tiny-malloc.c

Newlib 嵌入式 C库 标准实现代码

/* A replacement malloc with:
   - Much reduced code size;
   - Smaller RAM footprint;
   - The ability to handle downward-growing heaps;
   but
   - Slower;
   - Probably higher memory fragmentation;
   - Doesn't support threads (but, if it did support threads,
     it wouldn't need a global lock, only a compare-and-swap instruction);
   - Assumes the maximum alignment required is the alignment of a pointer;
   - Assumes that memory is already there and doesn't need to be allocated.

* Synopsis of public routines

  malloc(size_t n);
     Return a pointer to a newly allocated chunk of at least n bytes, or null
     if no space is available.
  free(void* p);
     Release the chunk of memory pointed to by p, or no effect if p is null.
  realloc(void* p, size_t n);
     Return a pointer to a chunk of size n that contains the same data
     as does chunk p up to the minimum of (n, p's size) bytes, or null
     if no space is available. The returned pointer may or may not be
     the same as p. If p is null, equivalent to malloc.  Unless the
     #define REALLOC_ZERO_BYTES_FREES below is set, realloc with a
     size argument of zero (re)allocates a minimum-sized chunk.
  memalign(size_t alignment, size_t n);
     Return a pointer to a newly allocated chunk of n bytes, aligned
     in accord with the alignment argument, which must be a power of
     two.  Will fail if 'alignment' is too large.
  calloc(size_t unit, size_t quantity);
     Returns a pointer to quantity * unit bytes, with all locations
     set to zero.
  cfree(void* p);
     Equivalent to free(p).
  malloc_trim(size_t pad);
     Release all but pad bytes of freed top-most memory back 
     to the system. Return 1 if successful, else 0.
  malloc_usable_size(void* p);
     Report the number usable allocated bytes associated with allocated
     chunk p. This may or may not report more bytes than were requested,
     due to alignment and minimum size constraints.
  malloc_stats();
     Prints brief summary statistics on stderr.
  mallinfo()
     Returns (by copy) a struct containing various summary statistics.
  mallopt(int parameter_number, int parameter_value)
     Changes one of the tunable parameters described below. Returns
     1 if successful in changing the parameter, else 0.  Actually, returns 0
     always, as no parameter can be changed.
*/

#ifdef __xstormy16__
#define MALLOC_DIRECTION -1
#endif

#ifndef MALLOC_DIRECTION
#define MALLOC_DIRECTION 1
#endif

#include <stddef.h>

void* malloc(size_t);
void    free(void*);
void* realloc(void*, size_t);
void* memalign(size_t, size_t);
void* valloc(size_t);
void* pvalloc(size_t);
void* calloc(size_t, size_t);
void    cfree(void*);
int     malloc_trim(size_t);
size_t  malloc_usable_size(void*);
void    malloc_stats(void);
int     mallopt(int, int);
struct mallinfo mallinfo(void);

typedef struct freelist_entry {
  size_t size;
  struct freelist_entry *next;
} *fle;

extern void * __malloc_end;
extern fle __malloc_freelist;

/* Return the number of bytes that need to be added to X to make it
   aligned to an ALIGN boundary.  ALIGN must be a power of 2.  */
#define M_ALIGN(x, align) (-(size_t)(x) & ((align) - 1))

/* Return the number of bytes that need to be subtracted from X to make it
   aligned to an ALIGN boundary.  ALIGN must be a power of 2.  */
#define M_ALIGN_SUB(x, align) ((size_t)(x) & ((align) - 1))

extern void __malloc_start;

/* This is the minimum gap allowed between __malloc_end and the top of
   the stack.  This is only checked for when __malloc_end is
   decreased; if instead the stack grows into the heap, silent data
   corruption will result.  */
#define MALLOC_MINIMUM_GAP 32

#ifdef __xstormy16__
register void * stack_pointer asm ("r15");
#define MALLOC_LIMIT stack_pointer
#else
#define MALLOC_LIMIT __builtin_frame_address (0)
#endif

#if MALLOC_DIRECTION < 0
#define CAN_ALLOC_P(required)				\
  (((size_t) __malloc_end - (size_t)MALLOC_LIMIT	\
    - MALLOC_MINIMUM_GAP) >= (required))
#else
#define CAN_ALLOC_P(required)				\
  (((size_t)MALLOC_LIMIT - (size_t) __malloc_end	\
    - MALLOC_MINIMUM_GAP) >= (required))
#endif

/* real_size is the size we actually have to allocate, allowing for
   overhead and alignment.  */
#define REAL_SIZE(sz)						\
  ((sz) < sizeof (struct freelist_entry) - sizeof (size_t)	\
   ? sizeof (struct freelist_entry)				\
   : sz + sizeof (size_t) + M_ALIGN(sz, sizeof (size_t)))

#ifdef DEFINE_MALLOC

void * __malloc_end = &__malloc_start;
fle __malloc_freelist;

void *
malloc (size_t sz)
{
  fle *nextfree;
  fle block;

  /* real_size is the size we actually have to allocate, allowing for
     overhead and alignment.  */
  size_t real_size = REAL_SIZE (sz);

  /* Look for the first block on the freelist that is large enough.  */
  for (nextfree = &__malloc_freelist; 
       *nextfree; 
       nextfree = &(*nextfree)->next)  
    {
      block = *nextfree;
      
      if (block->size >= real_size)
	{
	  /* If the block found is just the right size, remove it from
	     the free list.  Otherwise, split it.  */
	  if (block->size < real_size + sizeof (struct freelist_entry))
	    {
	      *nextfree = block->next;
	      return (void *)&block->next;
	    }
	  else
	    {
	      size_t newsize = block->size - real_size;
	      fle newnext = block->next;
	      *nextfree = (fle)((size_t)block + real_size);
	      (*nextfree)->size = newsize;
	      (*nextfree)->next = newnext;
	      goto done;
	    }
	}

      /* If this is the last block on the freelist, and it was too small,
	 enlarge it.  */
      if (! block->next
	  && __malloc_end == (void *)((size_t)block + block->size))
	{
	  size_t moresize = real_size - block->size;
	  if (! CAN_ALLOC_P (moresize))
	    return NULL;
	  
	  *nextfree = NULL;
	  if (MALLOC_DIRECTION < 0)
	    {
	      block = __malloc_end = (void *)((size_t)block - moresize);
	    }
	  else
	    {
	      __malloc_end = (void *)((size_t)block + real_size);
	    }

	  goto done;
	}
    }

  /* No free space at the end of the free list.  Allocate new space
     and use that.  */

  if (! CAN_ALLOC_P (real_size))
    return NULL;

  if (MALLOC_DIRECTION > 0)
    {
      block = __malloc_end;
      __malloc_end = (void *)((size_t)__malloc_end + real_size);
    }
  else
    {
      block = __malloc_end = (void *)((size_t)__malloc_end - real_size);
    }
 done:
  block->size = real_size;
  return (void *)&block->next;
}

#endif

#ifdef DEFINE_FREE

void
free (void *block_p)
{
  fle *nextfree;
  fle block = (fle)((size_t) block_p - offsetof (struct freelist_entry, next));

  if (block_p == NULL)
    return;
  
  /* Look on the freelist to see if there's a free block just before
     or just after this block.  */
  for (nextfree = &__malloc_freelist; 
       *nextfree; 
       nextfree = &(*nextfree)->next)
    {
      fle thisblock = *nextfree;
      if ((size_t)thisblock + thisblock->size == (size_t) block)
	{
	  thisblock->size += block->size;
	  if (MALLOC_DIRECTION > 0
	      && thisblock->next
	      && (size_t) block + block->size == (size_t) thisblock->next)
	    {
	      thisblock->size += thisblock->next->size;
	      thisblock->next = thisblock->next->next;
	    }
	  return;
	}
      else if ((size_t) thisblock == (size_t) block + block->size)
	{
	  if (MALLOC_DIRECTION < 0
	      && thisblock->next
	      && (size_t) block == ((size_t) thisblock->next 
				    + thisblock->next->size))
	    {
	      *nextfree = thisblock->next;
	      thisblock->next->size += block->size + thisblock->size;
	    }
	  else
	    {
	      block->size += thisblock->size;
	      block->next = thisblock->next;
	      *nextfree = block;
	    }
	  return;
	}
      else if ((MALLOC_DIRECTION > 0
		&& (size_t) thisblock > (size_t) block)
	       || (MALLOC_DIRECTION < 0
		   && (size_t) thisblock < (size_t) block))
	break;
    }

  block->next = *nextfree;
  *nextfree = block;
  return;
}
#endif

#ifdef DEFINE_REALLOC
void *
realloc (void *block_p, size_t sz)
{
  fle block = (fle)((size_t) block_p - offsetof (struct freelist_entry, next));
  size_t real_size = REAL_SIZE (sz);
  size_t old_real_size;

  if (block_p == NULL)
    return malloc (sz);

  old_real_size = block->size;

  /* Perhaps we need to allocate more space.  */
  if (old_real_size < real_size)
    {
      void *result;
      size_t old_size = old_real_size - sizeof (size_t);
      
      /* Need to allocate, copy, and free.  */
      result = malloc (sz);
      if (result == NULL)
	return NULL;
      memcpy (result, block_p, old_size < sz ? old_size : sz);
      free (block_p);
      return result;
    }
  /* Perhaps we can free some space.  */