xTaskNotify() with value in FreeRTOS - Step-by-Step Execution

Concept Flow - xTaskNotify() with value

Task A calls xTaskNotify() with value

↓

Kernel stores notification value for Task B

↓

Task B unblocked or notified

↓

Task B reads notification value

↓

Task B processes notification

↓

Notification cleared or updated

Task A sends a notification with a value to Task B, which then receives and processes it.

Execution Sample

FreeRTOS

xTaskNotify(taskBHandle, 42, eSetValueWithOverwrite);
// Task B waits and reads notification value
uint32_t value;
xTaskNotifyWait(0, 0xffffffffUL, &value, portMAX_DELAY);

Task A sends value 42 to Task B using xTaskNotify(), Task B waits and receives the value.

Execution Table

Step	Action	Task A State	Task B State	Notification Value	Result
1	Task A calls xTaskNotify(taskBHandle, 42, eSetValueWithOverwrite)	Running	Blocked (waiting)	None	Notification value 42 stored for Task B
2	Kernel unblocks Task B	Ready	Ready	42	Task B ready to run with notification
3	Task B returns from xTaskNotifyWait(0, 0xffffffffUL, &value, portMAX_DELAY)	Ready	Running	42	Task B reads notification value 42
4	Notification cleared after read	Ready	Running	None	Notification cleared for Task B
5	Task B processes value 42	Ready	Running	None	Task B uses notification value
6	No further notifications	Ready	Running	None	Execution continues normally

💡 No more notifications sent; Task B continues running after processing value

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
Notification Value	None	42	42	None	None
Task A State	Ready	Running	Ready	Ready	Ready
Task B State	Blocked	Blocked	Running	Running	Running
value (in Task B)	Undefined	Undefined	42	42	42

Key Moments - 3 Insights

Why does Task B get unblocked after Task A calls xTaskNotify()?

Because xTaskNotify() sends a notification to Task B, the kernel unblocks Task B so it can process the notification, as shown in execution_table step 2.

What happens to the notification value after Task B reads it with xTaskNotifyWait()?

The notification value is cleared after Task B reads it, so it doesn't remain set, as shown in execution_table step 4.

Can Task A overwrite the notification value if it calls xTaskNotify() again before Task B reads it?

Yes, using eSetValueWithOverwrite allows Task A to overwrite the previous notification value before Task B reads it.

Visual Quiz - 3 Questions

Look at the execution_table, what is the notification value stored for Task B immediately after Task A calls xTaskNotify()?

A 42

B None

C 0

D Undefined

💡 Hint

Check execution_table row 1 under 'Notification Value'

At which step does Task B read the notification value?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at execution_table rows and find when Task B returns from xTaskNotifyWait()

If Task A calls xTaskNotify() twice with eSetValueWithOverwrite before Task B reads, what happens to the notification value?

A The first value is kept, second ignored

B The notification value is overwritten with the second value

C Task B receives both values separately

D Notification value is cleared

💡 Hint

Refer to key_moments about eSetValueWithOverwrite behavior

Concept Snapshot

xTaskNotify(taskHandle, value, action) sends a notification with a value to a task.
The notified task can wait and read the value using xTaskNotifyWait().
Using eSetValueWithOverwrite overwrites previous notification values.
Notification unblocks the task if it was waiting.
After reading, the notification value is cleared.

Full Transcript

This visual execution shows how xTaskNotify() sends a value from one task to another in FreeRTOS. Task A calls xTaskNotify() with a value and an action to overwrite. The kernel stores this value and unblocks Task B if it was waiting. Task B then calls xTaskNotifyWait() to receive the value. After reading, the notification is cleared. This process allows tasks to communicate simple data efficiently. The execution table tracks each step, showing task states and notification values. Key moments clarify why tasks unblock and how values are handled. The quiz tests understanding of notification timing and overwriting behavior.

Practice

1.

What does the xTaskNotify() function primarily do in FreeRTOS?

easy

A. Creates a new task in the system

B. Deletes a task from the scheduler

C. Sends a notification value from one task to another

D. Suspends the current task indefinitely

Solution

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

   The function is designed to send a notification value or signal from one task to another in FreeRTOS.

2. Step 2: Compare with other task functions

   Creating, deleting, or suspending tasks are done by other FreeRTOS functions, not xTaskNotify().

3. Final Answer:

   Sends a notification value from one task to another -> Option C

4. Quick Check:

   xTaskNotify() sends notification value [OK]

Hint: Remember: xTaskNotify() sends signals between tasks [OK]

Common Mistakes:

• Confusing xTaskNotify() with task creation functions
• Thinking it suspends or deletes tasks
• Assuming it handles task priorities

2.

Which of the following is the correct syntax to send a notification with a value using xTaskNotify()?

BaseType_t xTaskNotify( TaskHandle_t xTaskToNotify, uint32_t ulValue, eNotifyAction eAction );

easy

A. xTaskNotify(taskHandle, 10, eSetBits);

B. xTaskNotify(10, taskHandle, eSetValueWithOverwrite);

C. xTaskNotify(taskHandle, eSetValueWithoutOverwrite, 10);

D. xTaskNotify(eSetValueWithOverwrite, taskHandle, 10);

Solution

1. Step 1: Identify parameter order in xTaskNotify()

   The correct order is: TaskHandle_t, uint32_t value, eNotifyAction.

2. Step 2: Match options to correct order

   xTaskNotify(taskHandle, 10, eSetBits); matches the correct order: taskHandle, 10, eSetBits. Others mix parameter order incorrectly.

3. Final Answer:

   xTaskNotify(taskHandle, 10, eSetBits); -> Option A

4. Quick Check:

   Correct parameter order: task, value, action [OK]

Hint: Remember parameter order: task, value, action [OK]

Common Mistakes:

• Swapping value and task handle parameters
• Using wrong enum for eNotifyAction
• Passing parameters in wrong order

3.

Consider the following code snippet:

uint32_t notifiedValue = 0;

// Task A sends notification
xTaskNotify(taskBHandle, 0x04, eSetBits);

// Task B waits for notification
xTaskNotifyWait(0x00, 0xFFFFFFFF, ¬ifiedValue, portMAX_DELAY);

// What is the value of notifiedValue after notification?

medium

A. 0x04

B. 0x00

C. 0xFFFFFFFF

D. Undefined

Solution

1. Step 1: Understand eSetBits action

   eSetBits sets bits in the notification value. Since initial is 0, setting bit 0x04 results in notifiedValue = 0x04.

2. Step 2: Check xTaskNotifyWait parameters

   It clears bits 0x00 (none) and waits indefinitely, storing the notified bits in notifiedValue.

3. Final Answer:

   0x04 -> Option A

4. Quick Check:

   eSetBits sets bits = 0x04 [OK]

Hint: eSetBits adds bits to notified value [OK]

Common Mistakes:

• Assuming notifiedValue remains zero
• Confusing eSetBits with eSetValueWithoutOverwrite
• Ignoring the effect of xTaskNotifyWait

4.

Identify the error in the following code snippet using xTaskNotify():

BaseType_t status;
status = xTaskNotify(NULL, 5, eSetValueWithOverwrite);
if(status == pdPASS) {
    // Notification sent
}

medium

A. Missing return statement in function

B. Using eSetValueWithOverwrite is invalid

C. Notification value must be zero

D. Passing NULL as task handle causes error

Solution

1. Step 1: Check task handle parameter

   xTaskNotify requires a valid task handle; NULL is invalid and causes failure.

2. Step 2: Validate other parameters

   eSetValueWithOverwrite and value 5 are valid; no return statement needed here.

3. Final Answer:

   Passing NULL as task handle causes error -> Option D

4. Quick Check:

   Task handle NULL invalid [OK]

Hint: Task handle must never be NULL [OK]

Common Mistakes:

• Passing NULL as task handle
• Thinking notification value must be zero
• Confusing eNotifyAction enums

5.

You want to send a notification value from Task A to Task B, but only if Task B's current notification value is zero. Which eNotifyAction should you use with xTaskNotify() to achieve this?

hard

A. eSetValueWithOverwrite

B. eSetValueWithoutOverwrite

C. eIncrement

D. eNoAction

Solution

1. Step 1: Understand eNotifyAction options

   eSetValueWithoutOverwrite sets the notification value only if it is currently zero.

2. Step 2: Compare with other actions

   eSetValueWithOverwrite always overwrites; eIncrement adds 1; eNoAction sends no value.

3. Final Answer:

   eSetValueWithoutOverwrite -> Option B

4. Quick Check:

   Set only if zero = eSetValueWithoutOverwrite [OK]

Hint: Use eSetValueWithoutOverwrite to avoid overwriting [OK]

Common Mistakes:

• Using eSetValueWithOverwrite and overwriting value
• Choosing eIncrement which changes value incorrectly
• Selecting eNoAction which sends no value