xQueueCreate() function in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a queue with 5 items, each of size sizeof(int).

FreeRTOS

QueueHandle_t queue = xQueueCreate([1], sizeof(int));

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

A-1

B10

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or negative numbers which are invalid queue lengths.

Confusing the item size with the queue length.

The first parameter of xQueueCreate() is the number of items the queue can hold. Here, 5 is correct.

2fill in blank

medium

Complete the code to create a queue that holds 10 items, each of size sizeof(char).

FreeRTOS

QueueHandle_t queue = xQueueCreate([1], sizeof(char));

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

A10

B5

C20

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the size of the item as the queue length.

Using a smaller or larger number than required.

The queue length is 10, so the first parameter should be 10.

3fill in blank

hard

Fix the error in the code to create a queue for 3 items of size sizeof(float).

FreeRTOS

QueueHandle_t queue = xQueueCreate([1], sizeof(float));

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

A3

Bsizeof(float)

C-3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero or negative numbers for queue length.

Using the size of the item as the queue length.

The queue length must be a positive integer. 3 is correct here.

4fill in blank

hard

Fill both blanks to create a queue that holds 7 items, each of size sizeof(uint32_t).

FreeRTOS

QueueHandle_t queue = xQueueCreate([1], [2]);

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

A7

Bsizeof(uint32_t)

Csizeof(int)

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of parameters.

Using wrong sizes for the item.

The first blank is the number of items (7), the second blank is the size of each item (sizeof(uint32_t)).

5fill in blank

hard

Fill all three blanks to create a queue for 4 items, each of size sizeof(char), and check if the queue was created successfully.

FreeRTOS

QueueHandle_t queue = xQueueCreate([1], [2]);
if (queue == [3]) {
    // Handle error
}

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

A4

Bsizeof(char)

CNULL

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking the queue handle against NULL.

Using 0 instead of NULL for the check.

Mixing up the order of parameters.

The queue length is 4, item size is sizeof(char), and the queue handle is checked against NULL to detect failure.

Practice

1. What does the xQueueCreate() function do in FreeRTOS?

easy

A. Sends data to a queue

B. Creates a new queue to hold data items between tasks

C. Deletes an existing queue

D. Receives data from a queue

Solution

1. Step 1: Understand the purpose of xQueueCreate()

   The function is designed to create a new queue in FreeRTOS for inter-task communication.

2. Step 2: Differentiate from other queue functions

   Functions like sending or receiving data operate on existing queues, not create them.

3. Final Answer:

   Creates a new queue to hold data items between tasks -> Option B

4. Quick Check:

   xQueueCreate() = create queue [OK]

Hint: xQueueCreate() always makes a new queue [OK]

Common Mistakes:

• Confusing creation with sending or receiving data
• Thinking it deletes a queue
• Assuming it initializes queue data

2. Which of the following is the correct syntax to create a queue for 10 items, each of size sizeof(int)?

easy

A. xQueueCreate(10, sizeof(int));

B. xQueueCreate(sizeof(int), 10);

C. xQueueCreate(10);

D. xQueueCreate(sizeof(int));

Solution

1. Step 1: Recall xQueueCreate() parameters

   The first parameter is the number of items, the second is the size of each item.

2. Step 2: Match parameters to options

   xQueueCreate(10, sizeof(int)); correctly uses 10 items and sizeof(int) as size per item.

3. Final Answer:

   xQueueCreate(10, sizeof(int)); -> Option A

4. Quick Check:

   Parameters = (number, size) [OK]

Hint: First param = count, second = size [OK]

Common Mistakes:

• Swapping the order of parameters
• Providing only one parameter
• Using raw numbers instead of sizeof()

3. Consider this code snippet:

QueueHandle_t q = xQueueCreate(5, sizeof(char));
if (q == NULL) {
    printf("Queue creation failed\n");
} else {
    printf("Queue created successfully\n");
}

What will be printed if the queue is created successfully?

medium

A. Queue creation failed

B. Compilation error

C. Queue created successfully

D. No output

Solution

1. Step 1: Check the return value of xQueueCreate()

   If queue creation succeeds, it returns a non-NULL handle.

2. Step 2: Analyze the if condition

   The code prints "Queue created successfully" when q is not NULL.

3. Final Answer:

   Queue created successfully -> Option C

4. Quick Check:

   q != NULL means success [OK]

Hint: Check if queue handle is NULL for failure [OK]

Common Mistakes:

• Assuming NULL means success
• Ignoring the if condition
• Confusing print messages

4. What is wrong with this code?

QueueHandle_t q = xQueueCreate(0, sizeof(int));
if (q == NULL) {
    // handle error
}

medium

A. Queue size cannot be zero, so creation fails

B. sizeof(int) is invalid parameter

C. Missing queue deletion after creation

D. QueueHandle_t is not the correct type

Solution

1. Step 1: Understand queue length parameter

   The first parameter must be greater than zero to hold items.

2. Step 2: Consequence of zero length

   Passing zero causes xQueueCreate() to fail and return NULL.

3. Final Answer:

   Queue size cannot be zero, so creation fails -> Option A

4. Quick Check:

   Queue length > 0 required [OK]

Hint: Queue length must be > 0 [OK]

Common Mistakes:

• Using zero length queue
• Confusing size of item with queue length
• Ignoring return NULL check

5. You want to create a queue that can hold 3 structures of type SensorData. Which is the correct way to create this queue?

hard

A. xQueueCreate(1, sizeof(SensorData) * 3);

B. xQueueCreate(sizeof(SensorData), 3);

C. xQueueCreate(3, sizeof(SensorData*));

D. xQueueCreate(3, sizeof(SensorData));

Solution

1. Step 1: Identify queue length and item size

   Queue length is 3, item size is size of one SensorData structure.

2. Step 2: Match parameters to xQueueCreate()

   First parameter is number of items (3), second is size of each item (sizeof(SensorData)).

3. Final Answer:

   xQueueCreate(3, sizeof(SensorData)); -> Option D

4. Quick Check:

   Length=3, size=sizeof(struct) [OK]

Hint: Length first, size of one item second [OK]

Common Mistakes:

• Swapping length and size parameters
• Using pointer size instead of struct size
• Creating queue for one large combined item