Queue Implementations in C

Three different ways to implement a Queue (FIFO) data structure in C, written while studying and preparing to teach the topic.

Files

array_queue.c — Simple Array-Based Queue

The most basic implementation: a fixed-size array with front and rear indices that only move forward.

Demonstrates the false overflow problem — once rear reaches the end of the array, no more elements can be added even if slots at the front have been freed by dequeue().

circular_queue.c — Circular Queue

Fixes the false overflow problem using modular arithmetic: rear = (rear + 1) % CAPACITY.

Once the end of the array is reached, the index wraps back around to 0, so freed slots at the front are reused. Still fixed capacity, but no wasted memory.

linked_list_queue.c — Linked List Queue

Uses dynamically allocated nodes (malloc/free) instead of a fixed-size array.

No capacity limit and no false overflow, at the cost of extra memory per node (a pointer) and the responsibility of managing allocation/deallocation correctly.

Build & Run

Each file is self-contained and can be compiled independently:

gcc -Wall -o array_queue array_queue.c && ./array_queue
gcc -Wall -o circular_queue circular_queue.c && ./circular_queue
gcc -Wall -o linked_list_queue linked_list_queue.c && ./linked_list_queue

Summary

	Array (simple)	Circular	Linked List
enqueue / dequeue	O(1)	O(1)	O(1)
Capacity	Fixed	Fixed	Dynamic
False overflow	Yes	No	No
malloc / free needed	No	No	Yes