wait then signal = Consumer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY); to wait, Producer calls xTaskNotifyGive(ConsumerHandle); to signal [OK]

Hint: Consumer waits, Producer signals with xTaskNotifyGive() [OK]

Common Mistakes:

• Reversing roles of Producer and Consumer
• Both tasks signaling without waiting
• Using wrong task handles in calls