/* Return a pointer to a zero-size object in memory.
   Copyright (C) 2009-2024 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.
   And OS/2 kLIBC has <sys/mman.h> and mprotect(), but not mmap().  */
#if HAVE_SYS_MMAN_H && HAVE_MPROTECT && !defined __KLIBC__
# 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 && !defined __KLIBC__
# 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
    {
      size_t pagesize = sysconf (_SC_PAGESIZE);
      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;
}