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annotate m4/link-follow.m4 @ 40203:30d91797009f

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author Karl Berry <karl@freefriends.org>
date Fri, 01 Mar 2019 08:42:42 -0800
parents b06060465f09
children
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Samuel Thibault <samuel.thibault@gnu.org>
parents: 18626
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1 # serial 20
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Paul Eggert <eggert@cs.ucla.edu>
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2 dnl Run a program to determine whether link(2) follows symlinks.
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Jim Meyering <jim@meyering.net>
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3 dnl Set LINK_FOLLOWS_SYMLINKS accordingly.
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4
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Paul Eggert <eggert@cs.ucla.edu>
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5 # Copyright (C) 1999-2001, 2004-2006, 2009-2019 Free Software Foundation, Inc.
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6 # This file is free software; the Free Software Foundation
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7 # gives unlimited permission to copy and/or distribute it,
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8 # with or without modifications, as long as this notice is preserved.
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9
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10 dnl This macro can be used to emulate POSIX linkat. If
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parents: 11929
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11 dnl LINK_FOLLOWS_SYMLINKS is 0, link matches linkat(,0), and
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Eric Blake <ebb9@byu.net>
parents: 11929
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12 dnl linkat(,AT_SYMLINK_FOLLOW) requires a readlink. If it is 1,
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13 dnl link matches linkat(,AT_SYMLINK_FOLLOW), and there is no way
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14 dnl to do linkat(,0) on symlinks (on all other file types,
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15 dnl link() is sufficient). If it is -1, use a Solaris specific
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16 dnl runtime test. If it is -2, use a generic runtime test.
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17 AC_DEFUN([gl_FUNC_LINK_FOLLOWS_SYMLINK],
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18 [dnl
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19 AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
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20 AC_CHECK_FUNCS_ONCE([readlink])
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21 dnl Mingw lacks link, although gnulib provides a good replacement.
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22 dnl However, it also lacks symlink, so there's nothing to test in
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23 dnl the first place, and no reason to need to distinguish between
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24 dnl linkat variants. So, we set LINK_FOLLOWS_SYMLINKS to 0.
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25 gl_link_follows_symlinks=0 # assume GNU behavior
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26 if test $ac_cv_func_readlink = yes; then
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27 dnl Solaris has an __xpg4 variable in libc, and it determines the
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28 dnl behaviour of link(): It dereferences a symlink if and only if
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29 dnl __xpg4 != 0.
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30 AC_CACHE_CHECK([for __xpg4], [gl_cv_have___xpg4],
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31 [AC_LINK_IFELSE(
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32 [AC_LANG_PROGRAM(
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33 [[extern int __xpg4;]],
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34 [[return __xpg4;]])],
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35 [gl_cv_have___xpg4=yes],
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36 [gl_cv_have___xpg4=no])
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37 ])
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38 if test $gl_cv_have___xpg4 = yes; then
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39 gl_link_follows_symlinks=-1
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40 else
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41 AC_CACHE_CHECK([whether link(2) dereferences a symlink],
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42 [gl_cv_func_link_follows_symlink],
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43 [
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44 # Create a regular file.
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45 echo > conftest.file
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46 AC_RUN_IFELSE(
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47 [AC_LANG_SOURCE([[
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48 # include <sys/types.h>
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49 # include <sys/stat.h>
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50 # include <unistd.h>
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51 # include <stdlib.h>
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52
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53 # define SAME_INODE(Stat_buf_1, Stat_buf_2) \
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54 ((Stat_buf_1).st_ino == (Stat_buf_2).st_ino \
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55 && (Stat_buf_1).st_dev == (Stat_buf_2).st_dev)
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56
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57 int
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58 main ()
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59 {
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60 const char *file = "conftest.file";
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61 const char *sym = "conftest.sym";
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62 const char *hard = "conftest.hard";
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63 struct stat sb_file, sb_hard;
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64
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65 /* Create a symlink to the regular file. */
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66 if (symlink (file, sym))
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67 return 2;
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68
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69 /* Create a hard link to that symlink. */
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70 if (link (sym, hard))
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71 return 3;
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72
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73 if (lstat (hard, &sb_hard))
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74 return 4;
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75 if (lstat (file, &sb_file))
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76 return 5;
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77
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78 /* If the dev/inode of hard and file are the same, then
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79 the link call followed the symlink. */
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80 return SAME_INODE (sb_hard, sb_file) ? 1 : 0;
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81 }
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82 ]])],
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83 [gl_cv_func_link_follows_symlink=no], dnl GNU behavior
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84 [gl_cv_func_link_follows_symlink=yes], dnl Followed link/compile failed
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85 [dnl We're cross compiling.
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86 dnl The past results are "yes" on Mac OS X, FreeBSD, NetBSD,
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87 dnl OpenBSD, Minix, AIX, HP-UX, OSF/1, and "no" on Linux, Cygwin.
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88 case "$host_os" in
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89 # On glibc/Linux we know the result.
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90 linux*-gnu* | gnu*) gl_cv_func_link_follows_symlink="guessing no" ;;
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91 # Otherwise, we don't know.
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92 *) gl_cv_func_link_follows_symlink=unknown ;;
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93 esac
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94 ])
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95 rm -f conftest.file conftest.sym conftest.hard
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96 ])
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97 case "$gl_cv_func_link_follows_symlink" in
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98 *yes) gl_link_follows_symlinks=1 ;;
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99 *no) ;; # already defaulted to 0
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100 *) gl_link_follows_symlinks=-2 ;;
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101 esac
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102 fi
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103 fi
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104 AC_DEFINE_UNQUOTED([LINK_FOLLOWS_SYMLINKS], [$gl_link_follows_symlinks],
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105 [Define to 1 if 'link(2)' dereferences symbolic links, 0 if it
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106 creates hard links to symlinks, -1 if it depends on the variable __xpg4,
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107 and -2 if unknown.])
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108 ])