Ring buffer implementation in Embedded C - Mini Project: Build & Apply

Ring buffer implementation

📖 Scenario: You are working on a small embedded system that needs to store sensor readings temporarily. To do this efficiently, you will use a ring buffer (also called a circular buffer). This buffer will hold a fixed number of readings and overwrite the oldest data when full.

🎯 Goal: Build a simple ring buffer in C that can store integer sensor readings. You will create the buffer, set its size, add new readings, and print the buffer contents.

📋 What You'll Learn

Create an integer array called buffer with size 5

Create integer variables head and tail to track positions

Create an integer variable buffer_size set to 5

Write a function add_to_buffer that adds a new integer to the buffer and updates head and tail correctly

Write a function print_buffer that prints all current values in the buffer from tail to head

Demonstrate adding 7 readings and printing the buffer contents

💡 Why This Matters

🌍 Real World

Ring buffers are used in embedded devices to store sensor data, communication bytes, or audio samples temporarily without delays.

💼 Career

Understanding ring buffers is important for embedded software engineers working on real-time systems, device drivers, or communication protocols.

1

Create the ring buffer data structure

Create an integer array called buffer with size 5. Also create integer variables head and tail and set both to 0.

Embedded C

# Create the buffer array and position variables
// Your code here

Hint

Think of buffer as a fixed-size box to hold numbers. head and tail tell you where to add or remove data.

2

Add buffer size configuration

Create an integer variable called buffer_size and set it to 5.

Embedded C

int buffer[5];
int head = 0;
int tail = 0;
// Set the buffer size
// Your code here

Hint

This variable helps us know the size of the buffer for wrapping around.

3

Write the function to add data to the buffer

Write a function called add_to_buffer that takes an integer value. It should store value at buffer[head], then increment head by 1 modulo buffer_size. If head equals tail after increment, also increment tail by 1 modulo buffer_size to overwrite oldest data.

Embedded C

int buffer[5];
int head = 0;
int tail = 0;
int buffer_size = 5;

// Write add_to_buffer function below
// Your code here

Hint

Remember to wrap head and tail using modulo to stay inside the buffer size.

4

Print the buffer contents

Write a function called print_buffer that prints all values currently in the buffer from tail to head. Then add 7 values (1 to 7) to the buffer using add_to_buffer and call print_buffer to display the buffer contents.

Embedded C

int buffer[5];
int head = 0;
int tail = 0;
int buffer_size = 5;

void add_to_buffer(int value) {
    buffer[head] = value;
    head = (head + 1) % buffer_size;
    if (head == tail) {
        tail = (tail + 1) % buffer_size;
    }
}

// Write print_buffer function and add values below
// Your code here

Hint

Use a loop starting from tail and stop before head. Use modulo to wrap around.

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