Choosing priorities for real applications in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS task priority example?

Consider two tasks created with different priorities. What will be the output order of the print statements?

FreeRTOS

void Task1(void *pvParameters) {
    printf("Task 1 running\n");
    printf("Task 1 finished\n");
    vTaskDelete(NULL);
}

void Task2(void *pvParameters) {
    printf("Task 2 running\n");
    printf("Task 2 finished\n");
    vTaskDelete(NULL);
}

int main() {
    xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    return 0;
}

A Task 1 running\nTask 2 running\nTask 2 finished\nTask 1 finished\n

B Task 1 running\nTask 1 finished\nTask 2 running\nTask 2 finished\n

C Task 2 running\nTask 2 finished\nTask 1 running\nTask 1 finished\n

D Task 2 running\nTask 1 running\nTask 1 finished\nTask 2 finished\n

💡 Hint

Remember, higher priority tasks preempt lower priority ones immediately.

Task2 has priority 3, higher than Task1's priority 2. So Task2 runs first, prints its messages, then Task1 runs.

🧠 Conceptual

intermediate

Which priority assignment best avoids priority inversion?

You have three tasks: A (high priority), B (medium priority), and C (low priority). Task C holds a resource needed by Task A. Which priority assignment helps avoid priority inversion?

A Assign all tasks the same priority.

B Keep Task C at the lowest priority always.

C Assign Task B the highest priority to preempt Task C.

D Assign Task C the highest priority temporarily while it holds the resource.

💡 Hint

Think about priority inheritance to prevent blocking.

Temporarily boosting Task C's priority prevents it from being preempted by Task B, avoiding priority inversion.

🔧 Debug

advanced

Why does this FreeRTOS system freeze with these priorities?

Given three tasks with priorities 1, 2, and 3, the system freezes after startup. What is the likely cause?

FreeRTOS

void TaskLow(void *pvParameters) {
    while(1) {
        // Does some work
    }
}

void TaskMedium(void *pvParameters) {
    while(1) {
        // Does some work
    }
}

void TaskHigh(void *pvParameters) {
    while(1) {
        // Does some work
    }
}

int main() {
    xTaskCreate(TaskLow, "Low", 1000, NULL, 1, NULL);
    xTaskCreate(TaskMedium, "Medium", 1000, NULL, 2, NULL);
    xTaskCreate(TaskHigh, "High", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    return 0;
}

A TaskHigh is starving lower priority tasks causing system freeze.

B TaskLow has a blocking call causing deadlock.

C TaskMedium has a syntax error preventing scheduler start.

D Stack sizes are too small causing overflow.

💡 Hint

Consider how higher priority tasks affect lower priority ones.

TaskHigh runs continuously without delay, starving lower priority tasks and freezing system progress.

📝 Syntax

advanced

Which option correctly sets a task priority in FreeRTOS?

Choose the correct way to set a task's priority to 5 after creation.

FreeRTOS

TaskHandle_t xHandle = NULL;
xTaskCreate(TaskFunction, "Task", 1000, NULL, 3, &xHandle);
// Set priority here

A xTaskSetPriority(xHandle, 5);

B vTaskPrioritySet(xHandle, 5);

C vTaskSetPriority(xHandle, 5);

D xTaskPriorityChange(xHandle, 5);

💡 Hint

Check the FreeRTOS API naming conventions.

The correct API to set a task priority is vTaskPrioritySet().

🚀 Application

expert

How many tasks will run concurrently with these priorities and delays?

Three tasks are created with priorities 4, 3, and 2. Task with priority 4 calls vTaskDelay(100) in its loop. How many tasks run concurrently during the delay?

FreeRTOS

void TaskHigh(void *pvParameters) {
    while(1) {
        // Do work
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

void TaskMid(void *pvParameters) {
    while(1) {
        // Do work
    }
}

void TaskLow(void *pvParameters) {
    while(1) {
        // Do work
    }
}

int main() {
    xTaskCreate(TaskHigh, "High", 1000, NULL, 4, NULL);
    xTaskCreate(TaskMid, "Mid", 1000, NULL, 3, NULL);
    xTaskCreate(TaskLow, "Low", 1000, NULL, 2, NULL);
    vTaskStartScheduler();
    return 0;
}

A Only one task runs during the delay: Mid.

B No tasks run during the delay.

C All three tasks run concurrently during the delay.

D Two tasks run concurrently during the delay: Mid and Low.

💡 Hint

Remember that vTaskDelay makes the task blocked, allowing lower priority tasks to run.

When TaskHigh delays, it is blocked. TaskMid (priority 3) runs next and never blocks or yields, starving TaskLow (priority 2).