EXCHANGEDATA(2) System Calls Manual EXCHANGEDATA(2)

exchangedata — atomically exchange data between two files

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

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

The () 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. 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 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.

WARNING: This system call is largely supported only by HFS and AFP file systems. Many other file systems, including APFS, do not support it. Further, it is not supported on iOS, tvOS, or watchOS. It is recommended that callers refer instead to () or () to conduct safe-save operations instead.

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 (). 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(). This includes APFS volumes. 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 VOL_CAP_INT_EXCHANGEDATA flag.

exchangedata() will fail if:

[]
The volume does not support exchangedata().
[]
A component of the path prefix is not a directory.
[]
A component of a path name exceeded NAME_MAX characters, or an entire path name exceeded PATH_MAX characters.
[]
Either file does not exist.
[]
Search permission is denied for a component of the path prefix.
[]
Too many symbolic links were encountered in translating the pathname.
[]
path1 or path2 points to an invalid address.
[]
path1 and path2 are on different volumes (mounted file systems).
[]
path1 or path2 reference the same file.
[]
You try to exchange something other than a regular file (for example, a directory).
[]
An I/O error occurred while reading from or writing to the file system.

getattrlist(2), rename(2)

A exchangedata() function call appeared in Darwin 1.3.1 (Mac OS X version 10.0). It was deprecated in macOS 10.13.

December 15, 2003 Darwin