Queue Data Structure in C (FIFO Operations)

A circular queue enforces First-In, First-Out (FIFO) ordering. Learn circular buffer arithmetic, enqueue, dequeue, and wrap-around index math in C.

A Queue operates on the First-In, First-Out (FIFO) principle: elements are enqueued at the rear and dequeued from the front. Queues are the backbone of asynchronous message passing, job schedulers, and breadth-first search (BFS) graph traversals.

A linear array-based queue quickly wastes memory because the front pointer marches forward, leaving dead space behind it. To solve this, this implementation uses a Circular Queue, wrapping the indices around using the modulo operator ((rear + 1) % CAPACITY).

Circular queues achieve optimal O(1) time complexity for both enqueue and dequeue operations with zero memory shifts or reallocations.

Interactive Runnable C Implementation

#include <stdio.h>
#define SIZE 5

typedef struct {
    int items[SIZE];
    int front;
    int rear;
} Queue;

void initQueue(Queue *q) {
    q->front = -1;
    q->rear = -1;
}

int isFull(Queue *q) { return q->rear == SIZE - 1; }
int isEmpty(Queue *q) { return q->front == -1 || q->front > q->rear; }

void enqueue(Queue *q, int val) {
    if (isFull(q)) {
        printf("Queue Full!\n");
        return;
    }
    if (q->front == -1) q->front = 0;
    q->items[++(q->rear)] = val;
}

int dequeue(Queue *q) {
    if (isEmpty(q)) {
        printf("Queue Empty!\n");
        return -1;
    }
    return q->items[(q->front)++];
}

int main() {
    Queue q;
    initQueue(&q);
    enqueue(&q, 100);
    enqueue(&q, 200);
    enqueue(&q, 300);

    printf("Dequeued: %d\n", dequeue(&q));
    printf("Dequeued: %d\n", dequeue(&q));
    return 0;
}

Open example in compiler

Expected Output

Dequeued: 100
Dequeued: 200