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       1                 :            : /* Return a pointer to a zero-size object in memory.
       2                 :            :    Copyright (C) 2009-2012 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                 :          2 : 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                 :          2 :   const int flags = MAP_ANONYMOUS | MAP_PRIVATE;
      51                 :          2 :   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                 :          2 :       int pagesize = getpagesize ();
      59                 :          2 :       char *two_pages =
      60                 :          2 :         (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,
      61                 :            :                        flags, fd, 0);
      62         [ +  - ]:          2 :       if (two_pages != (char *)(-1)
      63         [ +  - ]:          2 :           && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)
      64                 :          2 :         return two_pages + pagesize;
      65                 :            :     }
      66                 :            : #endif
      67                 :          2 :   return NULL;
      68                 :            : }