xTaskNotify() with value in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS task notification example?

Consider the following FreeRTOS code snippet where a task is notified with a value. What will be the value printed by the task?

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

TaskHandle_t xTaskHandle = NULL;

void vTaskFunction(void *pvParameters) {
    uint32_t ulNotificationValue = 0;
    xTaskNotifyWait(0x00, 0xffffffff, &ulNotificationValue, portMAX_DELAY);
    printf("Notification value: %lu\n", ulNotificationValue);
    vTaskDelete(NULL);
}

int main(void) {
    xTaskCreate(vTaskFunction, "Task", configMINIMAL_STACK_SIZE, NULL, 1, &xTaskHandle);
    vTaskStartScheduler();
    vTaskDelay(10);
    xTaskNotify(xTaskHandle, 1234, eSetValueWithOverwrite);
    return 0;
}

A Notification value: 0

B Notification value: 1234

C Notification value: 1

D Notification value: 4294967295

💡 Hint

Remember that xTaskNotifyWait receives the notification value sent by xTaskNotify when using eSetValueWithOverwrite.

The xTaskNotify function with eSetValueWithOverwrite sets the notification value directly. The task waits for the notification and receives the value 1234, which is then printed.

🧠 Conceptual

intermediate

Which notification action updates the task notification value by adding the new value?

In FreeRTOS, xTaskNotify() can perform different actions on the task's notification value. Which action adds the new value to the current notification value?

A eSetBits

B eSetValueWithoutOverwrite

C eSetValueWithOverwrite

D eIncrement

💡 Hint

Think about bitwise operations and how notification values can be combined.

The eSetBits action performs a bitwise OR between the current notification value and the new value, effectively adding bits. This is different from incrementing or overwriting.

❓ Predict Output

advanced

What happens if you call xTaskNotify() with eSetValueWithoutOverwrite when the notification value is already non-zero?

Given a task with a current notification value of 5, what will be the notification value after calling xTaskNotify() with value 10 and action eSetValueWithoutOverwrite?

FreeRTOS

uint32_t ulValue = 5;
// Assume task notification value is currently 5
// Call:
xTaskNotify(xTaskHandle, 10, eSetValueWithoutOverwrite);
// Then task reads notification value

A Notification value remains 5

B Notification value becomes 10

C Notification value becomes 15

D Notification value resets to 0

💡 Hint

Check what eSetValueWithoutOverwrite does if the notification value is not zero.

The eSetValueWithoutOverwrite action only sets the notification value if it is zero. Since the current value is 5 (non-zero), the notification value remains unchanged.

🔧 Debug

advanced

Why does this code cause the task to never unblock after xTaskNotify()?

Examine the code below. The task waits for a notification, but never prints the notification value. What is the likely cause?

FreeRTOS

void vTaskFunction(void *pvParameters) {
    uint32_t ulNotificationValue = 0;
    xTaskNotifyWait(0x00, 0xffffffff, &ulNotificationValue, 1000 / portTICK_PERIOD_MS);
    printf("Notification value: %lu\n", ulNotificationValue);
    vTaskDelete(NULL);
}

// In main or another task:
xTaskNotify(xTaskHandle, 42, eSetValueWithOverwrite);
// But task never prints anything

A The notification value is overwritten with zero inside the task

B The notification wait timeout is too long, blocking indefinitely

C The task handle passed to xTaskNotify is NULL

D The task notification is sent before the task starts waiting, so it misses the notification

💡 Hint

Consider the timing of when the notification is sent versus when the task waits.

If the notification is sent before the task calls xTaskNotifyWait, the notification is lost because the task was not yet waiting. The task then times out and prints zero.

🚀 Application

expert

How to atomically increment a task's notification value and unblock the task?

You want to safely increment a task's notification value by 1 and unblock the task if it is waiting. Which xTaskNotify() action should you use?

A eSetBits

B eSetValueWithOverwrite

C eIncrement

D eSetValueWithoutOverwrite

💡 Hint

Think about which action increments the notification value atomically.

The eIncrement action atomically increments the task's notification value by 1 and unblocks the task if it was waiting. This is the correct choice for incrementing safely.