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.\"     @(#)exchangedata.2
.
.Dd December 15, 2003
.Dt EXCHANGEDATA 2
.Os Darwin
.Sh NAME
.Nm exchangedata
.Nd atomically exchange data between two files
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn exchangedata "const char * path1" "const char * path2" "unsigned long options"
.
.Sh DESCRIPTION
The
.Fn exchangedata
function swaps the contents of the files referenced by 
.Fa path1 
and 
.Fa path2 
in an atomic fashion. 
That is, all concurrent processes will either see the pre-exchanged state or the 
post-exchanged state; they can never see the files in an inconsistent state.
The data in all forks is swapped in this way. 
The 
.Fa options 
parameter lets you control specific aspects of the function's behaviour.
.Pp
.
Open file descriptors follow the swapped data.
Thus, a descriptor that previously referenced 
.Fa path1 
will now reference the data that's accessible via 
.Fa path2 ,
and vice versa.
.Pp
.
In general, the file attributes (metadata) are not exchanged.
Specifically, the object identifier attributes (that is, the 
.Dv ATTR_CMN_OBJID 
and
.Dv ATTR_CMN_OBJPERMANENTID 
attributes as defined by the 
.Xr getattrlist 2 
function) are not swapped.
An exception to this general rule is that the modification time attribute (
.Dv ATTR_CMN_MODTIME
) is swapped.
.Pp
.
When combined, these features allow you to implement a 'safe save' function that 
does not break references to the file (for example, aliases).
You first save the new contents to a temporary file and then 
exchange the data of the original file and the temporary.
Programs that reference the file via an object identifier will continue to 
reference the original file, but now it has the new data.
.Pp
.
.\" path1 and path2 parameters
.
The
.Fa path1 
and 
.Fa path2 
parameters must both reference valid files.
All directories listed in the path names leading to these files must be 
searchable.
You must have write access to the files.
.Pp
.
.\" options parameter
.
The
.Fa options
parameter is a bit set that controls the behaviour of
.Fn exchangedata .
The following option bits are defined.
.
.Bl -tag -width FSOPT_NOFOLLOW
.
.It FSOPT_NOFOLLOW
If this bit is set, 
.Fn exchangedata 
will not follow a symlink if it occurs as 
the last component of
.Fa path1 
or 
.Fa path2 .
.
.El
.
.Sh RETURN VALUES
Upon successful completion a value of 0 is returned.
Otherwise, a value of -1 is returned and
.Va errno
is set to indicate the error.
.
.Sh COMPATIBILITY
Not all volumes support 
.Fn exchangedata .
You can test whether a volume supports 
.Fn exchangedata 
by using 
.Xr getattrlist 2 
to get the volume capabilities attribute 
.Dv ATTR_VOL_CAPABILITIES ,
and then testing the 
.Dv VOL_CAP_INT_EXCHANGEDATA 
flag.
.Pp
.
.Sh ERRORS
.Fn exchangedata
will fail if:
.Bl -tag -width Er
.
.It Bq Er ENOTSUP
The volume does not support
.Fn exchangedata .
.
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.
.It Bq Er ENAMETOOLONG
A component of a path name exceeded 
.Dv NAME_MAX
characters, or an entire path name exceeded 
.Dv PATH_MAX
characters.
.
.It Bq Er ENOENT
Either file does not exist.
.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.
.It Bq Er EFAULT
.Fa path1
or
.Em path2
points to an invalid address.
.
.It Bq Er EXDEV
.Fa path1
and
.Em path2
are on different volumes (mounted file systems).
.
.It Bq Er EINVAL
.Fa path1
or
.Em path2
reference the same file.
.
.It Bq Er EINVAL
You try to exchange something other than a regular file (for example, a directory).
.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.El
.Pp
.
.Sh SEE ALSO
.
.Xr getattrlist 2
.
.Sh HISTORY
A
.Fn exchangedata
function call appeared in Darwin 1.3.1 (Mac OS X version 10.0).
.