Queue blocking behavior (timeout) in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to send data to a queue with a blocking timeout.

FreeRTOS

xQueueSend(queueHandle, &data, [1]);

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

A0

BportMAX_DELAY

CNULL

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 means no wait, so the send might fail immediately.

Using NULL or pdFALSE are not valid timeout values.

Using portMAX_DELAY makes the task wait indefinitely until space is available in the queue.

2fill in blank

medium

Complete the code to receive data from a queue with a 100 tick timeout.

FreeRTOS

xQueueReceive(queueHandle, &receivedData, [1]);

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

ApdTRUE

B0

CportMAX_DELAY

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 means no wait, so the receive might fail immediately.

Using portMAX_DELAY waits forever, not 100 ticks.

Passing 100 ticks makes the task wait up to 100 ticks for data.

3fill in blank

hard

Fix the error in the code to block on queue send with a timeout of 50 ticks.

FreeRTOS

xQueueSend(queueHandle, &data, [1]);

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

A50

BpdFALSE

CNULL

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE or NULL causes incorrect behavior or compile errors.

Using 0 means no wait, so the send might fail immediately.

The timeout parameter must be a tick count like 50 to block for that duration.

4fill in blank

hard

Fill both blanks to create a queue receive call that waits up to 200 ticks and stores data in variable 'msg'.

FreeRTOS

xQueueReceive([1], &[2], 200);

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

AqueueHandle

Bmsg

Cdata

Dqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the queue handle and data variable.

Using an undefined variable for the data.

The first blank is the queue handle, and the second blank is the variable to store received data.

5fill in blank

hard

Fill all three blanks to send 'value' to 'queue1' with a 10 tick timeout and check if it was successful.

FreeRTOS

BaseType_t result = xQueueSend([1], &[2], [3]);
if (result == pdPASS) {
    // success
}

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

Aqueue1

Bvalue

C10

DportMAX_DELAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using portMAX_DELAY instead of 10 for timeout.

Mixing up variable names.

Use the queue name, the variable to send, and the timeout in ticks.

Practice

1. What does the xTicksToWait parameter control in FreeRTOS queue functions like xQueueReceive() or xQueueSend()?

easy

A. The number of items currently in the queue.

B. The size of the queue in bytes.

C. The maximum time a task will wait for the queue operation before timing out.

D. The priority of the task accessing the queue.

Solution

1. Step 1: Understand the role of xTicksToWait

   This parameter specifies how long a task should wait if the queue is full (for send) or empty (for receive).

2. Step 2: Relate to queue blocking behavior

   If the queue operation cannot proceed immediately, the task blocks for up to xTicksToWait ticks before timing out.

3. Final Answer:

   The maximum time a task will wait for the queue operation before timing out. -> Option C

4. Quick Check:

   Timeout duration = xTicksToWait [OK]

Hint: Remember: xTicksToWait sets wait time before giving up [OK]

Common Mistakes:

• Confusing xTicksToWait with queue size
• Thinking it sets task priority
• Assuming it counts queue items

2. Which of the following is the correct way to call xQueueReceive() with a timeout of 10 ticks?

easy

A. xQueueReceive(queueHandle, &data);

B. xQueueReceive(queueHandle, data, 10);

C. xQueueReceive(queueHandle, &data, "10");

D. xQueueReceive(queueHandle, &data, 10);

Solution

1. Step 1: Check function parameters

   xQueueReceive() requires the queue handle, pointer to data buffer, and timeout in ticks as a number.

2. Step 2: Validate correct syntax

   xQueueReceive(queueHandle, &data, 10); correctly passes the address of data and numeric timeout 10 ticks.

3. Final Answer:

   xQueueReceive(queueHandle, &data, 10); -> Option D

4. Quick Check:

   Correct parameters and timeout type = xQueueReceive(queueHandle, &data, 10); [OK]

Hint: Use &data and numeric timeout for xQueueReceive [OK]

Common Mistakes:

• Passing data instead of &data
• Using string "10" instead of number 10
• Omitting timeout parameter

3. Consider this code snippet:

if(xQueueReceive(queue, &item, 5) == pdPASS) {
    process(item);
} else {
    handleTimeout();
}

What happens if the queue is empty and no item arrives within 5 ticks?

medium

A. The function returns after 5 ticks with a failure code, and handleTimeout() is called.

B. The task blocks indefinitely waiting for an item.

C. The function returns immediately with pdPASS.

D. The queue is deleted automatically.

Solution

1. Step 1: Analyze xQueueReceive behavior with timeout

   If the queue is empty, the task waits up to 5 ticks for an item to arrive.

2. Step 2: Understand return values

   If no item arrives in 5 ticks, xQueueReceive() returns a failure code (not pdPASS), so handleTimeout() runs.

3. Final Answer:

   The function returns after 5 ticks with a failure code, and handleTimeout() is called. -> Option A

4. Quick Check:

   Timeout triggers failure return = The function returns after 5 ticks with a failure code, and handleTimeout() is called. [OK]

Hint: Timeout means failure return, so else branch runs [OK]

Common Mistakes:

• Assuming indefinite block without timeout
• Thinking pdPASS is returned on timeout
• Believing queue deletes itself

4. Which of the following correctly describes this code snippet:

BaseType_t result = xQueueSend(queue, &data, 0);
if(result == pdFAIL) {
    // handle failure
}

medium

A. Using 0 as timeout means the task will wait forever.

B. Using 0 as timeout means the function returns immediately if the queue is full.

C. The timeout parameter should be a pointer, not a number.

D. The return value should be compared to pdPASS, not pdFAIL.

Solution

1. Step 1: Understand timeout=0 behavior in xQueueSend

   A timeout of 0 means the function does not block; it returns immediately if the queue is full.

2. Step 2: Check the error handling logic

   If the queue is full, xQueueSend() returns pdFAIL immediately, so the failure handling code runs correctly.

3. Final Answer:

   Using 0 as timeout means the function returns immediately if the queue is full. -> Option B

4. Quick Check:

   Timeout 0 means no wait, immediate return = Using 0 as timeout means the function returns immediately if the queue is full. [OK]

Hint: Timeout 0 means no wait, immediate fail if full [OK]

Common Mistakes:

• Thinking 0 means wait forever
• Confusing timeout type with pointer
• Comparing return to wrong constant

5. You want a task to send data to a queue but only wait up to 20 ticks if the queue is full. If the queue remains full after 20 ticks, the task should perform an alternative action. Which code snippet correctly implements this behavior?

hard

A. if(xQueueSend(queue, &data, 20) == pdPASS) {
// sent successfully
} else {
// alternative action
}

B. if(xQueueSend(queue, &data, 0) == pdFAIL) {
// alternative action
} else {
// sent successfully
}

C. if(xQueueSend(queue, &data, portMAX_DELAY) == pdPASS) {
// sent successfully
} else {
// alternative action
}

D. if(xQueueSend(queue, &data, 20) == pdFAIL) {
// sent successfully
} else {
// alternative action
}

Solution

1. Step 1: Understand desired behavior

   The task should wait up to 20 ticks to send data, then do alternative action if it fails.

2. Step 2: Analyze each option

   if(xQueueSend(queue, &data, 20) == pdPASS) { // sent successfully } else { // alternative action } waits 20 ticks and checks for pdPASS to confirm success; else runs alternative action. if(xQueueSend(queue, &data, 0) == pdFAIL) { // alternative action } else { // sent successfully } waits 0 ticks (no wait). if(xQueueSend(queue, &data, portMAX_DELAY) == pdPASS) { // sent successfully } else { // alternative action } waits forever. if(xQueueSend(queue, &data, 20) == pdFAIL) { // sent successfully } else { // alternative action } reverses success/failure logic.

3. Final Answer:

   if(xQueueSend(queue, &data, 20) == pdPASS) { // sent successfully } else { // alternative action } -> Option A

4. Quick Check:

   Wait 20 ticks, success check = if(xQueueSend(queue, &data, 20) == pdPASS) { // sent successfully } else { // alternative action } [OK]

Hint: Wait 20 ticks, check for pdPASS to confirm send success [OK]

Common Mistakes:

• Using 0 ticks instead of 20
• Waiting forever with portMAX_DELAY
• Mixing up pdPASS and pdFAIL checks