Ring buffer for UART receive in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this ring buffer read function?

Consider this simplified ring buffer read function for UART data. What will be the returned value when ring_buffer_read() is called?

Embedded C

typedef struct {
    unsigned char buffer[4];
    unsigned int head;
    unsigned int tail;
} RingBuffer;

RingBuffer rb = {{'A', 'B', 'C', 'D'}, 2, 0};

unsigned char ring_buffer_read(RingBuffer *rb) {
    if (rb->head == rb->tail) {
        return 0; // Buffer empty
    }
    unsigned char data = rb->buffer[rb->tail];
    rb->tail = (rb->tail + 1) % 4;
    return data;
}

int main() {
    unsigned char c = ring_buffer_read(&rb);
    printf("%c", c);
    return 0;
}

A A

B B

C C

D D

💡 Hint

Remember that tail points to the next byte to read.

The tail is at 0, so the first byte read is buffer[0] which is 'A'.

❓ Predict Output

intermediate

What is the value of tail after reading one byte?

Given the same ring buffer as before, what is the value of rb.tail after calling ring_buffer_read(&rb) once?

Embedded C

typedef struct {
    unsigned char buffer[4];
    unsigned int head;
    unsigned int tail;
} RingBuffer;

RingBuffer rb = {{'A', 'B', 'C', 'D'}, 2, 0};

unsigned char ring_buffer_read(RingBuffer *rb) {
    if (rb->head == rb->tail) {
        return 0; // Buffer empty
    }
    unsigned char data = rb->buffer[rb->tail];
    rb->tail = (rb->tail + 1) % 4;
    return data;
}

int main() {
    ring_buffer_read(&rb);
    printf("%u", rb.tail);
    return 0;
}

A 1

B 2

C 3

D 0

💡 Hint

Tail moves forward by one position modulo buffer size.

Tail was 0, after reading one byte it becomes (0 + 1) % 4 = 1.

❓ Predict Output

advanced

What happens if head equals tail in this ring buffer?

What will ring_buffer_read() return if rb.head equals rb.tail?

Embedded C

typedef struct {
    unsigned char buffer[4];
    unsigned int head;
    unsigned int tail;
} RingBuffer;

unsigned char ring_buffer_read(RingBuffer *rb) {
    if (rb->head == rb->tail) {
        return 0; // Buffer empty
    }
    unsigned char data = rb->buffer[rb->tail];
    rb->tail = (rb->tail + 1) % 4;
    return data;
}

int main() {
    RingBuffer rb = {{'A', 'B', 'C', 'D'}, 1, 1};
    unsigned char c = ring_buffer_read(&rb);
    printf("%d", c);
    return 0;
}

A A

B 1

C 0

D Undefined behavior

💡 Hint

When head equals tail, the buffer is empty.

The function returns 0 to indicate the buffer is empty.

❓ Predict Output

advanced

What is the output after writing and reading in this ring buffer?

Given this code that writes a byte then reads one, what is printed?

Embedded C

typedef struct {
    unsigned char buffer[4];
    unsigned int head;
    unsigned int tail;
} RingBuffer;

void ring_buffer_write(RingBuffer *rb, unsigned char data) {
    rb->buffer[rb->head] = data;
    rb->head = (rb->head + 1) % 4;
}

unsigned char ring_buffer_read(RingBuffer *rb) {
    if (rb->head == rb->tail) {
        return 0;
    }
    unsigned char data = rb->buffer[rb->tail];
    rb->tail = (rb->tail + 1) % 4;
    return data;
}

int main() {
    RingBuffer rb = {{0}, 0, 0};
    ring_buffer_write(&rb, 'X');
    unsigned char c = ring_buffer_read(&rb);
    printf("%c", c);
    return 0;
}

A 0

B X

C A

D Undefined

💡 Hint

Write adds 'X' at head, then read returns from tail.

Write puts 'X' at buffer[0], head moves to 1. Read returns buffer[0] which is 'X'.

❓ Predict Output

expert

How many bytes can this ring buffer hold before head catches tail?

Given a ring buffer of size 4, what is the maximum number of bytes it can hold before it appears full (head catches tail)?

Embedded C

typedef struct {
    unsigned char buffer[4];
    unsigned int head;
    unsigned int tail;
} RingBuffer;

A 1

B 4

C 2

D 3

💡 Hint

One slot is left empty to distinguish full from empty.

To avoid confusion between full and empty, one slot remains unused, so max is size - 1 = 3.