Ring buffer implementation in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the ring buffer size constant.

Embedded C

const int BUFFER_SIZE = [1];

Drag options to blanks, or click blank then click option'

A128

B256

Csize

Dbuffer_size

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of a numeric constant.

The ring buffer size is set as a constant integer value. Here, 128 is a common buffer size.

2fill in blank

medium

Complete the code to declare the ring buffer array.

Embedded C

char ring_buffer[[1]];

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ABUFFER_SIZE

Bbuffer

Csize

D128

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not defined.

The ring buffer array is declared with the size constant BUFFER_SIZE.

3fill in blank

hard

Fix the error in the function to check if the ring buffer is empty.

Embedded C

int is_empty() {
    return [1] == tail;
}

Drag options to blanks, or click blank then click option'

Asize

Bbuffer

Ccount

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing wrong variables like buffer or count.

The ring buffer is empty when the head index equals the tail index.

4fill in blank

hard

Fill both blanks to correctly advance the head index in the ring buffer.

Embedded C

head = (head [1] 1) [2] BUFFER_SIZE;

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A+

B-

C%

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or division instead of addition and modulo.

The head index is incremented by 1 and wrapped around using modulo BUFFER_SIZE.

5fill in blank

hard

Fill all three blanks to implement the function that adds a character to the ring buffer safely.

Embedded C

int enqueue(char c) {
    if ((head [1] 1) [2] BUFFER_SIZE == tail) {
        return 0; // Buffer full
    }
    ring_buffer[head] = c;
    head = (head [3] 1) % BUFFER_SIZE;
    return 1;
}

Drag options to blanks, or click blank then click option'

A+

B-

C%

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition and modulo.

The function checks if the buffer is full by seeing if incrementing head wraps to tail. It uses addition and modulo to update the head index safely.

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