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/* 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
{
int pagesize = getpagesize ();
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;
}
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