ISR-safe API functions (FromISR suffix) in FreeRTOS - Time & Space Complexity

Time Complexity: ISR-safe API functions (FromISR suffix)

O(1)

Understanding Time Complexity

When using ISR-safe API functions in FreeRTOS, it's important to know how their execution time changes as input grows.

We want to see how the cost of these functions scales when called from interrupts.

Scenario Under Consideration

Analyze the time complexity of this ISR-safe queue send function.

void vExampleISR(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    int data = 42;
    xQueueSendFromISR(xQueue, &data, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}
    

This code sends data to a queue safely from an interrupt service routine.

Identify Repeating Operations

Look for loops or repeated steps inside the ISR-safe function.

• Primary operation: The queue send checks and updates queue state.
• How many times: It runs a fixed number of steps, no loops over input size.

How Execution Grows With Input

The function's work stays about the same no matter how many items are in the queue.

Input Size (n)	Approx. Operations
10	Constant steps
100	Constant steps
1000	Constant steps

Pattern observation: The execution time stays constant regardless of input size.

Final Time Complexity

Time Complexity: O(1)

This means the function takes about the same time no matter how big the queue is.

Common Mistake

[X] Wrong: "The ISR-safe queue send takes longer if the queue has more items."

[OK] Correct: The function only updates pointers and flags, not scanning all items, so time stays constant.

Interview Connect

Understanding how ISR-safe functions behave helps you write reliable real-time code and explain your choices clearly.

Self-Check

"What if the ISR-safe function had to copy multiple items instead of one? How would the time complexity change?"

Practice

1.

What is the main reason to use FreeRTOS API functions with the FromISR suffix inside an interrupt service routine (ISR)?

easy

A. They disable all interrupts during execution.

B. They are safe to call from ISRs and handle interrupt context correctly.

C. They run tasks faster than normal API functions.

D. They automatically create new tasks inside the ISR.

Solution

1. Step 1: Understand ISR context constraints

   Inside an ISR, normal FreeRTOS API functions are unsafe because they may block or cause race conditions.

2. Step 2: Role of FromISR functions

   Functions with the FromISR suffix are designed to be safe in interrupt context by avoiding blocking and managing critical sections properly.

3. Final Answer:

   They are safe to call from ISRs and handle interrupt context correctly. -> Option B

4. Quick Check:

   FromISR functions = ISR safe calls [OK]

Hint: Use FromISR functions only inside interrupts for safety [OK]

Common Mistakes:

• Calling normal API functions inside ISR causing crashes
• Assuming FromISR functions disable interrupts
• Thinking FromISR creates tasks automatically

2.

Which of the following is the correct way to declare a variable to track if a higher priority task was woken inside an ISR?

?

easy

A. int higherPriorityTaskWoken = 0;

B. bool higherPriorityTaskWoken = false;

C. BaseType_t higherPriorityTaskWoken = pdFALSE;

D. volatile int higherPriorityTaskWoken = 1;

Solution

1. Step 1: Identify FreeRTOS type for ISR task wake flag

   FreeRTOS uses BaseType_t for portability and defines pdFALSE and pdTRUE for boolean values.

2. Step 2: Correct initialization

   The variable must start as pdFALSE to indicate no task was woken yet.

3. Final Answer:

   BaseType_t higherPriorityTaskWoken = pdFALSE; -> Option C

4. Quick Check:

   Use BaseType_t and pdFALSE for ISR flags [OK]

Hint: Use BaseType_t and pdFALSE to track task wake in ISR [OK]

Common Mistakes:

• Using plain int instead of BaseType_t
• Initializing flag to 1 instead of pdFALSE
• Using bool which may not be portable

3.

Consider this ISR code snippet:

BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken);
if (higherPriorityTaskWoken) {
    portYIELD_FROM_ISR();
}

What happens if higherPriorityTaskWoken is set to pdTRUE after xQueueSendFromISR?

medium

A. The ISR requests a context switch to the higher priority task immediately after ISR.

B. The ISR disables all interrupts until the higher priority task runs.

C. The queue send operation fails silently.

D. The higher priority task is blocked until the ISR finishes.

Solution

1. Step 1: Understand higherPriorityTaskWoken flag

   This flag indicates if a task with higher priority than the current one was made ready by the ISR.

2. Step 2: Role of portYIELD_FROM_ISR()

   If the flag is true, calling portYIELD_FROM_ISR() requests a context switch to that higher priority task immediately after ISR exit.

3. Final Answer:

   The ISR requests a context switch to the higher priority task immediately after ISR. -> Option A

4. Quick Check:

   higherPriorityTaskWoken true triggers context switch [OK]

Hint: If higherPriorityTaskWoken true, call portYIELD_FROM_ISR() [OK]

Common Mistakes:

• Thinking interrupts are disabled by portYIELD_FROM_ISR()
• Assuming queue send fails if flag is true
• Believing higher priority task waits for ISR to finish

4.

Identify the error in this ISR code snippet:

void ISR_Handler(void) {
    BaseType_t higherPriorityTaskWoken = pdFALSE;
    xSemaphoreGive(semaphore);
    if (higherPriorityTaskWoken) {
        portYIELD_FROM_ISR();
    }
}

What is the problem here?

medium

A. The variable higherPriorityTaskWoken should be global.

B. The semaphore is given twice causing overflow.

C. The portYIELD_FROM_ISR() call is outside the ISR.

D. The higherPriorityTaskWoken variable is not passed to xSemaphoreGiveFromISR.

Solution

1. Step 1: Check function used for semaphore give

   The code uses xSemaphoreGive, which is not safe inside ISR context.

2. Step 2: Correct ISR-safe function usage

   Inside ISR, xSemaphoreGiveFromISR must be used with higherPriorityTaskWoken passed by reference to track task wake.

3. Final Answer:

   The higherPriorityTaskWoken variable is not passed to xSemaphoreGiveFromISR. -> Option D

4. Quick Check:

   Use FromISR functions with higherPriorityTaskWoken [OK]

Hint: Always pass higherPriorityTaskWoken to FromISR functions [OK]

Common Mistakes:

• Using normal API functions inside ISR
• Not passing higherPriorityTaskWoken pointer
• Declaring higherPriorityTaskWoken as global unnecessarily

5.

You want to send data from an ISR to a task using a queue. Which sequence correctly uses FreeRTOS ISR-safe API functions to do this and ensure the higher priority task runs immediately if woken?

?

hard

A. BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken);
if (higherPriorityTaskWoken) portYIELD_FROM_ISR();

B. xQueueSend(queue, &data, 0);
BaseType_t higherPriorityTaskWoken = pdFALSE;
if (higherPriorityTaskWoken) portYIELD_FROM_ISR();

C. BaseType_t higherPriorityTaskWoken = pdTRUE;
xQueueSendFromISR(queue, &data, NULL);
portYIELD_FROM_ISR();

D. BaseType_t higherPriorityTaskWoken = pdFALSE;
xQueueSend(queue, &data, 0);
portYIELD_FROM_ISR();

Solution

1. Step 1: Use ISR-safe queue send function

   Inside ISR, xQueueSendFromISR must be used with a pointer to higherPriorityTaskWoken.

2. Step 2: Check higherPriorityTaskWoken and yield

   If higherPriorityTaskWoken is set to pdTRUE, call portYIELD_FROM_ISR() to request immediate context switch.

3. Final Answer:

   BaseType_t higherPriorityTaskWoken = pdFALSE; xQueueSendFromISR(queue, &data, &higherPriorityTaskWoken); if (higherPriorityTaskWoken) portYIELD_FROM_ISR(); -> Option A

4. Quick Check:

   Use FromISR + check flag + yield [OK]

Hint: Use xQueueSendFromISR with flag and yield if true [OK]

Common Mistakes:

• Using normal xQueueSend inside ISR
• Not passing higherPriorityTaskWoken pointer
• Calling portYIELD_FROM_ISR unconditionally