exchangedata(2) BSD System Calls Manual exchangedata(2)
NAME
exchangedata -- atomically exchange data between two files
SYNOPSIS
#include <unistd.h>
#include <sys/attr.h>
int
exchangedata(const char * path1, const char * path2,
unsigned int options);
DESCRIPTION
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 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 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. Specifi-
cally, the object identifier attributes (that is, the ATTR_CMN_OBJID and
ATTR_CMN_OBJPERMANENTID attributes as defined by the getattrlist(2) func-
tion) 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' func-
tion 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.
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 sym-
link if it occurs as the last component of path1 or
path2.
RETURN VALUES
Upon successful completion a value of 0 is returned. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
COMPATIBILITY
Not all volumes support exchangedata(). You can test whether a volume
supports exchangedata() by using getattrlist(2) to get the volume capa-
bilities attribute ATTR_VOL_CAPABILITIES, and then testing the
VOL_CAP_INT_EXCHANGEDATA flag.
ERRORS
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 charac-
ters, or an entire path name exceeded PATH_MAX charac-
ters.
[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 translat-
ing 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.
SEE ALSO
getattrlist(2)
HISTORY
A exchangedata() function call appeared in Darwin 1.3.1 (Mac OS X version
10.0).
Darwin December 15, 2003 Darwin
Mac OS X 10.9.1 - Generated Sun Jan 5 20:32:38 CST 2014