/* Return a pointer to a zero-size object in memory.

   Copyright (C) 2009-2018 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify

   it under the terms of the GNU General Public License as published by

   the Free Software Foundation; either version 3 of the License, or

   (at your option) any later version.

   This program is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License

   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* ISO C 99 does not allow memcmp(), memchr() etc. to be invoked with a NULL

   argument.  Therefore this file produces a non-NULL pointer which cannot

   be dereferenced, if possible.  */

/* On Android, when targeting Android 4.4 or older with a GCC toolchain,

   prevent a compilation error

     "error: call to 'mmap' declared with attribute error: mmap is not

      available with _FILE_OFFSET_BITS=64 when using GCC until android-21.

      Either raise your minSdkVersion, disable _FILE_OFFSET_BITS=64, or

      switch to Clang."

   The files that we access in this compilation unit are less than 2 GB

   large.  */

#if defined __ANDROID__

# undef _FILE_OFFSET_BITS

# undef __USE_FILE_OFFSET64

#endif

#include <stdlib.h>

/* Test whether mmap() and mprotect() are available.

   We don't use HAVE_MMAP, because AC_FUNC_MMAP would not define it on HP-UX.

   HAVE_MPROTECT is not enough, because mingw does not have mmap() but has an

   mprotect() function in libgcc.a.  */

#if HAVE_SYS_MMAN_H && HAVE_MPROTECT

# include <fcntl.h>

# include <unistd.h>

# include <sys/types.h>

# include <sys/mman.h>

/* Define MAP_FILE when it isn't otherwise.  */

# ifndef MAP_FILE

#  define MAP_FILE 0

# endif

#endif

/* Return a pointer to a zero-size object in memory (that is, actually, a

   pointer to a page boundary where the previous page is readable and writable

   and the next page is neither readable not writable), if possible.

   Return NULL otherwise.  */

static void *

zerosize_ptr (void)

{

/* Use mmap and mprotect when they exist.  Don't test HAVE_MMAP, because it is

   not defined on HP-UX 11 (since it does not support MAP_FIXED).  */

#if HAVE_SYS_MMAN_H && HAVE_MPROTECT

# if HAVE_MAP_ANONYMOUS

  const int flags = MAP_ANONYMOUS | MAP_PRIVATE;

  const int fd = -1;

# else /* !HAVE_MAP_ANONYMOUS */

  const int flags = MAP_FILE | MAP_PRIVATE;

  int fd = open ("/dev/zero", O_RDONLY, 0666);

  if (fd >= 0)

# endif

    {

      int pagesize = getpagesize ();

      char *two_pages =

        (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,

                       flags, fd, 0);

      if (two_pages != (char *)(-1)

          && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)

        return two_pages + pagesize;

    }

#endif

  return NULL;

}