dos/malloc.c

   1 /*
   2  * malloc.c
   3  *
   4  * Very simple linked-list based malloc()/free().
   5  */
   6
   7 #include <stdlib.h>
   8 #include "malloc.h"
   9
  10 struct free_arena_header __malloc_head =
  11 {
  12   {
  13     ARENA_TYPE_HEAD,
  14     0,
  15     &__malloc_head,
  16     &__malloc_head,
  17   },
  18   &__malloc_head,
  19   &__malloc_head
  20 };
  21
  22 /* This is extern so it can be overridden by the user application */
  23 const size_t __stack_size = 4096;
  24
  25 static inline size_t sp(void)
  26 {
  27   uint32_t sp;
  28   asm volatile("movl %%esp,%0" : "=rm" (sp));
  29   return sp;
  30 }
  31
  32 extern void *__mem_end;
  33
  34 void __init_memory_arena(void)
  35 {
  36   struct free_arena_header *fp;
  37   size_t start, total_space;
  38
  39   start = (size_t)ARENA_ALIGN_UP(__mem_end);
  40   total_space = sp() - start;
  41
  42   fp = (struct free_arena_header *)start;
  43   fp->a.type = ARENA_TYPE_FREE;
  44   fp->a.size = total_space - __stack_size;
  45
  46   /* Insert into chains */
  47   fp->a.next = fp->a.prev = &__malloc_head;
  48   fp->next_free = fp->prev_free = &__malloc_head;
  49   __malloc_head.a.next = __malloc_head.a.prev = fp;
  50   __malloc_head.next_free = __malloc_head.prev_free = fp;
  51 }
  52
  53 static void *__malloc_from_block(struct free_arena_header *fp, size_t size)
  54 {
  55   size_t fsize;
  56   struct free_arena_header *nfp, *na;
  57
  58   fsize = fp->a.size;
  59
  60   /* We need the 2* to account for the larger requirements of a free block */
  61   if ( fsize >= size+2*sizeof(struct arena_header) ) {
  62     /* Bigger block than required -- split block */
  63     nfp = (struct free_arena_header *)((char *)fp + size);
  64     na = fp->a.next;
  65
  66     nfp->a.type = ARENA_TYPE_FREE;
  67     nfp->a.size = fsize-size;
  68     fp->a.type  = ARENA_TYPE_USED;
  69     fp->a.size  = size;
  70
  71     /* Insert into all-block chain */
  72     nfp->a.prev = fp;
  73     nfp->a.next = na;
  74     na->a.prev = nfp;
  75     fp->a.next = nfp;
  76
  77     /* Replace current block on free chain */
  78     nfp->next_free = fp->next_free;
  79     nfp->prev_free = fp->prev_free;
  80     fp->next_free->prev_free = nfp;
  81     fp->prev_free->next_free = nfp;
  82   } else {
  83     /* Allocate the whole block */
  84     fp->a.type = ARENA_TYPE_USED;
  85
  86     /* Remove from free chain */
  87     fp->next_free->prev_free = fp->prev_free;
  88     fp->prev_free->next_free = fp->next_free;
  89   }
  90
  91   return (void *)(&fp->a + 1);
  92 }
  93
  94 void *malloc(size_t size)
  95 {
  96   struct free_arena_header *fp;
  97
  98   if ( size == 0 )
  99     return NULL;
 100
 101   /* Add the obligatory arena header, and round up */
 102   size = (size+2*sizeof(struct arena_header)-1) & ~ARENA_SIZE_MASK;
 103
 104   for ( fp = __malloc_head.next_free ; fp->a.type != ARENA_TYPE_HEAD ;
 105         fp = fp->next_free ) {
 106     if ( fp->a.size >= size ) {
 107       /* Found fit -- allocate out of this block */
 108       return __malloc_from_block(fp, size);
 109     }
 110   }
 111
 112   /* Nothing found... need to request a block from the kernel */
 113   return NULL;                  /* No kernel to get stuff from */
 114 }