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

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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.