Colas de prioridad en lenguaje C. Se toma como valor "nulo" al entero negativo -1, esto puede modificarse sin alterar el funcionamiento del programa.

Queues.c

#include <stdio.h>
#include <stdlib.h>

int i, size_queue = 5;

void queue_overflow()
{
    printf("Overflow!");
    exit(0);
}

void queue_underflow()
{
    printf("Underflow!");
    exit(0);
}

void print_queue(int *queue)
{
    for (i = 0; i < size_queue; i++)
    {
        if (*(queue + i) == -1) printf(" Nil ");
        else printf(" %i ", *(queue + i));
    }
}

void enqueue(int *queue, int *tail, int val)
{
    *tail = *tail + 1;
    if (*tail > size_queue) *tail = 1;
    if (*(queue + (*tail - 1)) == -1) *(queue + (*tail - 1)) = val;
    else queue_overflow();
}

void dequeue(int *queue, int *head)
{
    if (*(queue + *head) == -1) queue_underflow();
    *(queue + *head) = -1;
    *head = *head + 1;
    if (*head > size_queue) *head = 0;
}

void main()
{
    int queue[size_queue + 1], head, tail;

    for (i = 0; i < size_queue; i++) *(queue + i) = -1;
    head = 0;
    tail = 0;

    enqueue(queue, &tail, 5);
    enqueue(queue, &tail, 6);
    enqueue(queue, &tail, 8);
    enqueue(queue, &tail, 4);
    enqueue(queue, &tail, 2);
    dequeue(queue, &head);
    dequeue(queue, &head);
    enqueue(queue, &tail, 5);
    //enqueue(queue, &tail, 6);

    print_queue(queue);
}