Queue from ISR (xQueueSendFromISR) in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of xQueueSendFromISR with full queue

What will be the output of the following code snippet when the queue is already full?

FreeRTOS

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

QueueHandle_t queue;
BaseType_t xHigherPriorityTaskWoken = pdFALSE;

void ISR_Handler() {
    int value = 10;
    BaseType_t result = xQueueSendFromISR(queue, &value, &xHigherPriorityTaskWoken);
    if(result == pdPASS) {
        printf("Sent\n");
    } else {
        printf("Failed\n");
    }
}

int main() {
    queue = xQueueCreate(1, sizeof(int));
    int initial = 5;
    xQueueSend(queue, &initial, 0); // Fill the queue
    ISR_Handler();
    return 0;
}

A Failed

B No output

C Compilation error

D Sent

💡 Hint

Think about what happens when you try to send to a full queue from an ISR.

xQueueSendFromISR returns errQUEUE_FULL if the queue is full and cannot accept new data. Since the queue size is 1 and already contains an item, the send fails and prints "Failed".

🧠 Conceptual

intermediate

Understanding the xHigherPriorityTaskWoken parameter

What is the purpose of the xHigherPriorityTaskWoken parameter in xQueueSendFromISR?

A It indicates if a higher priority task was woken by the send operation.

B It sets the priority of the task sending the data.

C It disables interrupts during the send operation.

D It specifies the maximum queue length.

💡 Hint

Consider what happens if sending to the queue unblocks a higher priority task.

xHigherPriorityTaskWoken is set to pdTRUE by xQueueSendFromISR if sending data to the queue causes a higher priority task to unblock. This allows the ISR to request a context switch after the ISR completes.

🔧 Debug

advanced

Why does xQueueSendFromISR cause a runtime error?

Given this ISR code snippet, why does the system crash at runtime? void ISR_Handler() { int value = 20; BaseType_t result = xQueueSendFromISR(NULL, &value, NULL); if(result == pdPASS) { // do something } }

A xQueueSendFromISR cannot be called from an ISR.

B Passing NULL as the queue handle causes a null pointer dereference.

C The value pointer must be volatile.

D xQueueSendFromISR requires a non-NULL xHigherPriorityTaskWoken pointer.

💡 Hint

Check the parameters passed to xQueueSendFromISR carefully.

Passing NULL as the queue handle causes the function to dereference a null pointer, leading to a runtime crash.

📝 Syntax

advanced

Identify the syntax error in xQueueSendFromISR usage

Which option contains a syntax error in the usage of xQueueSendFromISR?

FreeRTOS

QueueHandle_t queue = xQueueCreate(5, sizeof(int));
int data = 42;
BaseType_t xHigherPriorityTaskWoken = pdFALSE;

A ;)nekoWksaTytiroirPrehgiHx& ,atad& ,eueuq(RSImorFdneSeueuQx

B xQueueSendFromISR(queue, &data, &xHigherPriorityTaskWoken);

C xQueueSendFromISR(queue, &data, NULL);

D xQueueSendFromISR(queue, data, &xHigherPriorityTaskWoken);

💡 Hint

Check the type of the second argument expected by xQueueSendFromISR.

The second argument must be a pointer to the data to send. Option D passes data by value, causing a syntax or type error.

🚀 Application

expert

Determining the number of items in the queue after ISR sends

Assume a queue of length 3 is initially empty. The following ISR is called three times in quick succession without the main task removing items: void ISR_Handler() { static int count = 0; BaseType_t xHigherPriorityTaskWoken = pdFALSE; xQueueSendFromISR(queue, &count, &xHigherPriorityTaskWoken); count++; } After these three ISR calls, how many items are in the queue?

A 2

B 1

C 3

D 0

💡 Hint

Consider if the queue can hold all three items without removal.

The queue has space for 3 items and starts empty. Each ISR call sends one item successfully, so after three calls, the queue holds 3 items.

Practice

1.

What is the primary purpose of xQueueSendFromISR() in FreeRTOS?

easy

A. To send data to a queue safely from an interrupt service routine (ISR)

B. To create a new queue in the system

C. To delete a queue from an ISR

D. To block a task until data is received

Solution

1. Step 1: Understand the function context

   xQueueSendFromISR() is designed to be called inside an ISR, where normal queue functions that can block are unsafe.

2. Step 2: Identify the function's role

   It safely sends data to a queue without blocking, allowing communication from ISR to tasks.

3. Final Answer:

   To send data to a queue safely from an interrupt service routine (ISR) -> Option A

4. Quick Check:

   Purpose of xQueueSendFromISR = safe queue send from ISR [OK]

Hint: Remember: xQueueSendFromISR is for sending data inside interrupts [OK]

Common Mistakes:

• Confusing it with queue creation or deletion
• Thinking it blocks the ISR
• Using normal queue send functions inside ISR

2.

Which of the following is the correct way to call xQueueSendFromISR()?

BaseType_t xQueueSendFromISR(QueueHandle_t xQueue, const void * pvItemToQueue, BaseType_t *pxHigherPriorityTaskWoken);

easy

A. xQueueSendFromISR(queue, data, NULL);

B. xQueueSendFromISR(queue, &data, NULL);

C. xQueueSendFromISR(&queue, &data, NULL);

D. xQueueSendFromISR(queue, &data);

Solution

1. Step 1: Check parameter types

   The function expects a queue handle, a pointer to the data, and a pointer to a BaseType_t variable for task wake flag.

2. Step 2: Match correct call

   xQueueSendFromISR(queue, &data, NULL); passes the queue handle, address of data, and NULL for the last parameter, which is valid if you don't care about task wake.

3. Final Answer:

   xQueueSendFromISR(queue, &data, NULL); -> Option B

4. Quick Check:

   Correct parameters and pointers used = xQueueSendFromISR(queue, &data, NULL); [OK]

Hint: Always pass data address and pxHigherPriorityTaskWoken pointer [OK]

Common Mistakes:

• Passing data instead of its address
• Passing queue address instead of handle
• Omitting the third parameter

3.

Given the following code snippet inside an ISR, what will be the value of result after execution?

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
int data = 10;
BaseType_t result = xQueueSendFromISR(myQueue, &data, &xHigherPriorityTaskWoken);

Assume myQueue has space available.

medium

A. pdFAIL (0)

B. pdFALSE (0)

C. pdTRUE (1)

D. pdPASS (1)

Solution

1. Step 1: Understand return values of xQueueSendFromISR

   The function returns pdPASS (1) if the item was successfully sent to the queue, pdFAIL (0) otherwise.

2. Step 2: Analyze queue state

   Since myQueue has space, the send will succeed, so result will be pdPASS (1).

3. Final Answer:

   pdPASS (1) -> Option D

4. Quick Check:

   Queue has space, send success = pdPASS (1) [OK]

Hint: If queue has space, xQueueSendFromISR returns pdPASS (1) [OK]

Common Mistakes:

• Confusing pdPASS with pdTRUE
• Assuming it returns pdFALSE on success
• Ignoring queue capacity

4.

Identify the error in this ISR code snippet using xQueueSendFromISR():

void ISR_Handler(void) {
    int value = 5;
    BaseType_t xHigherPriorityTaskWoken;
    xQueueSendFromISR(myQueue, &value, &xHigherPriorityTaskWoken);
    if(xHigherPriorityTaskWoken) {
        portYIELD_FROM_ISR();
    }
}

medium

A. value should be a pointer, not an int

B. portYIELD_FROM_ISR() is called incorrectly

C. xHigherPriorityTaskWoken is not initialized before use

D. xQueueSendFromISR() cannot be called inside ISR

Solution

1. Step 1: Check variable initialization

   xHigherPriorityTaskWoken must be initialized to pdFALSE before calling xQueueSendFromISR() to avoid unpredictable behavior.

2. Step 2: Verify other parts

   Calling portYIELD_FROM_ISR() conditionally is correct; value is passed by address; calling xQueueSendFromISR() inside ISR is valid.

3. Final Answer:

   xHigherPriorityTaskWoken is not initialized before use -> Option C

4. Quick Check:

   Initialize xHigherPriorityTaskWoken = pdFALSE before use [OK]

Hint: Always init xHigherPriorityTaskWoken to pdFALSE before sending [OK]

Common Mistakes:

• Not initializing xHigherPriorityTaskWoken
• Misusing portYIELD_FROM_ISR
• Passing data incorrectly

5.

You want to send multiple sensor readings from an ISR to a task using xQueueSendFromISR(). Which approach correctly ensures the task runs immediately if it has higher priority?

void SensorISR(void) {
    static int sensorData = 0;
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    sensorData++;
    xQueueSendFromISR(sensorQueue, &sensorData, &xHigherPriorityTaskWoken);
    // What should be done here?
}

hard

A. Call portYIELD_FROM_ISR(xHigherPriorityTaskWoken); after sending

B. Call vTaskDelay(1); inside the ISR

C. Call taskYIELD(); inside the ISR unconditionally

D. Do nothing; the task will run eventually

Solution

1. Step 1: Understand task wake mechanism

   After sending data from ISR, if a higher priority task was woken, you must request a context switch immediately.

2. Step 2: Use correct yield function

   portYIELD_FROM_ISR(xHigherPriorityTaskWoken); triggers a context switch only if needed, which is the correct approach.

3. Final Answer:

   Call portYIELD_FROM_ISR(xHigherPriorityTaskWoken); after sending -> Option A

4. Quick Check:

   Use portYIELD_FROM_ISR with xHigherPriorityTaskWoken to yield [OK]

Hint: Use portYIELD_FROM_ISR with flag to yield only if needed [OK]

Common Mistakes:

• Calling blocking functions like vTaskDelay in ISR
• Calling taskYIELD unconditionally inside ISR
• Ignoring portYIELD_FROM_ISR call