Semaphore timeout handling in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to take a semaphore with a timeout.

FreeRTOS

if (xSemaphoreTake(xSemaphore, [1]) == pdTRUE) {
    // Semaphore taken successfully
}

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

A1000

B0

CportMAX_DELAY

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 means no wait, so the semaphore might not be taken.

Using pdFALSE is incorrect as it is not a valid timeout value.

Using portMAX_DELAY makes the task wait indefinitely for the semaphore.

2fill in blank

medium

Complete the code to check if the semaphore was taken or timed out.

FreeRTOS

if (xSemaphoreTake(xSemaphore, 100) == [1]) {
    // Got the semaphore
} else {
    // Timeout occurred
}

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

ApdFALSE

BpdPASS

CpdFAIL

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE means the semaphore was not taken.

Using pdPASS or pdFAIL are not valid return values here.

pdTRUE indicates the semaphore was successfully taken.

3fill in blank

hard

Fix the error in the semaphore take call to correctly wait for 500 ticks.

FreeRTOS

if (xSemaphoreTake(xSemaphore, [1]) == pdTRUE) {
    // Semaphore acquired
}

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

A500

BpdFALSE

CportMAX_DELAY

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE or 0 causes immediate return without waiting.

Using portMAX_DELAY waits forever, not 500 ticks.

Use the number of ticks (500) to wait for the semaphore.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps task names to their semaphore wait results with a timeout of 10 ticks.

FreeRTOS

results = {task: xSemaphoreTake(semaphores[task], [1]) for task in tasks if task [2] 'Task1'}

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

A10

B!=

C==

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using == includes only 'Task1', not others.

Using 5 as timeout waits less than required.

Wait 10 ticks for each semaphore and exclude 'Task1' from the dictionary.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps task names in uppercase to their semaphore wait results, only if the wait was successful (pdTRUE).

FreeRTOS

results = {task[1]: xSemaphoreTake(semaphores[task], [2]) for task in tasks if xSemaphoreTake(semaphores[task], [3]) == pdTRUE}

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

A.upper()

B50

D.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using .lower() instead of .upper().

Using different timeout values for the two calls.

Convert task names to uppercase and wait 50 ticks for the semaphore twice to check success and store result.

Practice

1.

What does the timeout parameter in xSemaphoreTake() control?

easy

A. How long the task waits for the semaphore before giving up

B. The priority of the task waiting for the semaphore

C. The number of times the semaphore can be taken

D. The size of the semaphore queue

Solution

1. Step 1: Understand the role of timeout in xSemaphoreTake()

   The timeout parameter specifies the maximum time the task will wait to obtain the semaphore.

2. Step 2: Relate timeout to task blocking

   If the semaphore is not available within this time, the function returns failure, preventing indefinite blocking.

3. Final Answer:

   How long the task waits for the semaphore before giving up -> Option A

4. Quick Check:

   Timeout controls wait duration = B [OK]

Hint: Timeout limits wait time to avoid blocking forever [OK]

Common Mistakes:

• Confusing timeout with task priority
• Thinking timeout changes semaphore count
• Assuming timeout affects queue size

2.

Which of the following is the correct way to call xSemaphoreTake() with a 100 tick timeout?

SemaphoreHandle_t sem;

easy

A. xSemaphoreTake(&sem, 100);

B. xSemaphoreTake(100, sem);

C. xSemaphoreTake(sem, &100);

D. xSemaphoreTake(sem, 100);

Solution

1. Step 1: Check function signature

   xSemaphoreTake takes the semaphore handle first, then the timeout as TickType_t.

2. Step 2: Match parameters correctly

   Passing sem as first argument and 100 as timeout matches the correct syntax.

3. Final Answer:

   xSemaphoreTake(sem, 100); -> Option D

4. Quick Check:

   Semaphore handle first, timeout second = A [OK]

Hint: Semaphore handle is first argument in xSemaphoreTake [OK]

Common Mistakes:

• Swapping arguments order
• Passing address of timeout
• Passing address of semaphore handle

3.

What will be the output of this code snippet?

SemaphoreHandle_t sem = xSemaphoreCreateBinary();
if(xSemaphoreTake(sem, 10) == pdTRUE) {
    printf("Got semaphore\n");
} else {
    printf("Timeout occurred\n");
}

medium

A. Got semaphore

B. No output

C. Timeout occurred

D. Compilation error

Solution

1. Step 1: Understand semaphore initial state

   xSemaphoreCreateBinary creates a semaphore initially empty (not given).

2. Step 2: Check xSemaphoreTake behavior

   Since semaphore is empty and no give is called, xSemaphoreTake waits 10 ticks then times out.

3. Final Answer:

   Timeout occurred -> Option C

4. Quick Check:

   Empty semaphore + timeout = timeout message [OK]

Hint: Binary semaphore starts empty, so take times out if not given [OK]

Common Mistakes:

• Assuming semaphore is initially given
• Ignoring timeout effect
• Expecting compilation error

4.

Identify the error in this code snippet that uses xSemaphoreTake() with timeout:

SemaphoreHandle_t sem = xSemaphoreCreateBinary();
if(xSemaphoreTake(sem, portMAX_DELAY)) {
    // Critical section
} else {
    // Handle timeout
}

medium

A. Using portMAX_DELAY disables timeout, so else block never runs

B. Semaphore handle is not initialized

C. Missing sem give before take

D. Incorrect if condition syntax

Solution

1. Step 1: Understand portMAX_DELAY meaning

   portMAX_DELAY means wait indefinitely for the semaphore.

2. Step 2: Analyze if-else behavior

   Since wait is infinite, timeout never occurs, so else block is unreachable.

3. Final Answer:

   Using portMAX_DELAY disables timeout, so else block never runs -> Option A

4. Quick Check:

   portMAX_DELAY means infinite wait = C [OK]

Hint: portMAX_DELAY means wait forever, no timeout occurs [OK]

Common Mistakes:

• Thinking else block handles timeout with portMAX_DELAY
• Assuming semaphore uninitialized causes error here
• Confusing syntax error with logic error

5.

You want to wait for a semaphore but only up to 50 ticks. If the semaphore is not available, you want to perform a fallback task immediately. Which code snippet correctly implements this behavior?

hard

A. if(xSemaphoreTake(sem, portMAX_DELAY)) {
// Use resource
} else {
// Fallback task
}

B. if(xSemaphoreTake(sem, 50) == pdTRUE) {
// Use resource
} else {
// Fallback task
}

C. if(xSemaphoreTake(sem, 0) == pdFALSE) {
// Use resource
} else {
// Fallback task
}

D. if(xSemaphoreTake(sem, 50)) {
// Fallback task
} else {
// Use resource
}

Solution

1. Step 1: Check timeout usage

   Using 50 ticks as timeout means wait up to 50 ticks for semaphore.

2. Step 2: Verify fallback logic

   If take returns pdTRUE, resource is used; else fallback task runs immediately after timeout.

3. Final Answer:

   Correctly waits 50 ticks and runs fallback if timeout -> Option B

4. Quick Check:

   Timeout 50 ticks + fallback on failure = A [OK]

Hint: Wait with timeout, fallback in else block after failure [OK]

Common Mistakes:

• Using portMAX_DELAY disables fallback
• Swapping if-else blocks
• Using zero timeout incorrectly