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annotate tests/zerosize-ptr.h @ 17249:e542fd46ad6f

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maint: update all copyright year number ranges Run "make update-copyright". Compare to commit 1602f0a from last year. Signed-off-by: Eric Blake <eblake@redhat.com>
author Eric Blake <eblake@redhat.com>
date Tue, 01 Jan 2013 00:50:58 +0000
parents 8250f2777afc
children
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1 /* Return a pointer to a zero-size object in memory.
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2 Copyright (C) 2009-2013 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 <http://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 #include <stdlib.h>
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22
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23 /* Test whether mmap() and mprotect() are available.
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24 We don't use HAVE_MMAP, because AC_FUNC_MMAP would not define it on HP-UX.
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25 HAVE_MPROTECT is not enough, because mingw does not have mmap() but has an
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26 mprotect() function in libgcc.a. */
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27 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT
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28 # include <fcntl.h>
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29 # include <unistd.h>
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30 # include <sys/types.h>
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31 # include <sys/mman.h>
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32 /* Define MAP_FILE when it isn't otherwise. */
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33 # ifndef MAP_FILE
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34 # define MAP_FILE 0
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35 # endif
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36 #endif
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37
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38 /* Return a pointer to a zero-size object in memory (that is, actually, a
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39 pointer to a page boundary where the previous page is readable and writable
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40 and the next page is neither readable not writable), if possible.
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41 Return NULL otherwise. */
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42
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43 static void *
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44 zerosize_ptr (void)
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45 {
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46 /* Use mmap and mprotect when they exist. Don't test HAVE_MMAP, because it is
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47 not defined on HP-UX 11 (since it does not support MAP_FIXED). */
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48 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT
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49 # if HAVE_MAP_ANONYMOUS
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50 const int flags = MAP_ANONYMOUS | MAP_PRIVATE;
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51 const int fd = -1;
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52 # else /* !HAVE_MAP_ANONYMOUS */
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53 const int flags = MAP_FILE | MAP_PRIVATE;
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54 int fd = open ("/dev/zero", O_RDONLY, 0666);
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55 if (fd >= 0)
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56 # endif
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57 {
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58 int pagesize = getpagesize ();
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59 char *two_pages =
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60 (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,
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61 flags, fd, 0);
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62 if (two_pages != (char *)(-1)
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63 && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)
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64 return two_pages + pagesize;
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65 }
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66 #endif
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67 return NULL;
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68 }