Ring buffer implementation in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of ring buffer enqueue and dequeue operations

Consider a ring buffer of size 4. After enqueuing 3 elements and dequeuing 1, what is the value returned by the next dequeue?

Embedded C

typedef struct {
    int buffer[4];
    int head;
    int tail;
    int max;
    int count;
} RingBuffer;

void enqueue(RingBuffer *rb, int val) {
    if (rb->count == rb->max) return; // full
    rb->buffer[rb->head] = val;
    rb->head = (rb->head + 1) % rb->max;
    rb->count++;
}

int dequeue(RingBuffer *rb) {
    if (rb->count == 0) return -1; // empty
    int val = rb->buffer[rb->tail];
    rb->tail = (rb->tail + 1) % rb->max;
    rb->count--;
    return val;
}

int main() {
    RingBuffer rb = {.head=0, .tail=0, .max=4, .count=0};
    enqueue(&rb, 10);
    enqueue(&rb, 20);
    enqueue(&rb, 30);
    dequeue(&rb);
    int result = dequeue(&rb);
    return result;
}

A 20

B 10

C 30

D -1

💡 Hint

Remember that dequeue removes the oldest element first.

After enqueuing 10, 20, 30, the buffer has [10, 20, 30]. Dequeue removes 10 first. The next dequeue returns 20.

🧠 Conceptual

intermediate

Ring buffer full condition check

Which condition correctly checks if a ring buffer is full?

A rb->count == rb->max

B rb->head == rb->tail

C (rb->head + 1) % rb->max == rb->tail

D rb->count == 0

💡 Hint

Count tracks how many elements are currently stored.

The buffer is full when the number of stored elements equals its maximum size.

🔧 Debug

advanced

Identify the bug in ring buffer enqueue function

What is the bug in this enqueue function for a ring buffer?

Embedded C

void enqueue(RingBuffer *rb, int val) {
    if (rb->count == rb->max) return;
    rb->buffer[rb->head] = val;
    rb->head = (rb->head + 1) % rb->max;
    rb->count++;
}

A The buffer array is accessed out of bounds

B The condition should be rb->count == rb->max, not >

C The count should be decremented, not incremented

D The head index should not wrap using modulo

💡 Hint

Check the condition that prevents adding when full.

The condition allows enqueue when count equals max, causing overflow. It should be == to prevent adding when full.

📝 Syntax

advanced

Syntax error in ring buffer initialization

Which option correctly initializes a ring buffer struct in C?

A RingBuffer rb = {head:0, tail:0, max:8, count:0};

B RingBuffer rb = [0, 0, 8, 0];

C RingBuffer rb = (head=0, tail=0, max:8, count=0);

D RingBuffer rb = {.head=0, .tail=0, .max=8, .count=0};

💡 Hint

Use designated initializers with dot notation in C.

Option D uses correct C99 designated initializer syntax. Others are invalid syntax.

🚀 Application

expert

Calculate number of items after mixed operations

Given a ring buffer of max size 5, starting empty, after these operations:

1. enqueue 1
2. enqueue 2
3. dequeue
4. enqueue 3
5. enqueue 4
6. enqueue 5
7. dequeue
8. enqueue 6

How many items are currently stored in the buffer?

A 3

B 5

C 4

D 2

💡 Hint

Track count carefully after each enqueue and dequeue.

Operations add or remove items, final count is 4.

Practice

1.

What is the main purpose of the head pointer in a ring buffer?

easy

A. To reset the buffer to empty

B. To indicate where data will be read next

C. To store the total size of the buffer

D. To indicate where new data will be written next

Solution

1. Step 1: Understand the role of head in ring buffer

   The head pointer marks the position where the next data element will be stored (written) in the buffer.

2. Step 2: Differentiate head from tail

   The tail pointer is used for reading data, while the head is for writing new data.

3. Final Answer:

   To indicate where new data will be written next -> Option D

4. Quick Check:

   Head = write position [OK]

Hint: Head points to write location, tail points to read location [OK]

Common Mistakes:

• Confusing head with tail pointer
• Thinking head stores buffer size
• Assuming head resets the buffer

2.

Which of the following is the correct way to advance the head pointer in a ring buffer of size BUFFER_SIZE after writing one element?

head = ?;

easy

A. head = BUFFER_SIZE - head;

B. head = head + 1;

C. head = (head + 1) % BUFFER_SIZE;

D. head = (head - 1) % BUFFER_SIZE;

Solution

1. Step 1: Understand circular increment

   To move head forward in a circular buffer, increment and wrap around using modulo with buffer size.

2. Step 2: Apply modulo operation

   The expression (head + 1) % BUFFER_SIZE ensures head wraps to 0 after reaching the end.

3. Final Answer:

   head = (head + 1) % BUFFER_SIZE; -> Option C

4. Quick Check:

   Use modulo to wrap index [OK]

Hint: Use modulo (%) to wrap pointer in ring buffer [OK]

Common Mistakes:

• Not using modulo causes overflow
• Subtracting instead of adding
• Incorrect wrap calculation

3.

Given this code snippet for a ring buffer of size 4, what will be the value of head and tail after these operations?

int head = 0, tail = 0;
int BUFFER_SIZE = 4;

// Write 3 elements
for (int i = 0; i < 3; i++) {
    head = (head + 1) % BUFFER_SIZE;
}

// Read 1 element
tail = (tail + 1) % BUFFER_SIZE;

medium

A. head = 3, tail = 0

B. head = 3, tail = 1

C. head = 2, tail = 1

D. head = 1, tail = 1

Solution

1. Step 1: Calculate head after writing 3 elements

   Starting at 0, head increments 3 times with modulo 4: head = (0+1)%4=1, then 2, then 3.

2. Step 2: Calculate tail after reading 1 element

   Tail starts at 0, increments once: tail = (0+1)%4=1.

3. Final Answer:

   head = 3, tail = 1 -> Option B

4. Quick Check:

   Head=3, Tail=1 after ops [OK]

Hint: Increment pointers with modulo for each operation [OK]

Common Mistakes:

• Forgetting modulo wrap
• Mixing head and tail values
• Counting increments incorrectly

4.

Identify the bug in this ring buffer write function and select the correct fix:

void write(int data) {
    if ((head + 1) == tail) {
        // Buffer full
        return;
    }
    buffer[head] = data;
    head = (head + 1) % BUFFER_SIZE;
}

medium

A. Change condition to ((head + 1) % BUFFER_SIZE) == tail

B. Remove the condition; buffer never gets full

C. Increment tail instead of head after write

D. Use head == tail to check full buffer

Solution

1. Step 1: Analyze buffer full condition

   The buffer is full when the next head position wraps to tail, so modulo is needed to wrap index.

2. Step 2: Correct condition with modulo

   Using ((head + 1) % BUFFER_SIZE) == tail correctly detects full buffer.

3. Final Answer:

   Change condition to ((head + 1) % BUFFER_SIZE) == tail -> Option A

4. Quick Check:

   Modulo needed for circular check [OK]

Hint: Always use modulo when comparing circular indices [OK]

Common Mistakes:

• Missing modulo causes wrong full detection
• Confusing head and tail roles
• Incorrectly incrementing tail on write

5.

You want to implement a ring buffer that stores bytes and can hold exactly 8 bytes. Which of these buffer sizes and pointer update methods correctly avoids ambiguity between full and empty states?

Options:

1. Buffer size = 8, full when (head + 1) % 8 == tail
2. Buffer size = 9, full when (head + 1) % 9 == tail
3. Buffer size = 8, full when head == tail
4. Buffer size = 9, full when head == tail

hard

A. Use buffer size 9 with full condition (head + 1) % 9 == tail

B. Use buffer size 8 with full condition (head + 1) % 8 == tail

C. Use buffer size 8 with full condition head == tail

D. Use buffer size 9 with full condition head == tail

Solution

1. Step 1: Understand full vs empty ambiguity

   When head == tail, buffer is empty. To distinguish full, buffer size is often set to capacity + 1.

2. Step 2: Choose buffer size and condition

   Using buffer size 9 for 8 bytes capacity and full condition (head + 1) % 9 == tail avoids ambiguity.

3. Final Answer:

   Use buffer size 9 with full condition (head + 1) % 9 == tail -> Option A

4. Quick Check:

   Extra slot avoids full/empty confusion [OK]

Hint: Use buffer size one more than capacity to avoid confusion [OK]

Common Mistakes:

• Using buffer size equal to capacity causes ambiguity
• Checking full with head == tail
• Not reserving one slot for full detection