| 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 | 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 | }
|
|---|