lib/gl/tests/zerosize-ptr.h

   1 /* Return a pointer to a zero-size object in memory.
   2    Copyright (C) 2009, 2010 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  16
  17 /* ISO C 99 does not allow memcmp(), memchr() etc. to be invoked with a NULL
  18    argument.  Therefore this file produces a non-NULL pointer which cannot
  19    be dereferenced, if possible.  */
  20
  21 #include <stdlib.h>
  22
  23 /* Test whether mmap() and mprotect() are available.
  24    We don't use HAVE_MMAP, because AC_FUNC_MMAP would not define it on HP-UX.
  25    HAVE_MPROTECT is not enough, because mingw does not have mmap() but has an
  26    mprotect() function in libgcc.a.  */
  27 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT
  28 # include <fcntl.h>
  29 # include <unistd.h>
  30 # include <sys/types.h>
  31 # include <sys/mman.h>
  32 /* Define MAP_FILE when it isn't otherwise.  */
  33 # ifndef MAP_FILE
  34 #  define MAP_FILE 0
  35 # endif
  36 #endif
  37
  38 /* Return a pointer to a zero-size object in memory (that is, actually, a
  39    pointer to a page boundary where the previous page is readable and writable
  40    and the next page is neither readable not writable), if possible.
  41    Return NULL otherwise.  */
  42
  43 static void *
  44 zerosize_ptr (void)
  45 {
  46 /* Use mmap and mprotect when they exist.  Don't test HAVE_MMAP, because it is
  47    not defined on HP-UX 11 (since it does not support MAP_FIXED).  */
  48 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT
  49 # if HAVE_MAP_ANONYMOUS
  50   const int flags = MAP_ANONYMOUS | MAP_PRIVATE;
  51   const int fd = -1;
  52 # else /* !HAVE_MAP_ANONYMOUS */
  53   const int flags = MAP_FILE | MAP_PRIVATE;
  54   int fd = open ("/dev/zero", O_RDONLY, 0666);
  55   if (fd >= 0)
  56 # endif
  57     {
  58       int pagesize = getpagesize ();
  59       char *two_pages =
  60         (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,
  61                        flags, fd, 0);
  62       if (two_pages != (char *)(-1)
  63           && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)
  64         return two_pages + pagesize;
  65     }
  66 #endif
  67   return NULL;
  68 }