Queue full and empty handling in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a queue that can hold 5 integers.

FreeRTOS

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

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

A0

B10

C1

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or 1 which creates a queue too small to hold 5 items.

Confusing the size of the item with the number of items.

The function xQueueCreate takes the number of items the queue can hold as the first argument. Here, 5 means the queue can hold 5 integers.

2fill in blank

medium

Complete the code to send an integer to the queue without blocking.

FreeRTOS

int value = 10;
BaseType_t result = xQueueSend(queue, &value, [1]);

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

A0

BportMAX_DELAY

C100

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using portMAX_DELAY which causes the function to block indefinitely.

Using a positive number which causes the function to wait.

Using 0 as the block time means the function will not wait if the queue is full and will return immediately.

3fill in blank

hard

Fix the error in the code to receive an item from the queue with a 10 tick timeout.

FreeRTOS

int receivedValue;
if (xQueueReceive(queue, &receivedValue, [1]) == pdPASS) {
    // Process receivedValue
}

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

A-1

B0

C10

DportMAX_DELAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 which causes no waiting and immediate return.

Using negative values which are invalid.

The block time parameter should be set to 10 ticks to wait up to 10 ticks for an item to be received.

4fill in blank

hard

Fill both blanks to check if the queue is full before sending an item.

FreeRTOS

if (uxQueueSpacesAvailable(queue) [1] 0) {
    xQueueSend(queue, &item, [2]);
}

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

A>

B0

DportMAX_DELAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' which reverses the logic.

Using 0 as block time which causes no waiting.

Checking if available spaces are greater than 0 ensures the queue is not full. Using portMAX_DELAY allows waiting indefinitely to send the item.

5fill in blank

hard

Fill all three blanks to create a non-blocking receive with a check for empty queue.

FreeRTOS

if (uxQueueMessagesWaiting(queue) [1] 0) {
    xQueueReceive(queue, &data, [2]);
} else {
    // Queue empty, handle accordingly
    vTaskDelay([3]);
}

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

A>

B0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' which reverses the check.

Using non-zero block time causing blocking behavior.

Checking if messages waiting is greater than 0 confirms the queue is not empty. Using 0 as block time makes receive non-blocking. Using 1 tick delay helps avoid busy waiting.

Practice

1. In FreeRTOS, what happens if you try to send an item to a queue that is already full without waiting?

easy

A. The send operation fails immediately and returns an error.

B. The queue automatically expands to fit the new item.

C. The oldest item in the queue is overwritten.

D. The task sending the item is suspended indefinitely.

Solution

1. Step 1: Understand queue full behavior

   When a queue is full, FreeRTOS does not allow new items to be added without waiting or handling the error.

2. Step 2: Check send operation without wait

   If the send function is called with zero wait time and the queue is full, it returns immediately with an error indicating failure.

3. Final Answer:

   The send operation fails immediately and returns an error. -> Option A

4. Quick Check:

   Queue full + no wait = send fails [OK]

Hint: Send with zero wait fails if queue is full [OK]

Common Mistakes:

• Assuming queue grows automatically
• Thinking oldest data is overwritten
• Believing task suspends indefinitely

2. Which of the following is the correct way to receive an item from a FreeRTOS queue with a block time of 10 ticks?

easy

A. xQueueReceive(queue, &item, 10);

B. xQueueReceive(queue, item, 10);

C. xQueueReceive(queue, &item, 0);

D. xQueueReceive(queue, &item, -1);

Solution

1. Step 1: Understand xQueueReceive parameters

   The function takes the queue handle, a pointer to the variable to store the item, and the block time in ticks.

2. Step 2: Identify correct syntax

   Passing &item as the second argument is correct to provide the address. Block time 10 means wait up to 10 ticks.

3. Final Answer:

   xQueueReceive(queue, &item, 10); -> Option A

4. Quick Check:

   Pointer + block time 10 = correct receive call [OK]

Hint: Use &item and positive ticks for blocking receive [OK]

Common Mistakes:

• Passing item instead of &item
• Using zero or negative block time incorrectly
• Confusing send and receive parameters

3. Consider the following code snippet:

QueueHandle_t queue = xQueueCreate(2, sizeof(int));
int val = 10;
xQueueSend(queue, &val, 0);
val = 20;
xQueueSend(queue, &val, 0);
val = 30;
BaseType_t result = xQueueSend(queue, &val, 0);

What is the value of result after the last send call?

medium

A. pdPASS (send succeeded)

B. errQUEUE_FULL (send failed due to full queue)

C. pdFAIL (generic failure)

D. Block indefinitely until space is available

Solution

1. Step 1: Analyze queue creation and sends

   The queue can hold 2 integers. Two sends with zero wait fill it completely.

2. Step 2: Evaluate last send with zero wait

   The third send tries to add an item when the queue is full and does not wait, so it fails returning errQUEUE_FULL.

3. Final Answer:

   errQUEUE_FULL (send failed due to full queue) -> Option B

4. Quick Check:

   Queue full + zero wait = send fails with errQUEUE_FULL [OK]

Hint: Queue size 2 + 3 sends with zero wait = last send fails [OK]

Common Mistakes:

• Assuming last send blocks
• Thinking last send overwrites data
• Confusing pdPASS and pdFAIL

4. Given this code snippet, why does xQueueReceive never return pdPASS?

QueueHandle_t queue = xQueueCreate(1, sizeof(int));
int data;

// Task A
xQueueReceive(queue, &data, 0);

// Task B
int val = 5;
xQueueSend(queue, &val, 0);

medium

A. xQueueSend is called with zero wait time, so it fails.

B. Queue size is too small to hold any data.

C. xQueueReceive is called with incorrect pointer.

D. Task A tries to receive before Task B sends, queue is empty, no wait time.

Solution

1. Step 1: Understand queue state at receive

   Task A calls receive immediately with zero wait, but queue is empty because Task B hasn't sent yet.

2. Step 2: Effect of zero wait on empty queue

   With zero wait, receive returns immediately with failure since no data is available yet.

3. Final Answer:

   Task A tries to receive before Task B sends, queue is empty, no wait time. -> Option D

4. Quick Check:

   Receive zero wait + empty queue = no pdPASS [OK]

Hint: Receive zero wait on empty queue always fails [OK]

Common Mistakes:

• Thinking queue size 1 means no data can be stored
• Assuming send fails with zero wait
• Confusing pointer usage in receive

5. You want to design a FreeRTOS task that sends sensor data to a queue but must never block if the queue is full. Which approach correctly handles the queue full condition?

hard

A. Increase the queue size dynamically when full.

B. Use xQueueSend with portMAX_DELAY to wait indefinitely until space is available.

C. Use xQueueSend with zero block time and check the return value to discard data if full.

D. Use xQueueOverwrite to replace the oldest data in the queue automatically.

Solution

1. Step 1: Understand non-blocking send requirement

   The task must never block, so waiting indefinitely or for a long time is not allowed.

2. Step 2: Choose correct send method and handling

   Using xQueueSend with zero block time returns immediately. Checking the return value lets the task discard data if the queue is full.

3. Step 3: Evaluate other options

   Use xQueueSend with portMAX_DELAY to wait indefinitely until space is available. blocks indefinitely, violating requirement. Use xQueueOverwrite to replace the oldest data in the queue automatically. overwrites data but may lose important info. Increase the queue size dynamically when full. is not supported by FreeRTOS.

4. Final Answer:

   Use xQueueSend with zero block time and check the return value to discard data if full. -> Option C

5. Quick Check:

   Non-blocking send + check return = safe discard [OK]

Hint: Send zero wait + check return to avoid blocking [OK]

Common Mistakes:

• Using blocking send when non-blocking needed
• Assuming queue size can grow dynamically
• Ignoring return value of send