#include <stdlib.h>

#include <stdio.h>

#define MAX_I 0x00ffffff

#define bool char
#define TRUE 1
#define FALSE 0

struct object_s {
    char val;
};

struct block_s {
    struct block_s *next;
    struct block_s *prev;
    struct object_s obj;
};

static struct block_s pool[MAX_I];

static struct block_s *free_pool = NULL,
	       *busy_pool = NULL;

/*
 * инициализация пула
 */
void init()
{
	size_t i;

	size_t max_i = MAX_I - 1;
	for (i = 1; i < max_i; i++) {
		pool[i].prev = &pool[i - 1];
		pool[i].next = &pool[i + 1];
	}

	pool[0].prev = NULL;
	pool[0].next = &pool[1];

	pool[MAX_I - 1].prev = &pool[MAX_I - 2];
	pool[MAX_I - 1].next = NULL;

	free_pool = &pool[0];
	busy_pool = NULL;
}

inline void *pool_alloc()
{
	if (!free_pool)
		return NULL;

	free_pool->prev = free_pool->next; // save free_pool->next in free_pool->prev
	
	free_pool->next = busy_pool;

	if (busy_pool)
		busy_pool->prev = free_pool;

	busy_pool = free_pool;

	free_pool = free_pool->prev;
	
	busy_pool->prev = NULL; // saved in free_pool->prev to NULL

	return &busy_pool->obj;
}

inline void pool_free(void *p)
{
	if (!p)
		return;

	struct block_s *b1 = (struct block_s *)(p - 2 * sizeof(struct block_s *)),
		       *b2 = b1->next;

	if (b1->prev)
		b1->prev->next = b2;
	if (b2)
		b2->prev = b1->prev;

	if (free_pool) {
		b1->next = free_pool;
		free_pool->prev = b1;
	}
	else
		b1->next = NULL;

	free_pool = b2;

	if (busy_pool == b1)
		busy_pool = b2;
}

int main()
{
	unsigned long long i;


	init();

	void **mem = malloc(MAX_I * sizeof(void *));
	
	for (i = 0; i < MAX_I; i++)
		if (!(mem[i] = pool_alloc())) {
			printf("не удалось выделить память под %lld блок\n", i);
			break;
		}

	for (i = 0; i < MAX_I; i++)
		pool_free(mem[i]);

	free(mem);

	return 0;
}