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 2, or (at your option)
   8    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, write to the Free Software Foundation,
  17    Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  18
  19 /* Written by James Yougnman <jay@gnu.org>. */
  20
  21 #include <config.h>
  22
  23
  24 #include <stddef.h>
  25 #include <stdlib.h>
  26 #include <assert.h>
  27 #include <errno.h>
  28
  29 #include "xalloc.h"
  30 #include "extendbuf.h"
  31
  32
  33 /* We initially use a small default size to ensure that this code
  34  * gets exercised.
  35  */
  36 #ifndef SIZE_DEFAULT
  37 # define SIZE_DEFAULT 16
  38 #endif
  39
  40 static size_t
  41 decide_size(size_t current, size_t wanted)
  42 {
  43   size_t newsize;
  44
  45   if (0 == current)
  46     newsize = SIZE_DEFAULT;
  47   else
  48     newsize = current;
  49
  50   while (newsize < wanted)
  51     {
  52       if (2 * newsize < newsize)
  53         xalloc_die ();
  54       newsize *= 2;
  55     }
  56   return newsize;
  57 }
  58
  59
  60 void *
  61 extendbuf(void* existing, size_t wanted, size_t *allocated)
  62 {
  63   int saved_errno;
  64   size_t newsize;
  65   void *result; /* leave uninitialised to allow static code checkers to identify bugs */
  66
  67   saved_errno = errno;
  68
  69   assert(wanted > 0u);
  70   newsize = decide_size(*allocated, wanted);
  71
  72   if ( (*allocated) == 0 )
  73     {
  74       /* Sanity check: If there is no existing allocation size, there
  75        * must be no existing allocated buffer.
  76        */
  77       assert(NULL == existing);
  78
  79       (*allocated) = newsize;
  80       result = xmalloc(newsize);
  81     }
  82   else
  83     {
  84       if (newsize != (*allocated) )
  85         {
  86           (*allocated) = newsize;
  87           result = xrealloc (existing, newsize);
  88
  89         }
  90       else
  91         {
  92           result = existing;
  93         }
  94     }
  95
  96   if (result)
  97     {
  98       /* xmalloc() or xrealloc() may have changed errno, but in the
  99          success case we want to preserve the previous value.
 100       */
 101       errno = saved_errno;
 102     }
 103   return result;
 104 }