ulTaskNotifyTake() for binary/counting notification in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of ulTaskNotifyTake() with clear count on binary notification?

Consider a FreeRTOS task that calls ulTaskNotifyTake(pdTRUE, 0) after receiving a single notification. What value does ulTaskNotifyTake() return?

FreeRTOS

/* Assume the task has received exactly one notification count before this call */
uint32_t count = ulTaskNotifyTake(pdTRUE, 0);
printf("%u", count);

A 1

B 2

C 0

D Error: function blocks and never returns

💡 Hint

Think about what pdTRUE does to the notification count.

When ulTaskNotifyTake(pdTRUE, 0) is called, it returns the current notification count and then clears it to zero. Since the task had exactly one notification, the function returns 1.

❓ Predict Output

intermediate

What does ulTaskNotifyTake() return after multiple notifications with clear count?

A task receives 3 notifications before calling ulTaskNotifyTake(pdTRUE, 0). What is the return value?

FreeRTOS

/* Notifications sent 3 times before this call */
uint32_t count = ulTaskNotifyTake(pdTRUE, 0);
printf("%u", count);

A Error: function blocks indefinitely

B 1

C 0

D 3

💡 Hint

Count accumulates with counting notifications.

Each notification increments the count. Calling ulTaskNotifyTake(pdTRUE, 0) returns the total count (3) and clears it.

❓ Predict Output

advanced

What happens if ulTaskNotifyTake() is called with clear count false on a counting notification?

A task has a notification count of 2. It calls ulTaskNotifyTake(pdFALSE, 0). What is the return value and the new notification count?

FreeRTOS

/* Notification count is 2 before call */
uint32_t count = ulTaskNotifyTake(pdFALSE, 0);
printf("%u", count);
// What is the notification count now?

A Returns 2; notification count resets to 0

B Returns 2; notification count remains 2

C Returns 0; notification count remains 2

D Returns 1; notification count decrements to 1

💡 Hint

Clear count flag controls if count resets after reading.

With pdFALSE, the count is returned but not cleared, so it stays at 2.

❓ Predict Output

advanced

What is the behavior of ulTaskNotifyTake() with timeout when no notification is pending?

A task calls ulTaskNotifyTake(pdTRUE, 10) but no notification is pending. What happens?

FreeRTOS

uint32_t count = ulTaskNotifyTake(pdTRUE, 10);
printf("%u", count);

A Returns immediately with 0

B Returns immediately with 1

C Blocks up to 10 ticks, then returns 0 if no notification received

D Blocks indefinitely until notification received

💡 Hint

Timeout controls how long the function waits for notification.

The function blocks up to the timeout (10 ticks). If no notification arrives, it returns 0.

🧠 Conceptual

expert

How does ulTaskNotifyTake() differ from xTaskNotifyWait() in counting notifications?

Which statement correctly describes the difference between ulTaskNotifyTake() and xTaskNotifyWait() when used for counting notifications?

A <code>ulTaskNotifyTake()</code> returns the current count and optionally clears it; <code>xTaskNotifyWait()</code> waits for a notification and returns a status but does not count.

B <code>ulTaskNotifyTake()</code> decrements the notification count by one per call; <code>xTaskNotifyWait()</code> reads and clears the notification value.

C <code>ulTaskNotifyTake()</code> only works for binary notifications; <code>xTaskNotifyWait()</code> only works for counting notifications.

D <code>ulTaskNotifyTake()</code> blocks indefinitely; <code>xTaskNotifyWait()</code> never blocks.

💡 Hint

Think about what each function returns and how they handle notification counts.

ulTaskNotifyTake() returns the notification count and optionally clears it, useful for counting notifications. xTaskNotifyWait() waits for a notification and returns a status indicating if one was received, but does not provide the count.

Practice

1. What does the ulTaskNotifyTake() function do in FreeRTOS?

easy

A. It deletes a task from the scheduler.

B. It creates a new task in the system.

C. It sends a notification to another task.

D. It blocks a task until a notification is received.

Solution

1. Step 1: Understand the purpose of ulTaskNotifyTake()

   This function is designed to make a task wait (block) until it receives a notification from another task or interrupt.

2. Step 2: Differentiate from other task functions

   Creating, sending, or deleting tasks are done by other FreeRTOS functions, not ulTaskNotifyTake().

3. Final Answer:

   It blocks a task until a notification is received. -> Option D

4. Quick Check:

   ulTaskNotifyTake() waits for notification [OK]

Hint: ulTaskNotifyTake() makes task wait for notification [OK]

Common Mistakes:

• Confusing ulTaskNotifyTake() with task creation functions
• Thinking it sends notifications instead of receiving
• Assuming it deletes tasks

2. Which parameter of ulTaskNotifyTake() controls whether the notification count resets on exit?

easy

A. xClearCountOnExit

B. xTicksToWait

C. ulNotificationValue

D. xNotifyOption

Solution

1. Step 1: Identify parameters of ulTaskNotifyTake()

   ulTaskNotifyTake() takes two parameters: xClearCountOnExit and xTicksToWait.

2. Step 2: Understand xClearCountOnExit role

   This parameter decides if the notification count resets to zero when the function exits (pdTRUE) or not (pdFALSE).

3. Final Answer:

   xClearCountOnExit -> Option A

4. Quick Check:

   xClearCountOnExit resets count [OK]

Hint: xClearCountOnExit resets notification count [OK]

Common Mistakes:

• Mixing xClearCountOnExit with xTicksToWait
• Assuming ulNotificationValue is a parameter
• Confusing with notification sending options

3. Consider this code snippet:

uint32_t count = ulTaskNotifyTake(pdFALSE, portMAX_DELAY);
// Assume 3 notifications were sent before this call

What will be the value of count after this call?

medium

A. 1

B. 0

C. 3

D. portMAX_DELAY

Solution

1. Step 1: Analyze xClearCountOnExit parameter

   pdFALSE means the notification count is not cleared on exit, so it counts all notifications received.

2. Step 2: Understand the count returned

   Since 3 notifications were sent before, ulTaskNotifyTake() returns the total count, which is 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   pdFALSE keeps count, so count = 3 [OK]

Hint: pdFALSE keeps count, so returns total notifications [OK]

Common Mistakes:

• Assuming count resets to 1 regardless
• Confusing portMAX_DELAY as return value
• Thinking count is zero if notifications sent before

4. What is wrong with this code snippet?

uint32_t count = ulTaskNotifyTake(pdTRUE, 100);
if(count == 0) {
    // No notification received
}
// Task expects to count multiple notifications

medium

A. Using pdTRUE clears count, so multiple notifications can't be counted.

B. Timeout value 100 is invalid and causes error.

C. ulTaskNotifyTake() cannot be used with if statements.

D. count variable should be int, not uint32_t.

Solution

1. Step 1: Check xClearCountOnExit usage

   pdTRUE clears the notification count on exit, so only a binary notification is counted, not multiple.

2. Step 2: Understand task expectation

   The task expects to count multiple notifications, but pdTRUE resets count, so it cannot accumulate multiple counts.

3. Final Answer:

   Using pdTRUE clears count, so multiple notifications can't be counted. -> Option A

4. Quick Check:

   pdTRUE clears count, no multiple counting [OK]

Hint: pdTRUE clears count; use pdFALSE for counting multiple [OK]

Common Mistakes:

• Thinking timeout 100 is invalid
• Believing ulTaskNotifyTake() can't be in if
• Confusing variable type for count

5. You want a task to sleep until it receives exactly 2 notifications, then proceed. Which ulTaskNotifyTake() usage is correct?

hard

A. Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) twice and sum results.

B. Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) once and check if return is 2.

C. Call ulTaskNotifyTake(pdTRUE, 0) in a loop until count is 2.

D. Call ulTaskNotifyTake(pdTRUE, portMAX_DELAY) once and check if return is 2.

Solution

1. Step 1: Understand counting notifications

   Using pdFALSE keeps the count of notifications, so the function returns the total number received.

2. Step 2: Use single call to wait indefinitely

   Calling ulTaskNotifyTake(pdFALSE, portMAX_DELAY) waits until at least one notification arrives and returns the count accumulated.

3. Step 3: Check if count equals 2

   After the call returns, check if the count is exactly 2 to proceed.

4. Final Answer:

   Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) once and check if return is 2. -> Option B

5. Quick Check:

   pdFALSE counts notifications; check return for 2 [OK]

Hint: Use pdFALSE and check return count for exact notifications [OK]

Common Mistakes:

• Calling ulTaskNotifyTake multiple times and summing counts
• Using pdTRUE which resets count
• Using zero timeout causing immediate return