1 /* Return a pointer to a zero-size object in memory.
2 Copyright (C) 2009-2021 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 <https://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 /* On Android, when targeting Android 4.4 or older with a GCC toolchain,
22 prevent a compilation error
23 "error: call to 'mmap' declared with attribute error: mmap is not
24 available with _FILE_OFFSET_BITS=64 when using GCC until android-21.
25 Either raise your minSdkVersion, disable _FILE_OFFSET_BITS=64, or
26 switch to Clang."
27 The files that we access in this compilation unit are less than 2 GB
28 large. */
29 #if defined __ANDROID__
30 # undef _FILE_OFFSET_BITS
31 # undef __USE_FILE_OFFSET64
32 #endif
33
34 #include <stdlib.h>
35
36 /* Test whether mmap() and mprotect() are available.
37 We don't use HAVE_MMAP, because AC_FUNC_MMAP would not define it on HP-UX.
38 HAVE_MPROTECT is not enough, because mingw does not have mmap() but has an
39 mprotect() function in libgcc.a.
40 And OS/2 kLIBC has <sys/mman.h> and mprotect(), but not mmap(). */
41 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT && !defined __KLIBC__
42 # include <fcntl.h>
43 # include <unistd.h>
44 # include <sys/types.h>
45 # include <sys/mman.h>
46 /* Define MAP_FILE when it isn't otherwise. */
47 # ifndef MAP_FILE
48 # define MAP_FILE 0
49 # endif
50 #endif
51
52 /* Return a pointer to a zero-size object in memory (that is, actually, a
53 pointer to a page boundary where the previous page is readable and writable
54 and the next page is neither readable not writable), if possible.
55 Return NULL otherwise. */
56
57 static void *
zerosize_ptr(void)58 zerosize_ptr (void)
59 {
60 /* Use mmap and mprotect when they exist. Don't test HAVE_MMAP, because it is
61 not defined on HP-UX 11 (since it does not support MAP_FIXED). */
62 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT && !defined __KLIBC__
63 # if HAVE_MAP_ANONYMOUS
64 const int flags = MAP_ANONYMOUS | MAP_PRIVATE;
65 const int fd = -1;
66 # else /* !HAVE_MAP_ANONYMOUS */
67 const int flags = MAP_FILE | MAP_PRIVATE;
68 int fd = open ("/dev/zero", O_RDONLY, 0666);
69 if (fd >= 0)
70 # endif
71 {
72 int pagesize = getpagesize ();
73 char *two_pages =
74 (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,
75 flags, fd, 0);
76 if (two_pages != (char *)(-1)
77 && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)
78 return two_pages + pagesize;
79 }
80 #endif
81 return NULL;
82 }
83