What is the history behind the name of the xdev option in find?

Question

I find the name of the xdev option counterintuitive. For me it seems like the abbreviation of 'cross device (search)'. However it does the opposite, it restricts the search to a single file system. The alternative but somewhat obsolete name is "mount", which is more memorizable. What is the history behind the naming of this option?

Answer 1

It most probably stands for cross-device indeed, though in effect it means do not cross devices.

In the original implementation on BSD in 1985, the code had:

int    Xdev = 1;       /* true if SHOULD cross devices (file systems) */
[...]
   else if (EQ(a, "-xdev")) {
           Xdev = 0;

Where Xdev was an internal variable used to track whether devices should be crossed. The -xdev predicate sets that to 0.

David Korn at AT&T added a similar predicate a few years later for SVR4 with a -mount alias for it (that mirrors the FTW_MOUNT flag of the new tree walking library).

I don't find the -mount option particularly better. Assuming it stands for single-mount, it's also misleading on Linux at least where you can have several mount points for a file system.

$ mkdir -p a/b b
$ sudo mount --bind a b

Now b is a mountpoint on the same device as . and a.

$ find . -xdev
.
./b
./b/b
./a
./a/b
$ find . -mount
.
./b
./b/b
./a
./a/b

find doesn't cross-devices when processing b, but it's a different mount point so -mount (as single-mount) is more misleading that -xdev (as long as you remember it's about preventing crossing devices).

Answer 2

It is even more complex that you might believe.

In the AT&T implementation for find, the feature is from nftw() which has been introduced with SVr4 and nftw() did come with a flag FTW_MOUNT that stopped at mount points and did not report files with a different st_dev entry.

The SVr4 find command did have both -mount and -xdev in 1988 and -xdev was an alias to -mount.

On the other side, SunOS and BSD had a -xdev in 1985 already and since the SunOS/ BSD findwas not implemented on top of *ftw(), it could use a different sematic and it did. find -xdev on SunOS stopped descending after the mount point with the different st_dev has been printed.

Then in 1992, the first POSIX standard that included more than just libc and file formats (like tar) mentioned find with -xdev only. The problem with that POSIX standard is that-xdev has to report mount points and to stop after reporting the moint points while the AT&T implementation stopped before reporting mount points.

GNU find is an implementation that follows the POSIX.1-1992 text.

As a result, we recently discussed a related bug report on the POSIX standard teleconference and decided to enhance the standard with a -mount primary and with a FTW_XDEV flag in nftw().

find -mount has to behave the way as find did behave on AT&T UNIX and must not report mount points.

find -xdev has to behave the way it was written in the POSIX standard and needs to report mount points and stop then with descending the tree.

This requiresAT&T UNIX based find implementations to change their implementation for -xdev and to let GNU find change the behavior of -mount.

BTW: AFAIK, sfind/libfind is currently the only implementation that already follows the new rules.

Since libfidnd implemented the needed code already before the final agreement for POSIX has been set up, it implements a -mount+ primary and -xdev now has become an alias to -mount+ which is easier to memorize for stop at mount points but in addition report them.