bsd/man/man2/exchangedata.2

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  17 .\"     @(#)exchangedata.2
  18 .
  19 .Dd December 15, 2003
  20 .Dt EXCHANGEDATA 2
  21 .Os Darwin
  22 .Sh NAME
  23 .Nm exchangedata
  24 .Nd atomically exchange data between two files
  25 .Sh SYNOPSIS
  26 .Fd #include <unistd.h>
  27 .Fd #include <sys/attr.h>
  28 .Ft int
  29 .Fn exchangedata "const char * path1" "const char * path2" "unsigned int options"
  30 .
  31 .Sh DESCRIPTION
  32 The
  33 .Fn exchangedata
  34 function swaps the contents of the files referenced by
  35 .Fa path1
  36 and
  37 .Fa path2
  38 in an atomic fashion.
  39 That is, all concurrent processes will either see the pre-exchanged state or the
  40 post-exchanged state; they can never see the files in an inconsistent state.
  41 The data in all forks is swapped in this way.
  42 The
  43 .Fa options
  44 parameter lets you control specific aspects of the function's behaviour.
  45 .Pp
  46 .
  47 Open file descriptors follow the swapped data.
  48 Thus, a descriptor that previously referenced
  49 .Fa path1
  50 will now reference the data that's accessible via
  51 .Fa path2 ,
  52 and vice versa.
  53 .Pp
  54 .
  55 In general, the file attributes (metadata) are not exchanged.
  56 Specifically, the object identifier attributes (that is, the
  57 .Dv ATTR_CMN_OBJID
  58 and
  59 .Dv ATTR_CMN_OBJPERMANENTID
  60 attributes as defined by the
  61 .Xr getattrlist 2
  62 function) are not swapped.
  63 An exception to this general rule is that the modification time attribute (
  64 .Dv ATTR_CMN_MODTIME
  65 ) is swapped.
  66 .Pp
  67 .
  68 When combined, these features allow you to implement a 'safe save' function that
  69 does not break references to the file (for example, aliases).
  70 You first save the new contents to a temporary file and then
  71 exchange the data of the original file and the temporary.
  72 Programs that reference the file via an object identifier will continue to
  73 reference the original file, but now it has the new data.
  74 .Pp
  75 .
  76 WARNING: This system call is largely supported only by HFS and AFP file systems.  Many other
  77 file systems, including APFS, do not support it.  Further, it is not supported on iOS, tvOS, or watchOS.
  78 It is recommended that callers refer
  79 instead to
  80 .Fn rename
  81 or
  82 .Fn renamex_np
  83 to conduct safe-save operations instead.
  84 .Pp
  85 .
  86 .\" path1 and path2 parameters
  87 .
  88 The
  89 .Fa path1
  90 and
  91 .Fa path2
  92 parameters must both reference valid files.
  93 All directories listed in the path names leading to these files must be
  94 searchable.
  95 You must have write access to the files.
  96 .Pp
  97 .
  98 .\" options parameter
  99 .
 100 The
 101 .Fa options
 102 parameter is a bit set that controls the behaviour of
 103 .Fn exchangedata .
 104 The following option bits are defined.
 105 .
 106 .Bl -tag -width FSOPT_NOFOLLOW
 107 .
 108 .It FSOPT_NOFOLLOW
 109 If this bit is set,
 110 .Fn exchangedata
 111 will not follow a symlink if it occurs as
 112 the last component of
 113 .Fa path1
 114 or
 115 .Fa path2 .
 116 .
 117 .El
 118 .
 119 .Sh RETURN VALUES
 120 Upon successful completion a value of 0 is returned.
 121 Otherwise, a value of -1 is returned and
 122 .Va errno
 123 is set to indicate the error.
 124 .
 125 .Sh COMPATIBILITY
 126 Not all volumes support
 127 .Fn exchangedata .
 128 This includes APFS volumes.
 129 You can test whether a volume supports
 130 .Fn exchangedata
 131 by using
 132 .Xr getattrlist 2
 133 to get the volume capabilities attribute
 134 .Dv ATTR_VOL_CAPABILITIES ,
 135 and then testing the
 136 .Dv VOL_CAP_INT_EXCHANGEDATA
 137 flag.
 138 .Pp
 139 .
 140 .Sh ERRORS
 141 .Fn exchangedata
 142 will fail if:
 143 .Bl -tag -width Er
 144 .
 145 .It Bq Er ENOTSUP
 146 The volume does not support
 147 .Fn exchangedata .
 148 .
 149 .It Bq Er ENOTDIR
 150 A component of the path prefix is not a directory.
 151 .
 152 .It Bq Er ENAMETOOLONG
 153 A component of a path name exceeded
 154 .Dv NAME_MAX
 155 characters, or an entire path name exceeded
 156 .Dv PATH_MAX
 157 characters.
 158 .
 159 .It Bq Er ENOENT
 160 Either file does not exist.
 161 .
 162 .It Bq Er EACCES
 163 Search permission is denied for a component of the path prefix.
 164 .
 165 .It Bq Er ELOOP
 166 Too many symbolic links were encountered in translating the pathname.
 167 .
 168 .It Bq Er EFAULT
 169 .Fa path1
 170 or
 171 .Em path2
 172 points to an invalid address.
 173 .
 174 .It Bq Er EXDEV
 175 .Fa path1
 176 and
 177 .Em path2
 178 are on different volumes (mounted file systems).
 179 .
 180 .It Bq Er EINVAL
 181 .Fa path1
 182 or
 183 .Em path2
 184 reference the same file.
 185 .
 186 .It Bq Er EINVAL
 187 You try to exchange something other than a regular file (for example, a directory).
 188 .
 189 .It Bq Er EIO
 190 An I/O error occurred while reading from or writing to the file system.
 191 .El
 192 .Pp
 193 .
 194 .Sh SEE ALSO
 195 .
 196 .Xr getattrlist 2 ,
 197 .Xr rename 2
 198 .
 199 .Sh HISTORY
 200 A
 201 .Fn exchangedata
 202 function call appeared in Darwin 1.3.1 (Mac OS X version 10.0).
 203 .
 204 It was deprecated in macOS 10.13.