vTaskPrioritySet() dynamic priority in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS task priority change?

Consider the following FreeRTOS code snippet where a task's priority is changed dynamically. What will be the printed output?

FreeRTOS

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

void vTaskFunction(void *pvParameters) {
    UBaseType_t initialPriority = uxTaskPriorityGet(NULL);
    printf("Initial Priority: %u\n", (unsigned int)initialPriority);
    vTaskPrioritySet(NULL, initialPriority + 2);
    UBaseType_t newPriority = uxTaskPriorityGet(NULL);
    printf("New Priority: %u\n", (unsigned int)newPriority);
    vTaskDelete(NULL);
}

int main(void) {
    TaskHandle_t xHandle = NULL;
    xTaskCreate(vTaskFunction, "Task", configMINIMAL_STACK_SIZE, NULL, 3, &xHandle);
    vTaskStartScheduler();
    return 0;
}

A

Initial Priority: 3
New Priority: 5

B

Initial Priority: 3
New Priority: 3

C

Initial Priority: 0
New Priority: 2

D

Initial Priority: 5
New Priority: 7

💡 Hint

Remember that vTaskPrioritySet() changes the priority of the task immediately.

The task is created with priority 3. The call to vTaskPrioritySet() increases it by 2, so the new priority is 5.

🧠 Conceptual

intermediate

What happens if you set a task priority higher than configMAX_PRIORITIES?

In FreeRTOS, what is the behavior when vTaskPrioritySet() is called with a priority value greater than configMAX_PRIORITIES - 1?

A The system crashes with a runtime error.

B The priority wraps around to zero.

C The priority is set to the maximum allowed priority, <code>configMAX_PRIORITIES - 1</code>.

D The priority is set to the requested value without any limit.

💡 Hint

FreeRTOS limits task priorities to a maximum defined by configMAX_PRIORITIES.

FreeRTOS clamps the priority to the maximum allowed value if a higher value is requested.

🔧 Debug

advanced

Why does this code cause a runtime error when changing task priority?

Examine the code below. It attempts to change the priority of a task but causes a runtime error. What is the cause?

FreeRTOS

void vTaskFunction(void *pvParameters) {
    vTaskPrioritySet(NULL, 10);
    for(;;) {}
}

int main(void) {
    xTaskCreate(vTaskFunction, "Task", configMINIMAL_STACK_SIZE, NULL, 5, NULL);
    vTaskStartScheduler();
    return 0;
}

A vTaskStartScheduler() must be called before creating tasks.

B vTaskPrioritySet() cannot be called from within a task function.

C The task stack size is too small causing stack overflow.

D Priority 10 exceeds configMAX_PRIORITIES, causing invalid priority assignment.

💡 Hint

Check the maximum allowed priority in FreeRTOS configuration.

Setting a priority higher than configMAX_PRIORITIES - 1 is invalid and can cause runtime errors.

📝 Syntax

advanced

Which option correctly changes a task's priority dynamically?

Given a task handle xTaskHandle, which code snippet correctly changes its priority to 4?

A vTaskPrioritySet(xTaskHandle, 4);

B vTaskPrioritySet(&xTaskHandle, 4);

C vTaskPrioritySet(4, xTaskHandle);

D vTaskPrioritySet(xTaskHandle, &4);

💡 Hint

Check the function signature: void vTaskPrioritySet(TaskHandle_t xTask, UBaseType_t uxNewPriority);

The first argument is the task handle, the second is the new priority value.

🚀 Application

expert

How many tasks have priority 3 after this code runs?

Given the following code, how many tasks will have priority 3 after execution?

FreeRTOS

TaskHandle_t xTask1, xTask2, xTask3;

void vTask1(void *pvParameters) { for(;;) {} }
void vTask2(void *pvParameters) { for(;;) {} }
void vTask3(void *pvParameters) { for(;;) {} }

int main(void) {
    xTaskCreate(vTask1, "Task1", configMINIMAL_STACK_SIZE, NULL, 2, &xTask1);
    xTaskCreate(vTask2, "Task2", configMINIMAL_STACK_SIZE, NULL, 3, &xTask2);
    xTaskCreate(vTask3, "Task3", configMINIMAL_STACK_SIZE, NULL, 1, &xTask3);

    vTaskPrioritySet(xTask1, 3);
    vTaskPrioritySet(xTask3, 3);

    vTaskStartScheduler();
    return 0;
}

A 2

B 3

C 1

D 0

💡 Hint

Check which tasks had their priority changed to 3.

Initially, Task2 has priority 3. Then Task1 and Task3 priorities are set to 3. So total tasks with priority 3 are 3.

But Task2 was already priority 3, so after changes, Task1 and Task3 also have priority 3, making 3 tasks total.