| 1 | /* Return a pointer to a zero-size object in memory.
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| 2 | Copyright (C) 2009-2021 Free Software Foundation, Inc.
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| 3 |
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| 4 | This program is free software: you can redistribute it and/or modify
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| 5 | it under the terms of the GNU General Public License as published by
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| 6 | the Free Software Foundation; either version 3 of the License, or
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| 7 | (at your option) any later version.
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| 8 |
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| 9 | This program is distributed in the hope that it will be useful,
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| 10 | but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 11 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 12 | GNU General Public License for more details.
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| 13 |
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| 14 | You should have received a copy of the GNU General Public License
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| 15 | along with this program. If not, see <https://www.gnu.org/licenses/>. */
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| 16 |
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| 17 | /* ISO C 99 does not allow memcmp(), memchr() etc. to be invoked with a NULL
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| 18 | argument. Therefore this file produces a non-NULL pointer which cannot
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| 19 | be dereferenced, if possible. */
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| 20 |
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| 21 | /* On Android, when targeting Android 4.4 or older with a GCC toolchain,
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| 22 | prevent a compilation error
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| 23 | "error: call to 'mmap' declared with attribute error: mmap is not
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| 24 | available with _FILE_OFFSET_BITS=64 when using GCC until android-21.
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| 25 | Either raise your minSdkVersion, disable _FILE_OFFSET_BITS=64, or
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| 26 | switch to Clang."
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| 27 | The files that we access in this compilation unit are less than 2 GB
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| 28 | large. */
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| 29 | #if defined __ANDROID__
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| 30 | # undef _FILE_OFFSET_BITS
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| 31 | # undef __USE_FILE_OFFSET64
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| 32 | #endif
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| 33 |
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| 34 | #include <stdlib.h>
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| 35 |
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| 36 | /* Test whether mmap() and mprotect() are available.
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| 37 | We don't use HAVE_MMAP, because AC_FUNC_MMAP would not define it on HP-UX.
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| 38 | HAVE_MPROTECT is not enough, because mingw does not have mmap() but has an
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| 39 | mprotect() function in libgcc.a.
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| 40 | And OS/2 kLIBC has <sys/mman.h> and mprotect(), but not mmap(). */
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| 41 | #if HAVE_SYS_MMAN_H && HAVE_MPROTECT && !defined __KLIBC__
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| 42 | # include <fcntl.h>
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| 43 | # include <unistd.h>
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| 44 | # include <sys/types.h>
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| 45 | # include <sys/mman.h>
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| 46 | /* Define MAP_FILE when it isn't otherwise. */
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| 47 | # ifndef MAP_FILE
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| 48 | # define MAP_FILE 0
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| 49 | # endif
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| 50 | #endif
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| 51 |
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| 52 | /* Return a pointer to a zero-size object in memory (that is, actually, a
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| 53 | pointer to a page boundary where the previous page is readable and writable
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| 54 | and the next page is neither readable not writable), if possible.
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| 55 | Return NULL otherwise. */
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| 56 |
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| 57 | static void *
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| 58 | zerosize_ptr (void)
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| 59 | {
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| 60 | /* Use mmap and mprotect when they exist. Don't test HAVE_MMAP, because it is
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| 61 | not defined on HP-UX 11 (since it does not support MAP_FIXED). */
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| 62 | #if HAVE_SYS_MMAN_H && HAVE_MPROTECT && !defined __KLIBC__
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| 63 | # if HAVE_MAP_ANONYMOUS
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| 64 | const int flags = MAP_ANONYMOUS | MAP_PRIVATE;
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| 65 | const int fd = -1;
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| 66 | # else /* !HAVE_MAP_ANONYMOUS */
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| 67 | const int flags = MAP_FILE | MAP_PRIVATE;
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| 68 | int fd = open ("/dev/zero", O_RDONLY, 0666);
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| 69 | if (fd >= 0)
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| 70 | # endif
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| 71 | {
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| 72 | int pagesize = getpagesize ();
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| 73 | char *two_pages =
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| 74 | (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,
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| 75 | flags, fd, 0);
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| 76 | if (two_pages != (char *)(-1)
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| 77 | && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)
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| 78 | return two_pages + pagesize;
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| 79 | }
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| 80 | #endif
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| 81 | return NULL;
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| 82 | }
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