Prime sieve using queues

sieve.c

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include "queue.h"

static int *numbers;

Queue *generate(int n)
{
	Queue *input = queue_new();
	int i;

	// Initial input is a queue of numbers from 2 to n
	for (i = 2; i <= n; i++) {
		numbers[i] = i;
		queue_enqueue(input, &numbers[i]);
	}

	return input;
}

Queue *sieve(Queue *input)
{
	Queue *output;
	int *n, prime;

	if (queue_empty(input)) {
		// End of recursion
		free(input);
		return NULL;
	}

	// First element of input is prime
	prime = *(int *)queue_dequeue(input);
	printf("%2d is prime\n", prime);

	output = queue_new();

	while ((n = (int *)queue_dequeue(input)) != NULL) {
		// Remove all multiples of prime; those are not prime
		if (*n % prime != 0) {
			queue_enqueue(output, n);
		} 
	}

	assert(queue_empty(input));
	free(input);

	return sieve(output);
}

int main(int argc, char *argv[])
{
	int n;

	if (argc != 2 || (n = atoi(argv[1])) < 2) {
		printf("Usage: %s <upper limit>\n", argv[0]);
		return 0;
	}
	
	numbers = malloc((n+1) * sizeof(int));
	if (!numbers) {
		fprintf(stderr, "Out of memory\n");
		return 1;
	}

	sieve(generate(n));

	free(numbers);

	return 0;
}