Task priority assignment in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS task priority code?

Consider the following FreeRTOS code snippet that creates two tasks with different priorities. What will be the output printed by the tasks?

FreeRTOS

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

void Task1(void *pvParameters) {
    printf("Task1 running with priority %lu\n", (unsigned long)uxTaskPriorityGet(NULL));
    vTaskDelete(NULL);
}

void Task2(void *pvParameters) {
    printf("Task2 running with priority %lu\n", (unsigned long)uxTaskPriorityGet(NULL));
    vTaskDelete(NULL);
}

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

A Task2 running with priority 2\nTask1 running with priority 3

B Task1 running with priority 2\nTask2 running with priority 3

C Task2 running with priority 3\nTask1 running with priority 2

D Task1 running with priority 3\nTask2 running with priority 2

💡 Hint

Remember that FreeRTOS runs the highest priority task that is ready first.

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

🧠 Conceptual

intermediate

Which FreeRTOS API sets the priority of a running task?

In FreeRTOS, which function is used to change the priority of a task that is already running?

A vTaskPrioritySet(TaskHandle_t xTask, UBaseType_t uxNewPriority)

B vTaskDelete(TaskHandle_t xTask)

C xTaskCreate(TaskFunction_t pxTaskCode, const char * const pcName, uint16_t usStackDepth, void *pvParameters, UBaseType_t uxPriority, TaskHandle_t *pxCreatedTask)

D uxTaskPriorityGet(TaskHandle_t xTask)

💡 Hint

Look for the function that explicitly mentions setting priority.

vTaskPrioritySet changes the priority of a specified task at runtime.

🔧 Debug

advanced

Why does this FreeRTOS task priority code cause starvation?

Examine the code below. Why does Task1 never get to run?

FreeRTOS

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

void Task1(void *pvParameters) {
    while(1) {
        printf("Task1 running\n");
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

void Task2(void *pvParameters) {
    while(1) {
        printf("Task2 running\n");
    }
}

int main(void) {
    xTaskCreate(Task1, "Task1", 1000, NULL, 1, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 2, NULL);
    vTaskStartScheduler();
    return 0;
}

A Task1 deletes itself immediately, so it never runs.

B Task1 has higher priority but calls vTaskDelay incorrectly.

C Both tasks have the same priority causing a deadlock.

D Task2 has higher priority and never blocks, so Task1 starves.

💡 Hint

Think about task priorities and blocking behavior.

Task2 has priority 2 and never blocks, so it always runs. Task1 with priority 1 never gets CPU time.

📝 Syntax

advanced

Identify the syntax error in this FreeRTOS task creation code

Which option contains the correct syntax to create a task with priority 5?

A xTaskCreate(TaskFunction, Task, 1000, NULL, 5, NULL);

B xTaskCreate(TaskFunction, "Task", 1000, NULL, 5, NULL);

C xTaskCreate(TaskFunction, "Task", 1000, NULL, "5", NULL);

D xTaskCreate(TaskFunction, "Task", 1000, NULL, 5);

💡 Hint

Check the parameter types and count.

Option B uses correct string for task name, numeric priority, and correct number of parameters.

🚀 Application

expert

How to dynamically change task priority to avoid starvation?

You have two tasks: TaskA with priority 3 and TaskB with priority 5. TaskB runs continuously and causes TaskA starvation. Which approach correctly changes TaskB's priority dynamically to allow TaskA to run?

A Use vTaskPrioritySet(NULL, 2) inside TaskB to lower its priority temporarily.

B Use vTaskPrioritySet(TaskAHandle, 6) to increase TaskA priority above TaskB.

C Delete TaskB and recreate it with lower priority.

D Use vTaskSuspend(TaskBHandle) to pause TaskB permanently.

💡 Hint

Think about temporarily lowering the running task's priority.

Lowering TaskB's priority temporarily allows TaskA to run without stopping TaskB permanently.