source: bootcd/isolinux/syslinux-6.03/dos/free.c

/*
 * free.c
 *
 * Very simple linked-list based malloc()/free().
 */

#include <stdlib.h>
#include "malloc.h"

static struct free_arena_header *__free_block(struct free_arena_header *ah)
{
    struct free_arena_header *pah, *nah;

    pah = ah->a.prev;
    nah = ah->a.next;
    if (pah->a.type == ARENA_TYPE_FREE &&
        (char *)pah + pah->a.size == (char *)ah) {
        /* Coalesce into the previous block */
        pah->a.size += ah->a.size;
        pah->a.next = nah;
        nah->a.prev = pah;

#ifdef DEBUG_MALLOC
        ah->a.type = ARENA_TYPE_DEAD;
#endif

        ah = pah;
        pah = ah->a.prev;
    } else {
        /* Need to add this block to the free chain */
        ah->a.type = ARENA_TYPE_FREE;

        ah->next_free = __malloc_head.next_free;
        ah->prev_free = &__malloc_head;
        __malloc_head.next_free = ah;
        ah->next_free->prev_free = ah;
    }

    /* In either of the previous cases, we might be able to merge
       with the subsequent block... */
    if (nah->a.type == ARENA_TYPE_FREE &&
        (char *)ah + ah->a.size == (char *)nah) {
        ah->a.size += nah->a.size;

        /* Remove the old block from the chains */
        nah->next_free->prev_free = nah->prev_free;
        nah->prev_free->next_free = nah->next_free;
        ah->a.next = nah->a.next;
        nah->a.next->a.prev = ah;

#ifdef DEBUG_MALLOC
        nah->a.type = ARENA_TYPE_DEAD;
#endif
    }

    /* Return the block that contains the called block */
    return ah;
}

void free(void *ptr)
{
    struct free_arena_header *ah;

    if (!ptr)
        return;

    ah = (struct free_arena_header *)
        ((struct arena_header *)ptr - 1);

    __free_block(ah);

    /* Here we could insert code to return memory to the system. */
}