freebsd-dev/gnu/lib/libmalloc/realloc.c

/* Change the size of a block allocated by `malloc'.
   Copyright 1990, 1991, 1992, 1993 Free Software Foundation, Inc.
		     Written May 1989 by Mike Haertel.

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with this library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.

   The author may be reached (Email) at the address mike@ai.mit.edu,
   or (US mail) as Mike Haertel c/o Free Software Foundation.  */

#ifndef	_MALLOC_INTERNAL
#define _MALLOC_INTERNAL
#include <malloc.h>
#endif

#define min(A, B) ((A) < (B) ? (A) : (B))

/* Debugging hook for realloc.  */
__ptr_t (*__realloc_hook) __P ((__ptr_t __ptr, size_t __size));

/* Resize the given region to the new size, returning a pointer
   to the (possibly moved) region.  This is optimized for speed;
   some benchmarks seem to indicate that greater compactness is
   achieved by unconditionally allocating and copying to a
   new region.  This module has incestuous knowledge of the
   internals of both free and malloc. */
__ptr_t
realloc (ptr, size)
     __ptr_t ptr;
     size_t size;
{
  __ptr_t result;
  int type;
  size_t block, blocks, oldlimit;

  if (size == 0)
    {
      free (ptr);
      return malloc (0);
    }
  else if (ptr == NULL)
    return malloc (size);

  if (__realloc_hook != NULL)
    return (*__realloc_hook) (ptr, size);

  block = BLOCK (ptr);

  type = _heapinfo[block].busy.type;
  switch (type)
    {
    case 0:
      /* Maybe reallocate a large block to a small fragment.  */
      if (size <= BLOCKSIZE / 2)
	{
	  result = malloc (size);
	  if (result != NULL)
	    {
	      memcpy (result, ptr, size);
	      free (ptr);
	      return result;
	    }
	}

      /* The new size is a large allocation as well;
	 see if we can hold it in place. */
      blocks = BLOCKIFY (size);
      if (blocks < _heapinfo[block].busy.info.size)
	{
	  /* The new size is smaller; return
	     excess memory to the free list. */
	  _heapinfo[block + blocks].busy.type = 0;
	  _heapinfo[block + blocks].busy.info.size
	    = _heapinfo[block].busy.info.size - blocks;
	  _heapinfo[block].busy.info.size = blocks;
	  free (ADDRESS (block + blocks));
	  result = ptr;
	}
      else if (blocks == _heapinfo[block].busy.info.size)
	/* No size change necessary.  */
	result = ptr;
      else
	{
	  /* Won't fit, so allocate a new region that will.
	     Free the old region first in case there is sufficient
	     adjacent free space to grow without moving. */
	  blocks = _heapinfo[block].busy.info.size;
	  /* Prevent free from actually returning memory to the system.  */
	  oldlimit = _heaplimit;
	  _heaplimit = 0;
	  free (ptr);
	  _heaplimit = oldlimit;
	  result = malloc (size);
	  if (result == NULL)
	    {
	      /* Now we're really in trouble.  We have to unfree
		 the thing we just freed.  Unfortunately it might
		 have been coalesced with its neighbors.  */
	      if (_heapindex == block)
	        (void) malloc (blocks * BLOCKSIZE);
	      else
		{
		  __ptr_t previous = malloc ((block - _heapindex) * BLOCKSIZE);
		  (void) malloc (blocks * BLOCKSIZE);
		  free (previous);
		}
	      return NULL;
	    }
	  if (ptr != result)
	    memmove (result, ptr, blocks * BLOCKSIZE);
	}
      break;

    default:
      /* Old size is a fragment; type is logarithm
	 to base two of the fragment size.  */
      if (size > (size_t) (1 << (type - 1)) && size <= (size_t) (1 << type))
	/* The new size is the same kind of fragment.  */
	result = ptr;
      else
	{
	  /* The new size is different; allocate a new space,
	     and copy the lesser of the new size and the old. */
	  result = malloc (size);
	  if (result == NULL)
	    return NULL;
	  memcpy (result, ptr, min (size, (size_t) 1 << type));
	  free (ptr);
	}
      break;
    }

  return result;
}