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

Using xSemaphoreCreateMutex() in FreeRTOS

📖 Scenario: You are working on a FreeRTOS project where two tasks share a resource. To avoid conflicts, you need to use a mutex semaphore.

🎯 Goal: Learn how to create a mutex semaphore using xSemaphoreCreateMutex() and use it to protect a shared variable.

📋 What You'll Learn

Create a mutex semaphore using xSemaphoreCreateMutex()

Create a shared integer variable called sharedCounter

Create a task that increments sharedCounter safely using the mutex

Print the value of sharedCounter after incrementing

💡 Why This Matters

🌍 Real World

Mutex semaphores are used in embedded systems to protect shared resources like sensors, communication buses, or memory from being accessed by multiple tasks at the same time.

💼 Career

Understanding how to use <code>xSemaphoreCreateMutex()</code> is essential for embedded software engineers working with FreeRTOS to write safe and reliable multitasking applications.

1

Create the shared variable

Create a global integer variable called sharedCounter and set it to 0.

FreeRTOS

# Create a global integer variable called sharedCounter and set it to 0
// Your code here

Hint

Use int sharedCounter = 0; to declare and initialize the variable.

2

Create the mutex semaphore

Create a global variable called mutex of type SemaphoreHandle_t. Then create the mutex using mutex = xSemaphoreCreateMutex();.

FreeRTOS

int sharedCounter = 0;
// Create a SemaphoreHandle_t variable called mutex
// Create the mutex using xSemaphoreCreateMutex()
// Your code here

Hint

Use SemaphoreHandle_t mutex; to declare and mutex = xSemaphoreCreateMutex(); to create the mutex.

3

Write a task to increment sharedCounter safely

Write a function called vTaskIncrement that tries to take the mutex using xSemaphoreTake(mutex, portMAX_DELAY). If successful, increment sharedCounter by 1, then give the mutex back using xSemaphoreGive(mutex).

FreeRTOS

int sharedCounter = 0;
SemaphoreHandle_t mutex;

void setupMutex() {
    mutex = xSemaphoreCreateMutex();
}

// Write vTaskIncrement function to safely increment sharedCounter
// Your code here

Hint

Use xSemaphoreTake to lock, increment sharedCounter, then xSemaphoreGive to unlock.

4

Print the sharedCounter value

Add a printf statement inside vTaskIncrement after incrementing sharedCounter to print its value as "sharedCounter: %d\n".

FreeRTOS

int sharedCounter = 0;
SemaphoreHandle_t mutex;

void setupMutex() {
    mutex = xSemaphoreCreateMutex();
}

void vTaskIncrement(void *pvParameters) {
    if (xSemaphoreTake(mutex, portMAX_DELAY) == pdTRUE) {
        sharedCounter++;
        // Print sharedCounter value here
        // Your code here
        xSemaphoreGive(mutex);
    }
    vTaskDelete(NULL);
}

Hint

Use printf("sharedCounter: %d\n", sharedCounter); to print the value.

Practice

1.

What is the primary purpose of the xSemaphoreCreateMutex() function in FreeRTOS?

easy

A. To delete an existing semaphore

B. To create a counting semaphore for task synchronization

C. To create a mutex for safe access to shared resources

D. To start a new task in the system

Solution

1. Step 1: Understand the function purpose

   xSemaphoreCreateMutex() is designed to create a mutex, which is a special type of semaphore used to protect shared resources.

2. Step 2: Compare with other options

   Options A, B, and C describe different FreeRTOS functions or concepts unrelated to mutex creation.

3. Final Answer:

   To create a mutex for safe access to shared resources -> Option C

4. Quick Check:

   Mutex creation = safe resource sharing [OK]

Hint: Mutexes protect shared resources safely [OK]

Common Mistakes:

• Confusing mutex with counting semaphore
• Thinking it creates a task
• Assuming it deletes semaphores

2.

Which of the following is the correct way to create a mutex using xSemaphoreCreateMutex() in FreeRTOS?

SemaphoreHandle_t mutex = ?;

easy

A. xSemaphoreCreateMutex();

B. xSemaphoreCreateCounting(1, 0);

C. xTaskCreateMutex();

D. xSemaphoreCreateBinary();

Solution

1. Step 1: Identify correct function call

   The function to create a mutex is exactly xSemaphoreCreateMutex(), which returns a handle.

2. Step 2: Eliminate incorrect options

   Options B, C, and D are either different semaphore types or invalid functions.

3. Final Answer:

   xSemaphoreCreateMutex(); -> Option A

4. Quick Check:

   Correct mutex creation syntax = xSemaphoreCreateMutex(); [OK]

Hint: Use exact function name xSemaphoreCreateMutex() [OK]

Common Mistakes:

• Using counting or binary semaphore functions
• Mistyping function name
• Confusing task creation with mutex creation

3.

Consider the following code snippet:

SemaphoreHandle_t mutex = xSemaphoreCreateMutex();
if (mutex == NULL) {
    printf("Mutex creation failed\n");
} else {
    printf("Mutex created successfully\n");
}

What will be the output if the mutex is created successfully?

medium

A. Mutex creation failed

B. Compilation error

C. No output

D. Mutex created successfully

Solution

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

   If the mutex is created successfully, the returned handle is not NULL.

2. Step 2: Follow the if-else logic

   Since mutex != NULL, the else block runs, printing "Mutex created successfully".

3. Final Answer:

   Mutex created successfully -> Option D

4. Quick Check:

   Non-NULL handle means success message [OK]

Hint: Check for NULL to confirm creation success [OK]

Common Mistakes:

• Assuming NULL means success
• Ignoring the if-else structure
• Expecting compilation errors

4.

Identify the error in the following code snippet that uses xSemaphoreCreateMutex():

SemaphoreHandle_t mutex;
mutex = xSemaphoreCreateMutex();
if (mutex != NULL) {
    xSemaphoreTake(mutex, 0);
}
// Use shared resource
xSemaphoreGive(mutex);

medium

A. Using zero block time in xSemaphoreTake() may cause immediate failure

B. Mutex handle is not initialized before use

C. xSemaphoreGive() should be called before xSemaphoreTake()

D. xSemaphoreCreateMutex() does not return a handle

Solution

1. Step 1: Analyze xSemaphoreTake() call with zero block time

   Using 0 as block time means the call will not wait and may fail if mutex is not available immediately.

2. Step 2: Understand implications for resource use

   If xSemaphoreTake() fails, the shared resource is accessed without protection, causing potential issues.

3. Final Answer:

   Using zero block time in xSemaphoreTake() may cause immediate failure -> Option A

4. Quick Check:

   Zero block time risks immediate take failure [OK]

Hint: Avoid zero block time unless sure mutex is free [OK]

Common Mistakes:

• Assuming zero block time always succeeds
• Ignoring mutex initialization
• Misordering take and give calls

5.

You want to protect a shared resource in a FreeRTOS application using a mutex created by xSemaphoreCreateMutex(). Which of the following sequences correctly shows how to use the mutex to safely access the resource?

1. xSemaphoreTake(mutex, portMAX_DELAY);
2. Access shared resource
3. xSemaphoreGive(mutex);
4. Create mutex with xSemaphoreCreateMutex()

hard

A. 1, 4, 2, 3

B. 4, 1, 2, 3

C. 4, 2, 1, 3

D. 1, 2, 3, 4

Solution

1. Step 1: Identify correct order of operations

   First, create the mutex (step 4), then take it before accessing the resource (step 1), access the resource (step 2), and finally give the mutex back (step 3).

2. Step 2: Verify sequence correctness

   Only 4, 1, 2, 3 follows the logical order: create, take, access, give.

3. Final Answer:

   4, 1, 2, 3 -> Option B

4. Quick Check:

   Create before take, then access, then give [OK]

Hint: Always create mutex before taking it [OK]

Common Mistakes:

• Taking mutex before creation
• Accessing resource without taking mutex
• Giving mutex before accessing resource