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 *
  11 __free_block(struct free_arena_header *ah)
  12 {
  13   struct free_arena_header *pah, *nah;
  14
  15   pah = ah->a.prev;
  16   nah = ah->a.next;
  17   if ( pah->a.type == ARENA_TYPE_FREE &&
  18        (char *)pah+pah->a.size == (char *)ah ) {
  19     /* Coalesce into the previous block */
  20     pah->a.size += ah->a.size;
  21     pah->a.next = nah;
  22     nah->a.prev = pah;
  23
  24 #ifdef DEBUG_MALLOC
  25     ah->a.type = ARENA_TYPE_DEAD;
  26 #endif
  27
  28     ah = pah;
  29     pah = ah->a.prev;
  30   } else {
  31     /* Need to add this block to the free chain */
  32     ah->a.type = ARENA_TYPE_FREE;
  33
  34     ah->next_free = __malloc_head.next_free;
  35     ah->prev_free = &__malloc_head;
  36     __malloc_head.next_free = ah;
  37     ah->next_free->prev_free = ah;
  38   }
  39
  40   /* In either of the previous cases, we might be able to merge
  41      with the subsequent block... */
  42   if ( nah->a.type == ARENA_TYPE_FREE &&
  43        (char *)ah+ah->a.size == (char *)nah ) {
  44     ah->a.size += nah->a.size;
  45
  46     /* Remove the old block from the chains */
  47     nah->next_free->prev_free = nah->prev_free;
  48     nah->prev_free->next_free = nah->next_free;
  49     ah->a.next = nah->a.next;
  50     nah->a.next->a.prev = ah;
  51
  52 #ifdef DEBUG_MALLOC
  53     nah->a.type = ARENA_TYPE_DEAD;
  54 #endif
  55   }
  56
  57   /* Return the block that contains the called block */
  58   return ah;
  59 }
  60
  61 void free(void *ptr)
  62 {
  63   struct free_arena_header *ah;
  64
  65   if ( !ptr )
  66     return;
  67
  68   ah = (struct free_arena_header *)
  69     ((struct arena_header *)ptr - 1);
  70
  71 #ifdef DEBUG_MALLOC
  72   assert( ah->a.type == ARENA_TYPE_USED );
  73 #endif
  74
  75   __free_block(ah);
  76
  77   /* Here we could insert code to return memory to the system. */
  78 }