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 .\" path1 and path2 parameters
  77 .
  78 The
  79 .Fa path1
  80 and
  81 .Fa path2
  82 parameters must both reference valid files.
  83 All directories listed in the path names leading to these files must be
  84 searchable.
  85 You must have write access to the files.
  86 .Pp
  87 .
  88 .\" options parameter
  89 .
  90 The
  91 .Fa options
  92 parameter is a bit set that controls the behaviour of
  93 .Fn exchangedata .
  94 The following option bits are defined.
  95 .
  96 .Bl -tag -width FSOPT_NOFOLLOW
  97 .
  98 .It FSOPT_NOFOLLOW
  99 If this bit is set,
 100 .Fn exchangedata
 101 will not follow a symlink if it occurs as
 102 the last component of
 103 .Fa path1
 104 or
 105 .Fa path2 .
 106 .
 107 .El
 108 .
 109 .Sh RETURN VALUES
 110 Upon successful completion a value of 0 is returned.
 111 Otherwise, a value of -1 is returned and
 112 .Va errno
 113 is set to indicate the error.
 114 .
 115 .Sh COMPATIBILITY
 116 Not all volumes support
 117 .Fn exchangedata .
 118 You can test whether a volume supports
 119 .Fn exchangedata
 120 by using
 121 .Xr getattrlist 2
 122 to get the volume capabilities attribute
 123 .Dv ATTR_VOL_CAPABILITIES ,
 124 and then testing the
 125 .Dv VOL_CAP_INT_EXCHANGEDATA
 126 flag.
 127 .Pp
 128 .
 129 .Sh ERRORS
 130 .Fn exchangedata
 131 will fail if:
 132 .Bl -tag -width Er
 133 .
 134 .It Bq Er ENOTSUP
 135 The volume does not support
 136 .Fn exchangedata .
 137 .
 138 .It Bq Er ENOTDIR
 139 A component of the path prefix is not a directory.
 140 .
 141 .It Bq Er ENAMETOOLONG
 142 A component of a path name exceeded
 143 .Dv NAME_MAX
 144 characters, or an entire path name exceeded
 145 .Dv PATH_MAX
 146 characters.
 147 .
 148 .It Bq Er ENOENT
 149 Either file does not exist.
 150 .
 151 .It Bq Er EACCES
 152 Search permission is denied for a component of the path prefix.
 153 .
 154 .It Bq Er ELOOP
 155 Too many symbolic links were encountered in translating the pathname.
 156 .
 157 .It Bq Er EFAULT
 158 .Fa path1
 159 or
 160 .Em path2
 161 points to an invalid address.
 162 .
 163 .It Bq Er EXDEV
 164 .Fa path1
 165 and
 166 .Em path2
 167 are on different volumes (mounted file systems).
 168 .
 169 .It Bq Er EINVAL
 170 .Fa path1
 171 or
 172 .Em path2
 173 reference the same file.
 174 .
 175 .It Bq Er EINVAL
 176 You try to exchange something other than a regular file (for example, a directory).
 177 .
 178 .It Bq Er EIO
 179 An I/O error occurred while reading from or writing to the file system.
 180 .El
 181 .Pp
 182 .
 183 .Sh SEE ALSO
 184 .
 185 .Xr getattrlist 2
 186 .
 187 .Sh HISTORY
 188 A
 189 .Fn exchangedata
 190 function call appeared in Darwin 1.3.1 (Mac OS X version 10.0).
 191 .