Ring buffer implementation in Embedded C - Step-by-Step Execution

Concept Flow - Ring buffer implementation

Start

↓

Check if buffer full?

Yes→Wait or overwrite

No↓

Write data at tail

↓

Move tail forward

↓

Check if buffer empty?

Yes→No data to read

No↓

Read data at head

↓

Move head forward

↓

Repeat or Stop

The ring buffer moves head and tail pointers to write and read data in a circular way, checking for full or empty conditions.

Execution Sample

Embedded C

char buffer[4];
int head=0, tail=0;
// Write 'A'
buffer[tail] = 'A';
tail = (tail + 1) % 4;

This code writes a character 'A' into the buffer at the tail position and moves the tail pointer forward circularly.

Execution Table

Step	Action	Head	Tail	Buffer State	Condition	Result
1	Initialize head and tail	0	0	['', '', '', '']	N/A	Start with empty buffer
2	Write 'A' at tail	0	0	['A', '', '', '']	Buffer full? No	Write allowed
3	Move tail forward	0	1	['A', '', '', '']	N/A	Tail updated
4	Write 'B' at tail	0	1	['A', 'B', '', '']	Buffer full? No	Write allowed
5	Move tail forward	0	2	['A', 'B', '', '']	N/A	Tail updated
6	Read data at head	0	2	['A', 'B', '', '']	Buffer empty? No	Read 'A'
7	Move head forward	1	2	['A', 'B', '', '']	N/A	Head updated
8	Write 'C' at tail	1	2	['A', 'B', 'C', '']	Buffer full? No	Write allowed
9	Move tail forward	1	3	['A', 'B', 'C', '']	N/A	Tail updated
10	Write 'D' at tail	1	3	['A', 'B', 'C', '']	Buffer full? Yes	Cannot write or overwrite
11	Read data at head	1	3	['A', 'B', 'C', '']	Buffer empty? No	Read 'B'
12	Move head forward	2	3	['A', 'B', 'C', '']	N/A	Head updated
13	Write 'D' at tail	2	3	['A', 'B', 'C', 'D']	Buffer full? No	Write allowed
14	Move tail forward	2	0	['A', 'B', 'C', 'D']	N/A	Tail wrapped to 0
15	Read data at head	2	0	['A', 'B', 'C', 'D']	Buffer empty? No	Read 'C'
16	Move head forward	3	0	['A', 'B', 'C', 'D']	N/A	Head updated
17	Read data at head	3	0	['A', 'B', 'C', 'D']	Buffer empty? No	Read 'D'
18	Move head forward	0	0	['A', 'B', 'C', 'D']	N/A	Head wrapped to 0
19	Buffer empty check	0	0	['A', 'B', 'C', 'D']	Head == Tail	Buffer empty now
20	Stop	0	0	['A', 'B', 'C', 'D']	N/A	No more data to read

💡 Execution stops when head equals tail, meaning buffer is empty.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	After Step 9	After Step 12	After Step 14	After Step 16	After Step 18	Final
head	0	0	0	1	1	2	2	3	0	0
tail	0	1	2	2	3	3	0	0	0	0
buffer	['', '', '', '']	['A', '', '', '']	['A', 'B', '', '']	['A', 'B', '', '']	['A', 'B', 'C', '']	['A', 'B', 'C', '']	['A', 'B', 'C', 'D']	['A', 'B', 'C', 'D']	['A', 'B', 'C', 'D']	['A', 'B', 'C', 'D']

Key Moments - 3 Insights

Why does the tail wrap back to 0 after reaching the buffer size?

Because the tail uses modulo operation with buffer size (tail = (tail + 1) % size), it wraps around to start to keep the buffer circular, as shown in step 14.

What happens when the buffer is full and we try to write more data?

The write is blocked or handled specially because tail + 1 would equal head, indicating full buffer (step 10). This prevents overwriting unread data.

How do we know when the buffer is empty?

When head equals tail, no unread data remains (step 19). This condition signals the buffer is empty.

Visual Quiz - 3 Questions

Look at the execution table at step 14, what is the value of tail?

A 4

B 3

C 0

D 2

💡 Hint

Check the 'Tail' column at step 14 in the execution_table.

At which step does the buffer become empty again?

A Step 10

B Step 19

C Step 7

D Step 3

💡 Hint

Look for the step where head equals tail in the execution_table.

If we increase buffer size to 5, how would step 10 change?

A Buffer would not be full, write allowed

B Buffer would still be full at step 10

C Head would move instead of tail

D Tail would not wrap

💡 Hint

Consider buffer full condition depends on buffer size and tail position in execution_table.

Concept Snapshot

Ring buffer stores data in a fixed-size array.
Use head and tail pointers to track read/write positions.
Tail moves forward on write; head moves forward on read.
Pointers wrap around using modulo to stay within buffer size.
Buffer full when (tail + 1) % size == head.
Buffer empty when head == tail.

Full Transcript

A ring buffer uses two pointers, head and tail, to manage data in a circular array. Writing data places it at the tail position and moves tail forward. Reading data takes from head and moves head forward. Both pointers wrap around to the start when reaching the buffer's end. The buffer is full when moving tail forward would collide with head, and empty when head equals tail. This trace shows writing and reading characters, pointer updates, and buffer full/empty checks step-by-step.