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.

Practice

1.

What does the FreeRTOS function vTaskPrioritySet() do?

easy

A. It changes the priority of a task while the program is running.

B. It creates a new task with a fixed priority.

C. It deletes a task permanently.

D. It suspends a task temporarily.

Solution

1. Step 1: Understand the function purpose

   vTaskPrioritySet() is designed to change the priority of an existing task dynamically during runtime.

2. Step 2: Compare with other task functions

   Creating, deleting, or suspending tasks are done by different functions, not vTaskPrioritySet().

3. Final Answer:

   It changes the priority of a task while the program is running. -> Option A

4. Quick Check:

   vTaskPrioritySet() changes priority [OK]

Hint: Remember: vTaskPrioritySet changes priority dynamically [OK]

Common Mistakes:

• Confusing vTaskPrioritySet with task creation functions
• Thinking it deletes or suspends tasks
• Assuming priority is fixed at task creation

2.

Which of the following is the correct syntax to change a task's priority using vTaskPrioritySet()?

TaskHandle_t xTaskHandle;
// Change priority to 3
_____;

easy

A. vTaskPrioritySetPriority(xTaskHandle, 3);

B. vTaskPrioritySet(xTaskHandle, 3);

C. vTaskPrioritySet(3, xTaskHandle);

D. vTaskSetPriority(xTaskHandle, 3);

Solution

1. Step 1: Recall the function signature

   The correct function is vTaskPrioritySet(TaskHandle_t xTask, UBaseType_t uxNewPriority), so the task handle comes first, then the new priority.

2. Step 2: Check the options

   vTaskPrioritySet(xTaskHandle, 3); matches the correct syntax exactly. Options A, B, and D either swap parameters or use incorrect function names.

3. Final Answer:

   vTaskPrioritySet(xTaskHandle, 3); -> Option B

4. Quick Check:

   Correct syntax = vTaskPrioritySet(task, priority) [OK]

Hint: Function name and parameter order matter: task handle first [OK]

Common Mistakes:

• Swapping the order of parameters
• Using incorrect function names
• Passing priority before task handle

3.

Consider the following code snippet:

TaskHandle_t xTask1;
// Assume xTask1 is created with priority 2
vTaskPrioritySet(xTask1, 5);
UBaseType_t newPriority = uxTaskPriorityGet(xTask1);
printf("%u", newPriority);

What will be printed?

medium

A. 0

B. 2

C. 5

D. Compilation error

Solution

1. Step 1: Understand priority change

   The call vTaskPrioritySet(xTask1, 5) changes the task's priority from 2 to 5.

2. Step 2: Check the priority retrieval

   uxTaskPriorityGet(xTask1) returns the current priority, which is now 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Priority updated to 5, so output is 5 [OK]

Hint: Priority changes immediately, so get returns new value [OK]

Common Mistakes:

• Assuming priority stays the same after set
• Confusing priority with task handle
• Thinking printf prints task handle

4.

What is wrong with this code snippet?

TaskHandle_t xTask2 = NULL;
vTaskPrioritySet(xTask2, 4);

medium

A. The function vTaskPrioritySet returns a value that is ignored.

B. Priority 4 is invalid; priorities must be 0 or 1 only.

C. vTaskPrioritySet requires a pointer to the task, not the handle.

D. xTask2 is NULL, so setting priority causes undefined behavior.

Solution

1. Step 1: Check the task handle value

   The variable xTask2 is set to NULL, meaning it does not point to a valid task.

2. Step 2: Understand effect of NULL handle

   Calling vTaskPrioritySet() with a NULL handle leads to undefined behavior or runtime error because no valid task exists to change priority.

3. Final Answer:

   xTask2 is NULL, so setting priority causes undefined behavior. -> Option D

4. Quick Check:

   NULL task handle causes error [OK]

Hint: Never pass NULL as task handle to vTaskPrioritySet [OK]

Common Mistakes:

• Ignoring NULL task handle
• Assuming priority range is only 0 or 1
• Confusing handle with pointer to handle

5.

You have two tasks: TaskA with priority 3 and TaskB with priority 5. You want TaskA to temporarily run with higher priority than TaskB during a critical section. Which code snippet correctly achieves this?

TaskHandle_t xTaskA, xTaskB;
// Initially priorities: TaskA=3, TaskB=5
// Critical section starts
_____;
// Critical section ends
vTaskPrioritySet(xTaskA, 3);

hard

A. vTaskPrioritySet(xTaskA, 6);

B. vTaskPrioritySet(xTaskB, 2);

C. vTaskPrioritySet(xTaskA, 5);

D. vTaskPrioritySet(xTaskB, 3);

Solution

1. Step 1: Identify priority requirements

   TaskA must have higher priority than TaskB (which is 5) during the critical section.

2. Step 2: Choose correct priority change

   Setting TaskA to 6 makes it higher than TaskB's 5. Options A and D lower TaskB's priority instead. vTaskPrioritySet(xTaskA, 5); sets TaskA to 5, equal to TaskB, not higher.

3. Step 3: Restore original priority after critical section

   After the critical section, TaskA's priority is reset to 3 as shown.

4. Final Answer:

   vTaskPrioritySet(xTaskA, 6); -> Option A

5. Quick Check:

   TaskA priority > TaskB priority during critical section [OK]

Hint: Raise TaskA priority above TaskB, then restore after [OK]

Common Mistakes:

• Setting TaskA priority equal to TaskB
• Lowering TaskB priority instead of raising TaskA
• Not restoring original priority after critical section