ISR-safe API functions (FromISR suffix) in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of xQueueSendFromISR usage

What is the output of this code snippet when called inside an ISR?

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
QueueHandle_t xQueue = xQueueCreate(1, sizeof(int));
int value = 10;
BaseType_t result = xQueueSendFromISR(xQueue, &value, &xHigherPriorityTaskWoken);
printf("Result: %d, Woken: %d\n", result, xHigherPriorityTaskWoken);

FreeRTOS

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
QueueHandle_t xQueue = xQueueCreate(1, sizeof(int));
int value = 10;
BaseType_t result = xQueueSendFromISR(xQueue, &value, &xHigherPriorityTaskWoken);
printf("Result: %d, Woken: %d\n", result, xHigherPriorityTaskWoken);

A Result: 1, Woken: 1

B Result: 0, Woken: 1

C Result: 1, Woken: 0

D Result: 0, Woken: 0

💡 Hint

Remember that xQueueSendFromISR returns pdTRUE (1) on success and pdFALSE (0) on failure. The Woken flag is set only if a higher priority task was unblocked.

The queue is empty and has space, so xQueueSendFromISR returns pdTRUE (1). Since no higher priority task was waiting, xHigherPriorityTaskWoken remains pdFALSE (0).

🧠 Conceptual

intermediate

Purpose of the FromISR suffix in FreeRTOS API

What is the main reason FreeRTOS provides API functions with the FromISR suffix?

A They allow blocking calls inside ISRs.

B They are optimized for use in interrupt service routines and do not block.

C They disable all interrupts globally.

D They automatically create tasks from ISRs.

💡 Hint

Think about what restrictions exist inside ISRs.

Functions with the FromISR suffix are designed to be safe to call from ISRs. They do not block and handle critical sections differently to avoid issues in interrupt context.

🔧 Debug

advanced

Identify the error in this ISR-safe queue send code

What error will occur when running this code inside an ISR?

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
int value = 5;
xQueueSend(xQueue, &value, 0);

A Compile error: Missing FromISR suffix function.

B No error; code runs correctly.

C Queue overflow error.

D Runtime error: Blocking call inside ISR.

💡 Hint

Consider what happens if you call a normal API function inside an ISR.

xQueueSend is not safe to call inside an ISR because it may block. Calling it inside an ISR can cause runtime errors or unpredictable behavior.

📝 Syntax

advanced

Correct usage of xSemaphoreGiveFromISR

Which option correctly uses xSemaphoreGiveFromISR to give a semaphore inside an ISR?

A xSemaphoreGiveFromISR(xSemaphore, &xHigherPriorityTaskWoken);

B xSemaphoreGiveFromISR(&xSemaphore, &xHigherPriorityTaskWoken);

C xSemaphoreGiveFromISR(xSemaphore);

D xSemaphoreGiveFromISR(xSemaphore, NULL);

💡 Hint

Check the function signature and parameters carefully.

xSemaphoreGiveFromISR requires a pointer to the semaphore handle and a pointer to a BaseType_t variable to indicate if a higher priority task was woken.

🚀 Application

expert

Determining task switch necessity after ISR

Inside an ISR, you call xQueueSendFromISR and the xHigherPriorityTaskWoken flag is set to pdTRUE. What should you do next to ensure the higher priority task runs immediately after the ISR?

A Call portYIELD_FROM_ISR(xHigherPriorityTaskWoken);

B Call vTaskDelay(1);

C Do nothing; the scheduler will switch tasks automatically.

D Call xTaskResumeAll();

💡 Hint

Think about how to request a context switch from an ISR.

When xHigherPriorityTaskWoken is pdTRUE, you must call portYIELD_FROM_ISR to request a context switch immediately after the ISR ends.