Queue set for monitoring multiple queues in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a queue set with space for 3 queues.

FreeRTOS

QueueSetHandle_t xQueueSet = xQueueCreateSet([1]);

Drag options to blanks, or click blank then click option'

A10

B5

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number smaller than the number of queues.

Using zero or negative numbers.

The queue set must be created with the total number of messages it can hold. Here, 3 queues are monitored, so 3 is correct.

2fill in blank

medium

Complete the code to add a queue handle to the queue set.

FreeRTOS

xQueueAddToSet([1], xQueueSet);

Drag options to blanks, or click blank then click option'

AxSemaphore

BxQueue1

CxTaskHandle

DxQueueSet

Attempts:

3 left

💡 Hint

Common Mistakes

Adding the queue set handle instead of a queue.

Adding a task handle or semaphore by mistake.

You add a queue handle (like xQueue1) to the queue set, not the set itself or other objects.

3fill in blank

hard

Fix the error in the code to receive from the queue set.

FreeRTOS

QueueSetMemberHandle_t xActivatedMember = xQueueSelectFromSet(xQueueSet, [1]);

Drag options to blanks, or click blank then click option'

AportMAX_DELAY

B-1

C0

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes immediate return without data.

Using NULL or -1 causes errors.

portMAX_DELAY waits indefinitely for a queue to have data. Using 0 would not wait, and -1 or NULL are invalid.

4fill in blank

hard

Fill both blanks to check if the activated member is a specific queue and then receive from it.

FreeRTOS

if (xActivatedMember == [1]) {
    xQueueReceive([2], &data, 0);
}

Drag options to blanks, or click blank then click option'

AxQueue1

BxQueue2

CxQueue3

DxQueueSet

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing to the queue set handle instead of a queue.

Receiving from a different queue than the one checked.

You compare the activated member to the queue handle (xQueue1) and then receive from the same queue.

5fill in blank

hard

Fill all three blanks to create two queues, add them to a queue set, and then select from the set.

FreeRTOS

QueueHandle_t [1] = xQueueCreate(5, sizeof(int));
QueueHandle_t [2] = xQueueCreate(5, sizeof(int));
QueueSetHandle_t xQueueSet = xQueueCreateSet(10);
xQueueAddToSet([3], xQueueSet);
xQueueAddToSet([2], xQueueSet);
QueueSetMemberHandle_t xActivatedMember = xQueueSelectFromSet(xQueueSet, portMAX_DELAY);

Drag options to blanks, or click blank then click option'

AxQueue1

BxQueue2

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing queue names or adding the wrong queue to the set.

Using the queue set handle instead of queue handles.

First, create xQueue1 and xQueue2. Then add xQueue1 and xQueue2 to the queue set. The blanks correspond to these names.

Practice

1. What is the main purpose of a Queue Set in FreeRTOS?

easy

A. To monitor multiple queues with a single wait call

B. To increase the size of a single queue

C. To create a queue that stores different data types

D. To prioritize tasks based on queue length

Solution

1. Step 1: Recall the purpose of a Queue Set

   A Queue Set allows monitoring multiple queues or semaphores with one blocking call.

2. Step 2: Match options to the Queue Set purpose

   Only "To monitor multiple queues with a single wait call" correctly describes this purpose; others describe unrelated features.

3. Final Answer:

   To monitor multiple queues with a single wait call -> Option A

4. Quick Check:

   Queue Set = single wait for many queues [OK]

Hint: Queue Set = watch many queues with one wait [OK]

Common Mistakes:

• Thinking Queue Set increases queue size
• Confusing Queue Set with task priority
• Believing Queue Set stores mixed data types

2. Which of the following is the correct way to create a queue set for monitoring two queues in FreeRTOS?

easy

A. xQueueSetCreate( queue1Length + queue2Length, itemSize )

B. xQueueSetCreate( queue1Length * queue2Length, itemSize )

C. xQueueSetCreate( queue1Length + queue2Length, sizeof(void*) )

D. xQueueSetCreate( queue1Length + queue2Length, 0 )

Solution

1. Step 1: Review parameters of xQueueSetCreate

   The first parameter is the total number of items the set can hold, usually sum of queue lengths. The second is the size of a pointer (sizeof(void*)) because queue sets hold references to queues.

2. Step 2: Evaluate each option

   xQueueSetCreate( queue1Length + queue2Length, sizeof(void*) ) correctly uses sum of lengths and pointer size. Others are incorrect: itemSize, multiplication, or zero size.

3. Final Answer:

   xQueueSetCreate( queue1Length + queue2Length, sizeof(void*) ) -> Option C

4. Quick Check:

   Queue set size = sum lengths, item size = pointer size [OK]

Hint: Queue set size = sum of queues, item size = pointer size [OK]

Common Mistakes:

• Using item size instead of pointer size
• Multiplying queue lengths instead of adding
• Setting item size to zero

3. Given the code snippet:

QueueHandle_t queue1 = xQueueCreate(5, sizeof(int));
QueueHandle_t queue2 = xQueueCreate(3, sizeof(int));
QueueSetHandle_t queueSet = xQueueSetCreate(8, sizeof(void*));
xQueueAddToSet(queue1, queueSet);
xQueueAddToSet(queue2, queueSet);
QueueSetMemberHandle_t activatedQueue = xQueueSelectFromSet(queueSet, portMAX_DELAY);

What does activatedQueue represent after xQueueSelectFromSet returns?

medium

A. A pointer to the data item received

B. A handle to the queue that has data available

C. The total number of items in all queues

D. NULL if no queues have data

Solution

1. Step 1: Analyze xQueueSelectFromSet behavior

   This function blocks until any queue in the set has data, then returns a handle to that queue.

2. Step 2: Determine what activatedQueue holds

   It holds the queue handle that has data ready to be read, not the data itself or count.

3. Final Answer:

   A handle to the queue that has data available -> Option B

4. Quick Check:

   xQueueSelectFromSet returns queue handle with data [OK]

Hint: xQueueSelectFromSet returns queue handle with data ready [OK]

Common Mistakes:

• Thinking it returns data pointer
• Confusing with total item count
• Expecting NULL when data is available

4. What is wrong with this code snippet for using a queue set?

QueueSetHandle_t set = xQueueSetCreate(5, sizeof(void*));
QueueHandle_t q1 = xQueueCreate(3, sizeof(int));
xQueueAddToSet(q1, set);
QueueSetMemberHandle_t member = xQueueSelectFromSet(set, portMAX_DELAY);
int value;
xQueueReceive(set, &value, 0);

medium

A. xQueueCreate uses wrong item size

B. Queue set size is too small for the queue length

C. xQueueAddToSet is missing for q1

D. xQueueReceive is called with a queue set handle instead of a queue handle

Solution

1. Step 1: Examine the xQueueReceive call

   The code calls xQueueReceive(set, &value, 0), passing the queue set handle 'set'.

2. Step 2: Recall the return value of xQueueSelectFromSet

   It returns 'member', a handle to the queue with data available.

3. Step 3: Identify the mistake

   xQueueReceive must use the queue handle 'member', not the queue set handle 'set'.

4. Final Answer:

   xQueueReceive is called with a queue set handle instead of a queue handle -> Option D

5. Quick Check:

   xQueueReceive uses queue handle from set, not set itself [OK]

Hint: xQueueSelectFromSet returns queue handle for xQueueReceive [OK]

Common Mistakes:

• Confusing queue set handle with queue handle
• Assuming queue set size must match queue length
• Missing xQueueAddToSet call

5. You want to monitor three queues with lengths 4, 5, and 6 using a queue set. Which of the following is the best practice to create and use the queue set correctly?

hard

A. Create queue set with size 15, add all queues, then call xQueueSelectFromSet to wait

B. Create queue set with size 6, add all queues, then call xQueueSelectFromSet to wait

C. Create queue set with size 4, add only the largest queue, then call xQueueSelectFromSet

D. Create queue set with size 15, add queues after calling xQueueSelectFromSet

Solution

1. Step 1: Calculate total queue set size

   The queue set size must be at least the sum of all queue lengths: 4 + 5 + 6 = 15.

2. Step 2: Add all queues before waiting

   All queues must be added to the set before calling xQueueSelectFromSet to monitor them properly.

3. Step 3: Evaluate options

   Create queue set with size 15, add all queues, then call xQueueSelectFromSet to wait follows best practice: correct size, all queues added, then wait. Others fail on size, timing, or completeness.

4. Final Answer:

   Create queue set with size 15, add all queues, then call xQueueSelectFromSet to wait -> Option A

5. Quick Check:

   Queue set size = sum lengths, add all queues first [OK]

Hint: Queue set size = sum of queues; add all before waiting [OK]

Common Mistakes:

• Using queue set size smaller than total queue lengths
• Adding queues after waiting
• Adding only some queues to the set