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.