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