xQueueCreate() function in FreeRTOS - Mini Project: Build & Apply

Using the xQueueCreate() Function in FreeRTOS

📖 Scenario: You are working on a simple embedded system project using FreeRTOS. You need to create a queue to hold integer data that tasks will send and receive.

🎯 Goal: Learn how to create a queue using the xQueueCreate() function and verify its creation by checking the returned handle.

📋 What You'll Learn

Create a queue to hold 5 integers using xQueueCreate()

Store the queue handle in a variable named myQueue

Check if the queue was created successfully by verifying myQueue is not NULL

Print a message indicating success or failure

💡 Why This Matters

🌍 Real World

Queues are used in embedded systems to safely pass data between tasks without conflicts.

💼 Career

Understanding how to create and manage queues is essential for embedded software developers working with FreeRTOS or similar real-time operating systems.

1

Create a queue to hold 5 integers

Write a line of code to create a queue called myQueue that can hold 5 items, each of size sizeof(int), using the xQueueCreate() function.

FreeRTOS

// Create a queue called myQueue to hold 5 integers
// Your code here

Hint

Use xQueueCreate(5, sizeof(int)) and assign it to myQueue.

2

Add a check for successful queue creation

Add an if statement to check if myQueue is not NULL, indicating the queue was created successfully.

FreeRTOS

#include "FreeRTOS.h"
#include "queue.h"

QueueHandle_t myQueue = xQueueCreate(5, sizeof(int));

// Check if myQueue is not NULL
// Your code here

Hint

Use if (myQueue != NULL) to check the queue handle.

3

Add print statements for success or failure

Inside the if block, add a line to print "Queue created successfully.". Add an else block that prints "Failed to create queue.".

FreeRTOS

#include "FreeRTOS.h"
#include "queue.h"
#include 

QueueHandle_t myQueue = xQueueCreate(5, sizeof(int));

if (myQueue != NULL) {
    // Print success message
    // Your code here
} else {
    // Print failure message
    // Your code here
}

Hint

Use printf() inside both if and else blocks.

4

Run the program and check the output

Run the program and observe the printed output. It should display Queue created successfully. if the queue was created.

FreeRTOS

#include "FreeRTOS.h"
#include "queue.h"
#include 

QueueHandle_t myQueue = xQueueCreate(5, sizeof(int));

if (myQueue != NULL) {
    printf("Queue created successfully.\n");
} else {
    printf("Failed to create queue.\n");
}

// Run the program and check output

Hint

If you see Queue created successfully., your queue was created correctly.

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