memory pool: fast allocate & deallocate a single item memory

mempool.c

#include <errno.h>

#include "mempool.h"


int
mempool_init(MemPool *m, size_t nmemb, size_t size)
{
	m->nmemb = nmemb;
	m->head = NULL;

	for (size_t i = 0; i < size; i++) {
		MemPoolNode *const node = malloc(sizeof(MemPoolNode) * nmemb);
		if (node == NULL)
			goto err;

		node->next = m->head;
		m->head = node;
	}

	return 0;

err:
	mempool_deinit(m);
	return -ENOMEM;
}


void
mempool_deinit(MemPool *m)
{
	MemPoolNode *head = m->head;
	while (head != NULL) {
		MemPoolNode *const next = head->next;
		free(head);

		head = next;
	}

	m->head = NULL;
}


void *
mempool_alloc(MemPool *m)
{
	MemPoolNode *head = m->head;
	if (head != NULL) {
		m->head = head->next;
	} else {
		head = malloc(sizeof(MemPoolNode) * m->nmemb);
		if (head == NULL)
			return NULL;
	}

	return &head->data;
}


void
mempool_reserve(MemPool *m, void *mem)
{
	// not tested yet!
	const typeof(((MemPoolNode *)0)->data) *ptr = mem;
	MemPoolNode *const node = (MemPoolNode *)(((char *)ptr) - offsetof(MemPoolNode, data));
	
	//MemPoolNode *const node = (MemPoolNode *)(((char *)mem) - sizeof(MemPoolNode));
	node->next = m->head;
	m->head = node;
}


void
mempool_free(MemPool *m, void *mem)
{
	(void)m;

	MemPoolNode *const node = (MemPoolNode *)(((char *)mem) - sizeof(MemPoolNode));
	free(node);
}

mempool.h

#ifndef __MEMPOOL_H__
#define __MEMPOOL_H__


#include <stdlib.h>


typedef struct mempool_node {
	struct mempool_node *next;
	char                 data[];
} MemPoolNode;

typedef struct {
	size_t       nmemb;
	MemPoolNode *head;
} MemPool;


int   mempool_init(MemPool *m, size_t nmemb, size_t size);
void  mempool_deinit(MemPool *m);
void *mempool_alloc(MemPool *m);
void  mempool_reserve(MemPool *m, void *mem);
void  mempool_free(MemPool *m, void *mem);


#endif