Queues (FIFO) Using Circular Buffers

A queue follows first‑in‑first‑out. A circular buffer efficiently implements a fixed‑size queue without moving elements.

#define CAP 5
int q[CAP];
int front = 0, rear = 0, size = 0;

int enqueue(int x) {
    if (size == CAP) return -1; // full
    q[rear] = x;
    rear = (rear + 1) % CAP;
    ++size;
    return 0;
}

int dequeue(int *out) {
    if (size == 0) return -1; // empty
    *out = q[front];
    front = (front + 1) % CAP;
    --size;
    return 0;
}