dos/free.c

   1 /*
   2  * free.c
   3  *
   4  * Very simple linked-list based malloc()/free().
   5  */
   6
   7 #include <stdlib.h>
   8 #include "malloc.h"
   9
  10 static struct free_arena_header *__free_block(struct free_arena_header *ah)
  11 {
  12     struct free_arena_header *pah, *nah;
  13
  14     pah = ah->a.prev;
  15     nah = ah->a.next;
  16     if (pah->a.type == ARENA_TYPE_FREE &&
  17         (char *)pah + pah->a.size == (char *)ah) {
  18         /* Coalesce into the previous block */
  19         pah->a.size += ah->a.size;
  20         pah->a.next = nah;
  21         nah->a.prev = pah;
  22
  23 #ifdef DEBUG_MALLOC
  24         ah->a.type = ARENA_TYPE_DEAD;
  25 #endif
  26
  27         ah = pah;
  28         pah = ah->a.prev;
  29     } else {
  30         /* Need to add this block to the free chain */
  31         ah->a.type = ARENA_TYPE_FREE;
  32
  33         ah->next_free = __malloc_head.next_free;
  34         ah->prev_free = &__malloc_head;
  35         __malloc_head.next_free = ah;
  36         ah->next_free->prev_free = ah;
  37     }
  38
  39     /* In either of the previous cases, we might be able to merge
  40        with the subsequent block... */
  41     if (nah->a.type == ARENA_TYPE_FREE &&
  42         (char *)ah + ah->a.size == (char *)nah) {
  43         ah->a.size += nah->a.size;
  44
  45         /* Remove the old block from the chains */
  46         nah->next_free->prev_free = nah->prev_free;
  47         nah->prev_free->next_free = nah->next_free;
  48         ah->a.next = nah->a.next;
  49         nah->a.next->a.prev = ah;
  50
  51 #ifdef DEBUG_MALLOC
  52         nah->a.type = ARENA_TYPE_DEAD;
  53 #endif
  54     }
  55
  56     /* Return the block that contains the called block */
  57     return ah;
  58 }
  59
  60 void free(void *ptr)
  61 {
  62     struct free_arena_header *ah;
  63
  64     if (!ptr)
  65         return;
  66
  67     ah = (struct free_arena_header *)
  68         ((struct arena_header *)ptr - 1);
  69
  70 #ifdef DEBUG_MALLOC
  71     assert(ah->a.type == ARENA_TYPE_USED);
  72 #endif
  73
  74     __free_block(ah);
  75
  76     /* Here we could insert code to return memory to the system. */
  77 }