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annotate tests/zerosize-ptr.h @ 40186:8964917f9574

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author Karl Berry <karl@freefriends.org>
date Mon, 18 Feb 2019 08:02:49 -0800
parents b06060465f09
children
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1 /* Return a pointer to a zero-size object in memory.
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2 Copyright (C) 2009-2019 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 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT
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41 # include <fcntl.h>
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42 # include <unistd.h>
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43 # include <sys/types.h>
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44 # include <sys/mman.h>
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45 /* Define MAP_FILE when it isn't otherwise. */
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46 # ifndef MAP_FILE
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47 # define MAP_FILE 0
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48 # endif
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49 #endif
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50
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51 /* Return a pointer to a zero-size object in memory (that is, actually, a
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52 pointer to a page boundary where the previous page is readable and writable
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53 and the next page is neither readable not writable), if possible.
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54 Return NULL otherwise. */
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55
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56 static void *
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57 zerosize_ptr (void)
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58 {
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59 /* Use mmap and mprotect when they exist. Don't test HAVE_MMAP, because it is
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60 not defined on HP-UX 11 (since it does not support MAP_FIXED). */
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61 #if HAVE_SYS_MMAN_H && HAVE_MPROTECT
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62 # if HAVE_MAP_ANONYMOUS
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63 const int flags = MAP_ANONYMOUS | MAP_PRIVATE;
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64 const int fd = -1;
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65 # else /* !HAVE_MAP_ANONYMOUS */
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66 const int flags = MAP_FILE | MAP_PRIVATE;
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67 int fd = open ("/dev/zero", O_RDONLY, 0666);
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68 if (fd >= 0)
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69 # endif
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70 {
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71 int pagesize = getpagesize ();
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72 char *two_pages =
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73 (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE,
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74 flags, fd, 0);
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75 if (two_pages != (char *)(-1)
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76 && mprotect (two_pages + pagesize, pagesize, PROT_NONE) == 0)
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77 return two_pages + pagesize;
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78 }
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79 #endif
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80 return NULL;
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81 }