xQueueSend() and xQueueReceive() in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS queue send and receive example?

Consider the following FreeRTOS code snippet that sends and receives an integer via a queue:

QueueHandle_t queue = xQueueCreate(1, sizeof(int));
int valueToSend = 42;
int valueReceived = 0;

xQueueSend(queue, &valueToSend, 0);
xQueueReceive(queue, &valueReceived, 0);
printf("%d", valueReceived);

What will be printed?

FreeRTOS

QueueHandle_t queue = xQueueCreate(1, sizeof(int));
int valueToSend = 42;
int valueReceived = 0;

xQueueSend(queue, &valueToSend, 0);
xQueueReceive(queue, &valueReceived, 0);
printf("%d", valueReceived);

A 42

B 0

C Garbage value (uninitialized)

D Program crashes

💡 Hint

Remember that xQueueSend copies the data into the queue, and xQueueReceive copies it out.

The queue holds one integer. The value 42 is sent and then received immediately, so the received value is 42.

🧠 Conceptual

intermediate

What happens if xQueueSend is called with a full queue and zero block time?

In FreeRTOS, if you call xQueueSend() on a queue that is already full and specify a block time of zero, what will happen?

A The function overwrites the oldest item in the queue

B The function blocks until space is available in the queue

C The function causes a system crash

D The function returns immediately with an error indicating the queue is full

💡 Hint

Think about what zero block time means for waiting.

With zero block time, xQueueSend() does not wait and returns immediately with an error if the queue is full.

🔧 Debug

advanced

Why does this xQueueReceive call block indefinitely?

Examine this code snippet:

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

xQueueReceive(queue, &receivedValue, portMAX_DELAY);
printf("%d", receivedValue);

The program blocks forever at xQueueReceive. Why?

FreeRTOS

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

xQueueReceive(queue, &receivedValue, portMAX_DELAY);
printf("%d", receivedValue);

A The queue size is zero, so receive cannot get data

B The queue was deleted before receive was called

C No item was ever sent to the queue, so receive waits forever

D The receive buffer pointer is NULL

💡 Hint

Think about what portMAX_DELAY means and the queue state.

The receive call waits indefinitely for data, but since no data was sent, it blocks forever.

📝 Syntax

advanced

Which option correctly sends a struct to a FreeRTOS queue?

Given the struct:

typedef struct {
  int id;
  float value;
} SensorData;

Which code snippet correctly sends a SensorData variable to a queue?

FreeRTOS

typedef struct {
  int id;
  float value;
} SensorData;

SensorData data = {1, 23.5};
QueueHandle_t queue = xQueueCreate(5, sizeof(SensorData));

A xQueueSend(queue, data, 0);

B xQueueSend(queue, &data, 0);

C xQueueSend(queue, *data, 0);

D xQueueSend(queue, &data.id, 0);

💡 Hint

Remember that xQueueSend expects a pointer to the data to copy.

Option B passes the address of the struct variable, which is correct. Others either pass the struct by value or wrong pointer.

🚀 Application

expert

How many items can this queue hold and why?

Consider this queue creation:

QueueHandle_t queue = xQueueCreate(3, sizeof(char));

How many characters can this queue hold at once, and why?

FreeRTOS

QueueHandle_t queue = xQueueCreate(3, sizeof(char));

A 3 characters, because the queue length is 3 and each item is one char

B 1 character, because the queue size is the size of one char only

C Depends on the memory available, not fixed by parameters

D 0 characters, because the queue is not initialized properly

💡 Hint

Think about the parameters of xQueueCreate: length and item size.

The first parameter is the number of items the queue can hold, the second is the size of each item. So this queue holds 3 chars.

Practice

1. What is the primary purpose of the xQueueSend() function in FreeRTOS?

easy

A. To remove an item from a queue

B. To add an item to a queue for other tasks to receive

C. To create a new queue

D. To delete an existing queue

Solution

1. Step 1: Understand the role of xQueueSend()

   xQueueSend() is used to place data into a queue so that other tasks can retrieve it later.

2. Step 2: Differentiate from other queue functions

   Functions like xQueueReceive() remove data, while xQueueCreate() and xQueueDelete() manage queue lifecycle, not data transfer.

3. Final Answer:

   To add an item to a queue for other tasks to receive -> Option B

4. Quick Check:

   xQueueSend() adds data [OK]

Hint: Remember: Send puts data in, Receive takes data out [OK]

Common Mistakes:

• Confusing sending with receiving
• Thinking it creates or deletes queues
• Ignoring the function's data transfer role

2. Which of the following is the correct syntax to send data to a queue using xQueueSend()?

easy

A. xQueueSend(queueHandle, data, 0);

B. xQueueSend(&queueHandle, data, portMAX_DELAY);

C. xQueueSend(queueHandle, &data, portMAX_DELAY);

D. xQueueSend(queueHandle, &data);

Solution

1. Step 1: Check parameter types for xQueueSend()

   The function expects the queue handle, a pointer to the data, and a timeout value.

2. Step 2: Match the correct syntax

   xQueueSend(queueHandle, &data, portMAX_DELAY); correctly passes the queue handle, address of data, and a timeout constant portMAX_DELAY.

3. Final Answer:

   xQueueSend(queueHandle, &data, portMAX_DELAY); -> Option C

4. Quick Check:

   Queue handle + &data + timeout = correct syntax [OK]

Hint: Always pass data by address and include timeout [OK]

Common Mistakes:

• Passing data by value instead of pointer
• Omitting the timeout parameter
• Passing queue handle by address

3. Consider the following code snippet:

int value = 10;
if (xQueueSend(queue, &value, 0) == pdPASS) {
    int receivedValue = 0;
    if (xQueueReceive(queue, &receivedValue, 0) == pdPASS) {
        printf("%d", receivedValue);
    }
}

What will be printed when this code runs?

medium

A. 10

B. 0

C. No output (queue empty)

D. Compilation error

Solution

1. Step 1: Analyze xQueueSend() call

   The value 10 is sent to the queue with zero wait time. If the queue is not full, it returns pdPASS.

2. Step 2: Analyze xQueueReceive() call

   The code tries to receive from the queue immediately. Since the value was just sent, it will receive 10 and print it.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Send 10, receive 10, print 10 [OK]

Hint: Send then receive immediately prints sent value [OK]

Common Mistakes:

• Assuming queue is empty at receive
• Confusing return values with data
• Ignoring zero timeout behavior

4. What is wrong with this code snippet?

int data = 5;
if (xQueueSend(queue, data, 10) != pdPASS) {
    // handle error
}

medium

A. Data should be passed by pointer, not by value

B. Timeout value 10 is invalid

C. Return value should not be compared to pdPASS

D. Queue handle is missing

Solution

1. Step 1: Check parameter types in xQueueSend()

   The second parameter must be a pointer to the data, but here data is passed by value.

2. Step 2: Identify the error cause

   Passing data by value causes a type mismatch and likely a compilation error or incorrect behavior.

3. Final Answer:

   Data should be passed by pointer, not by value -> Option A

4. Quick Check:

   Pass &data, not data [OK]

Hint: Always pass address of data to queue functions [OK]

Common Mistakes:

• Passing data directly instead of pointer
• Misunderstanding timeout parameter
• Ignoring return value check

5. You want to send multiple integers to a queue and then receive them in order. Which approach correctly uses xQueueSend() and xQueueReceive() to achieve this without losing data?

hard

A. Use xQueueReceive() to send data and xQueueSend() to receive data

B. Send all integers at once by passing an array pointer to xQueueSend(), then receive with one xQueueReceive()

C. Send integers without checking return values, then receive once expecting all data

D. Send each integer with xQueueSend() using a timeout, then receive each with xQueueReceive() using a timeout, checking return values each time

Solution

1. Step 1: Understand queue data handling

   Queues handle one item at a time, so each integer must be sent individually with xQueueSend().

2. Step 2: Properly receive data

   Each item must be received individually with xQueueReceive(), checking return values to avoid errors or data loss.

3. Step 3: Eliminate incorrect options

   Send all integers at once by passing an array pointer to xQueueSend(), then receive with one xQueueReceive() is wrong because queues do not send arrays as single items. Send integers without checking return values, then receive once expecting all data risks data loss by ignoring return values. Use xQueueReceive() to send data and xQueueSend() to receive data reverses function roles.

4. Final Answer:

   Send each integer with xQueueSend() using a timeout, then receive each with xQueueReceive() using a timeout, checking return values each time -> Option D

5. Quick Check:

   Send and receive items one by one, check returns [OK]

Hint: Send and receive items individually, always check returns [OK]

Common Mistakes:

• Trying to send multiple items at once
• Ignoring return values causing lost data
• Mixing up send and receive functions