Queue full and empty handling in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when sending to a full queue?

Consider a FreeRTOS queue with length 2. The queue is already full. What will be the return value of xQueueSend when trying to add another item without waiting?

FreeRTOS

QueueHandle_t queue = xQueueCreate(2, sizeof(int));
int item = 10;
int ret1 = xQueueSend(queue, &item, 0);
int ret2 = xQueueSend(queue, &item, 0);
int ret3 = xQueueSend(queue, &item, 0); // Queue is full here
// What is ret3?

A errQUEUE_FULL (0) indicating the queue is full

B Block indefinitely until space is available

C pdPASS (1) indicating success

D pdFAIL (0) indicating failure

💡 Hint

Check the behavior of xQueueSend with zero block time when the queue is full.

When xQueueSend is called with a block time of 0 and the queue is full, it returns errQUEUE_FULL (0) immediately without blocking.

❓ Predict Output

intermediate

What happens when receiving from an empty queue with zero wait?

Given a FreeRTOS queue that is empty, what will xQueueReceive return if called with zero block time?

FreeRTOS

QueueHandle_t queue = xQueueCreate(3, sizeof(int));
int receivedValue;
int ret = xQueueReceive(queue, &receivedValue, 0); // Queue is empty
// What is ret?

A Block indefinitely until data is available

B pdPASS (1) and receivedValue contains data

C pdFAIL (0) indicating failure

D errQUEUE_EMPTY (0) indicating the queue is empty

💡 Hint

Think about what happens if you try to receive from an empty queue without waiting.

xQueueReceive returns 0 (errQUEUE_EMPTY) immediately if the queue is empty and block time is zero.

🧠 Conceptual

advanced

Which FreeRTOS API call blocks until space is available in a full queue?

You want to send data to a queue that might be full. Which xQueueSend variant will block until there is space to add the item?

A xQueueSend with block time set to 0

B xQueueOverwrite

C xQueueSend with block time set to portMAX_DELAY

D xQueueReceive with block time set to portMAX_DELAY

💡 Hint

Consider how block time affects waiting behavior in FreeRTOS queue functions.

Setting block time to portMAX_DELAY causes xQueueSend to block indefinitely until space is available.

❓ Predict Output

advanced

What is the output of this code snippet handling queue empty condition?

Analyze the following FreeRTOS code snippet. What will be printed?

FreeRTOS

QueueHandle_t queue = xQueueCreate(1, sizeof(int));
int val = 42;
xQueueSend(queue, &val, 0);
int received = 0;
if(xQueueReceive(queue, &received, 0) == pdPASS) {
    printf("Received: %d\n", received);
}
if(xQueueReceive(queue, &received, 0) == pdPASS) {
    printf("Received again: %d\n", received);
} else {
    printf("Queue empty\n");
}

A Received: 42\nQueue empty

B Received: 42\nReceived again: 42

C Queue empty\nQueue empty

D Received again: 42\nQueue empty

💡 Hint

Remember that the queue has only one item and the second receive tries to get from empty queue.

The first receive gets the value 42 and prints it. The second receive fails because the queue is empty and prints "Queue empty".

🔧 Debug

expert

Why does this FreeRTOS queue send block indefinitely?

Given the code below, why does the task block forever on xQueueSend?

FreeRTOS

QueueHandle_t queue = xQueueCreate(1, sizeof(int));
int data = 5;
// Task A
xQueueSend(queue, &data, portMAX_DELAY);
// Task B
int received;
xQueueReceive(queue, &received, 0);
// Task A tries to send again
xQueueSend(queue, &data, portMAX_DELAY); // Blocks here

A Queue size is zero, so no items can be added

B Task B never runs to remove item, so queue stays full

C xQueueSend with portMAX_DELAY never blocks

D xQueueReceive with zero block time blocks indefinitely

💡 Hint

Think about task scheduling and whether Task B runs before Task A tries to send again.

If Task B does not run to remove the item, the queue remains full. Task A blocks indefinitely waiting for space.

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