Problem: Implement a circular queue of integers of user-specified size using a simple array. Provide routines to initialize(), enqueue() and dequeue() the queue. Make it thread safe. Please implement this with your own code and do not use any class libraries or library calls.

circular_queue.c

/*
Problem

Implement a circular queue of integers of user-specified size using a simple array. 
Provide routines to initialize(), enqueue() and dequeue() the queue. Make it thread safe. 
Please implement this with your own code and do not use any class libraries or library calls.
*/

#include <malloc.h>

#define OK 0
#define ERROR -1

struct _Queue {
    int capacity;
    int head;
    int tail;
    int *buffer;
};

typedef struct _Queue Queue;

/* interface */

Queue* initialize(unsigned int capacity);
int destroy(Queue* queue);
int enqueue(Queue* queue, int element);
int dequeue(Queue* queue, int *out_element);
int size(Queue* queue);


/* implementation */

Queue* initialize(unsigned int capacity) {
    Queue* queue = (Queue*)malloc(sizeof(Queue));
    if (NULL == queue) {
        return NULL;
    }

    queue->capacity = capacity;
    queue->head = 0;
    queue->tail = 0;
    queue->buffer = (int*)malloc(sizeof(int) * (1 + capacity));
    if (NULL == queue->buffer) {
        free(queue);
        return NULL;
    }

    return queue;
}

int destroy(Queue *queue) {
    free(queue->buffer);
    free(queue);
    return OK;
}

int enqueue(Queue *queue, int element) {
    if (size(queue) >= queue->capacity) {
        return ERROR;
    }
    
    queue->buffer[queue->tail] = element;
    queue->tail = (queue->tail + 1) % (queue->capacity + 1);

    return OK;
}

int dequeue(Queue *queue, int *out_element) {
    if (size(queue) <= 0) {
        return ERROR;
    }

    *out_element = queue->buffer[queue->head];
    queue->head = (queue->head + 1) % (queue->capacity + 1);
    return OK;
}

int size(Queue *queue) {
    return (queue->tail - queue->head + (1 + queue->capacity)) % (1 + queue->capacity);
}