gnu/lib/libmalloc/realloc.c

   1 /* Change the size of a block allocated by `malloc'.
   2    Copyright 1990, 1991, 1992, 1993 Free Software Foundation, Inc.
   3                      Written May 1989 by Mike Haertel.
   4
   5 This library is free software; you can redistribute it and/or
   6 modify it under the terms of the GNU Library General Public License as
   7 published by the Free Software Foundation; either version 2 of the
   8 License, or (at your option) any later version.
   9
  10 This library is distributed in the hope that it will be useful,
  11 but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13 Library General Public License for more details.
  14
  15 You should have received a copy of the GNU Library General Public
  16 License along with this library; see the file COPYING.LIB.  If
  17 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  18 Cambridge, MA 02139, USA.
  19
  20    The author may be reached (Email) at the address mike@ai.mit.edu,
  21    or (US mail) as Mike Haertel c/o Free Software Foundation.  */
  22
  23 #ifndef _MALLOC_INTERNAL
  24 #define _MALLOC_INTERNAL
  25 #include <malloc.h>
  26 #endif
  27
  28 #define min(A, B) ((A) < (B) ? (A) : (B))
  29
  30 /* Debugging hook for realloc.  */
  31 __ptr_t (*__realloc_hook) __P ((__ptr_t __ptr, size_t __size));
  32
  33 /* Resize the given region to the new size, returning a pointer
  34    to the (possibly moved) region.  This is optimized for speed;
  35    some benchmarks seem to indicate that greater compactness is
  36    achieved by unconditionally allocating and copying to a
  37    new region.  This module has incestuous knowledge of the
  38    internals of both free and malloc. */
  39 __ptr_t
  40 realloc (ptr, size)
  41      __ptr_t ptr;
  42      size_t size;
  43 {
  44   __ptr_t result;
  45   int type;
  46   size_t block, blocks, oldlimit;
  47
  48   if (size == 0)
  49     {
  50       free (ptr);
  51       return malloc (0);
  52     }
  53   else if (ptr == NULL)
  54     return malloc (size);
  55
  56   if (__realloc_hook != NULL)
  57     return (*__realloc_hook) (ptr, size);
  58
  59   block = BLOCK (ptr);
  60
  61   type = _heapinfo[block].busy.type;
  62   switch (type)
  63     {
  64     case 0:
  65       /* Maybe reallocate a large block to a small fragment.  */
  66       if (size <= BLOCKSIZE / 2)
  67         {
  68           result = malloc (size);
  69           if (result != NULL)
  70             {
  71               memcpy (result, ptr, size);
  72               free (ptr);
  73               return result;
  74             }
  75         }
  76
  77       /* The new size is a large allocation as well;
  78          see if we can hold it in place. */
  79       blocks = BLOCKIFY (size);
  80       if (blocks < _heapinfo[block].busy.info.size)
  81         {
  82           /* The new size is smaller; return
  83              excess memory to the free list. */
  84           _heapinfo[block + blocks].busy.type = 0;
  85           _heapinfo[block + blocks].busy.info.size
  86             = _heapinfo[block].busy.info.size - blocks;
  87           _heapinfo[block].busy.info.size = blocks;
  88           free (ADDRESS (block + blocks));
  89           result = ptr;
  90         }
  91       else if (blocks == _heapinfo[block].busy.info.size)
  92         /* No size change necessary.  */
  93         result = ptr;
  94       else
  95         {
  96           /* Won't fit, so allocate a new region that will.
  97              Free the old region first in case there is sufficient
  98              adjacent free space to grow without moving. */
  99           blocks = _heapinfo[block].busy.info.size;
 100           /* Prevent free from actually returning memory to the system.  */
 101           oldlimit = _heaplimit;
 102           _heaplimit = 0;
 103           free (ptr);
 104           _heaplimit = oldlimit;
 105           result = malloc (size);
 106           if (result == NULL)
 107             {
 108               /* Now we're really in trouble.  We have to unfree
 109                  the thing we just freed.  Unfortunately it might
 110                  have been coalesced with its neighbors.  */
 111               if (_heapindex == block)
 112                 (void) malloc (blocks * BLOCKSIZE);
 113               else
 114                 {
 115                   __ptr_t previous = malloc ((block - _heapindex) * BLOCKSIZE);
 116                   (void) malloc (blocks * BLOCKSIZE);
 117                   free (previous);
 118                 }
 119               return NULL;
 120             }
 121           if (ptr != result)
 122             memmove (result, ptr, blocks * BLOCKSIZE);
 123         }
 124       break;
 125
 126     default:
 127       /* Old size is a fragment; type is logarithm
 128          to base two of the fragment size.  */
 129       if (size > (size_t) (1 << (type - 1)) && size <= (size_t) (1 << type))
 130         /* The new size is the same kind of fragment.  */
 131         result = ptr;
 132       else
 133         {
 134           /* The new size is different; allocate a new space,
 135              and copy the lesser of the new size and the old. */
 136           result = malloc (size);
 137           if (result == NULL)
 138             return NULL;
 139           memcpy (result, ptr, min (size, (size_t) 1 << type));
 140           free (ptr);
 141         }
 142       break;
 143     }
 144
 145   return result;
 146 }