lib/extendbuf.c

   1 /* extendbuf.c -- manage a dynamically-allocated buffer
   2
   3    Copyright 2004 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation, either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program 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
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  17 */
  18 /* Written by James Yougnman <jay@gnu.org>. */
  19
  20 #include <config.h>
  21
  22
  23 #include <stddef.h>
  24 #include <stdlib.h>
  25 #include <assert.h>
  26 #include <errno.h>
  27
  28 #include "xalloc.h"
  29 #include "extendbuf.h"
  30
  31
  32 /* We initially use a small default size to ensure that this code
  33  * gets exercised.
  34  */
  35 #ifndef SIZE_DEFAULT
  36 # define SIZE_DEFAULT 16
  37 #endif
  38
  39 static size_t
  40 decide_size(size_t current, size_t wanted)
  41 {
  42   size_t newsize;
  43
  44   if (0 == current)
  45     newsize = SIZE_DEFAULT;
  46   else
  47     newsize = current;
  48
  49   while (newsize < wanted)
  50     {
  51       if (2 * newsize < newsize)
  52         xalloc_die ();
  53       newsize *= 2;
  54     }
  55   return newsize;
  56 }
  57
  58
  59 void *
  60 extendbuf(void* existing, size_t wanted, size_t *allocated)
  61 {
  62   int saved_errno;
  63   size_t newsize;
  64   void *result; /* leave uninitialised to allow static code checkers to identify bugs */
  65
  66   saved_errno = errno;
  67
  68   assert (wanted > 0u);
  69   newsize = decide_size(*allocated, wanted);
  70
  71   if ( (*allocated) == 0 )
  72     {
  73       /* Sanity check: If there is no existing allocation size, there
  74        * must be no existing allocated buffer.
  75        */
  76       assert (NULL == existing);
  77
  78       (*allocated) = newsize;
  79       result = xmalloc(newsize);
  80     }
  81   else
  82     {
  83       if (newsize != (*allocated) )
  84         {
  85           (*allocated) = newsize;
  86           result = xrealloc (existing, newsize);
  87
  88         }
  89       else
  90         {
  91           result = existing;
  92         }
  93     }
  94
  95   if (result)
  96     {
  97       /* xmalloc() or xrealloc() may have changed errno, but in the
  98          success case we want to preserve the previous value.
  99       */
 100       errno = saved_errno;
 101     }
 102   return result;
 103 }