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EXCHANGEDATA(2)             BSD System Calls Manual            EXCHANGEDATA(2)

     exchangedata -- atomically exchange data between two files

     #include <unistd.h>
     #include <sys/attr.h>

     exchangedata(const char * path1, const char * path2, unsigned int options);

     The exchangedata() function swaps the contents of the files referenced by path1 and path2 in an atomic
     fashion.  That is, all concurrent processes will either see the pre-exchanged state or the post-exchanged postexchanged
     exchanged state; they can never see the files in an inconsistent state.  The data in all forks is
     swapped in this way.  The options parameter lets you control specific aspects of the function's behav-iour. behaviour.

     Open file descriptors follow the swapped data.  Thus, a descriptor that previously referenced path1
     will now reference the data that's accessible via path2, and vice versa.

     In general, the file attributes (metadata) are not exchanged.  Specifically, the object identifier
     attributes (that is, the ATTR_CMN_OBJID and ATTR_CMN_OBJPERMANENTID attributes as defined by the
     getattrlist(2) function) are not swapped.  An exception to this general rule is that the modification
     time attribute ( ATTR_CMN_MODTIME ) is swapped.

     When combined, these features allow you to implement a 'safe save' function that does not break refer-ences references
     ences 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.

     The path1 and 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.

     The options parameter is a bit set that controls the behaviour of exchangedata().  The following option
     bits are defined.

     FSOPT_NOFOLLOW  If this bit is set, exchangedata() will not follow a symlink if it occurs as the last
                     component of path1 or path2.

     Upon successful completion a value of 0 is returned.  Otherwise, a value of -1 is returned and errno is
     set to indicate the error.

     Not all volumes support exchangedata().  You can test whether a volume supports exchangedata() by using
     getattrlist(2) to get the volume capabilities attribute ATTR_VOL_CAPABILITIES, and then testing the

     exchangedata() will fail if:

     [ENOTSUP]          The volume does not support exchangedata().

     [ENOTDIR]          A component of the path prefix is not a directory.

     [ENAMETOOLONG]     A component of a path name exceeded NAME_MAX characters, or an entire path name
                        exceeded PATH_MAX characters.

     [ENOENT]           Either file does not exist.

     [EACCES]           Search permission is denied for a component of the path prefix.

     [ELOOP]            Too many symbolic links were encountered in translating the pathname.

     [EFAULT]           path1 or path2 points to an invalid address.

     [EXDEV]            path1 and path2 are on different volumes (mounted file systems).

     [EINVAL]           path1 or path2 reference the same file.

     [EINVAL]           You try to exchange something other than a regular file (for example, a directory).

     [EIO]              An I/O error occurred while reading from or writing to the file system.


     A exchangedata() function call appeared in Darwin 1.3.1 (Mac OS X version 10.0).

Darwin                         December 15, 2003                        Darwin

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