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 #if HAVE_CONFIG_H
  22 # include <config.h>
  23 #endif
  24
  25 #include <stddef.h>
  26 #include <stdlib.h>
  27 #include <assert.h>
  28 #include <errno.h>
  29
  30 #include "xalloc.h"
  31 #include "extendbuf.h"
  32
  33
  34 /* We initially use a small default size to ensure that this code
  35  * gets exercised.
  36  */
  37 #ifndef SIZE_DEFAULT
  38 # define SIZE_DEFAULT 16
  39 #endif
  40
  41 static size_t
  42 decide_size(size_t current, size_t wanted)
  43 {
  44   size_t newsize;
  45
  46   if (0 == current)
  47     newsize = SIZE_DEFAULT;
  48   else
  49     newsize = current;
  50
  51   while (newsize < wanted)
  52     {
  53       if (2 * newsize < newsize)
  54         xalloc_die ();
  55       newsize *= 2;
  56     }
  57   return newsize;
  58 }
  59
  60
  61 void *
  62 extendbuf(void* existing, size_t wanted, size_t *allocated)
  63 {
  64   int saved_errno;
  65   size_t newsize;
  66   void *result; /* leave uninitialised to allow static code checkers to identify bugs */
  67
  68   saved_errno = errno;
  69
  70   assert(wanted > 0u);
  71   newsize = decide_size(*allocated, wanted);
  72
  73   if ( (*allocated) == 0 )
  74     {
  75       /* Sanity check: If there is no existing allocation size, there
  76        * must be no existing allocated buffer.
  77        */
  78       assert(NULL == existing);
  79
  80       (*allocated) = newsize;
  81       result = xmalloc(newsize);
  82     }
  83   else
  84     {
  85       if (newsize != (*allocated) )
  86         {
  87           (*allocated) = newsize;
  88           result = xrealloc (existing, newsize);
  89
  90         }
  91       else
  92         {
  93           result = existing;
  94         }
  95     }
  96
  97   if (result)
  98     {
  99       /* xmalloc() or xrealloc() may have changed errno, but in the
 100          success case we want to preserve the previous value.
 101       */
 102       errno = saved_errno;
 103     }
 104   return result;
 105 }