Static vs dynamic allocation (configSUPPORT_STATIC_ALLOCATION) in FreeRTOS - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS task creation code?

Consider this FreeRTOS code snippet where configSUPPORT_STATIC_ALLOCATION is set to 1. What will be the output when the task is created?

FreeRTOS

StaticTask_t xTaskBuffer;
StackType_t xStack[100];

void vTaskCode(void * pvParameters) {
    for(;;) {}
}

int main() {
    TaskHandle_t xHandle = xTaskCreateStatic(
        vTaskCode,          // Task function
        "Task1",           // Name
        100,                // Stack size
        NULL,               // Parameters
        1,                  // Priority
        xStack,             // Stack buffer
        &xTaskBuffer        // Task buffer
    );
    if (xHandle != NULL) {
        printf("Task created successfully\n");
    } else {
        printf("Task creation failed\n");
    }
    return 0;
}

A Task created successfully

B Task creation failed

C Compilation error due to missing heap

D Runtime error due to NULL stack pointer

💡 Hint

Static allocation requires providing buffers explicitly. Check if buffers are correctly passed.

When configSUPPORT_STATIC_ALLOCATION is enabled, xTaskCreateStatic creates a task using the provided stack and task buffers. Since buffers are valid, the task is created successfully.

❓ Predict Output

intermediate

What happens if configSUPPORT_STATIC_ALLOCATION is 0 and xTaskCreateStatic is called?

Assuming configSUPPORT_STATIC_ALLOCATION is set to 0 in FreeRTOSConfig.h, what will happen if the following code is run?

FreeRTOS

StaticTask_t xTaskBuffer;
StackType_t xStack[100];

TaskHandle_t xHandle = xTaskCreateStatic(
    vTaskCode,
    "Task1",
    100,
    NULL,
    1,
    xStack,
    &xTaskBuffer
);

A Compilation error because xTaskCreateStatic is undefined

B Task is created successfully using static buffers

C Runtime error due to NULL heap

D Task is created dynamically ignoring static buffers

💡 Hint

Check if the static allocation API is available when the config option is disabled.

If configSUPPORT_STATIC_ALLOCATION is 0, the static allocation API xTaskCreateStatic is not compiled in, causing a compilation error.

🔧 Debug

advanced

Why does this statically allocated task crash at runtime?

Given this FreeRTOS task creation code with static allocation enabled, the system crashes at runtime. Identify the cause.

FreeRTOS

StaticTask_t *pxTaskBuffer;
StackType_t *pxStackBuffer;

void createTask() {
    TaskHandle_t xHandle = xTaskCreateStatic(
        vTaskCode,
        "TaskCrash",
        100,
        NULL,
        1,
        pxStackBuffer,
        pxTaskBuffer
    );
    if (xHandle == NULL) {
        printf("Task creation failed\n");
    }
}

int main() {
    createTask();
    vTaskStartScheduler();
    return 0;
}

A Task priority is invalid causing scheduler failure

B Stack size is too small causing overflow

C pxTaskBuffer and pxStackBuffer are uninitialized pointers causing invalid memory access

D vTaskStartScheduler is called before task creation

💡 Hint

Check if the buffers passed to xTaskCreateStatic are valid memory locations.

The pointers pxTaskBuffer and pxStackBuffer are declared but never assigned valid memory. Passing them to xTaskCreateStatic causes undefined behavior and crashes.

🧠 Conceptual

advanced

Which statement about static vs dynamic allocation in FreeRTOS is TRUE?

Choose the correct statement regarding static and dynamic allocation in FreeRTOS when configSUPPORT_STATIC_ALLOCATION is enabled.

A Dynamic allocation is only possible if static allocation is disabled

B Static allocation requires the user to provide memory buffers for task control blocks and stacks

C Static allocation automatically allocates memory from the heap at runtime

D Dynamic allocation requires the user to provide buffers explicitly

💡 Hint

Think about who manages memory in static vs dynamic allocation.

Static allocation means the user provides memory buffers for tasks, while dynamic allocation uses the heap managed by FreeRTOS. Both can be enabled independently.

❓ Predict Output

expert

What is the number of items in the task list after these mixed allocations?

Assuming configSUPPORT_STATIC_ALLOCATION is 1 and configSUPPORT_DYNAMIC_ALLOCATION is 1, consider this code:

TaskHandle_t h1, h2;

StaticTask_t xTaskBuffer;
StackType_t xStack[50];

h1 = xTaskCreateStatic(vTaskCode, "StaticTask", 50, NULL, 1, xStack, &xTaskBuffer);
h2 = NULL;
xTaskCreate(vTaskCode, "DynamicTask", 50, NULL, 1, &h2);

// After both tasks are created, how many tasks exist in the scheduler?

A 3

B 1

C 0

D 2

💡 Hint

Both static and dynamic task creation APIs add tasks to the scheduler.

Both xTaskCreateStatic and xTaskCreate create tasks that are added to the scheduler. So after both calls, there are 2 tasks.