The history of file type information being available in Unix directories
The two things that Unix directory entries absolutely have to have
are the name of the directory entry and its 'inode', by which we
generically mean some stable kernel identifier for the file that
will persist if it gets renamed, linked to other directories, and
so on. Unsurprisingly, directory entries have had these since the
days when you read the raw bytes of directories with
read(), and for a long time that was all they had; if you
wanted more than the name and the inode number, you had to
the file, not just read the directory. Then, well, I'll quote
myself from an old entry on a
[...], Unix filesystem developers realized that it was very common for programs reading directories to need to know a bit more about directory entries than just their names, especially their file types (
findis the obvious case, but also consider things like '
ls -F'). Given that the type of an active inode never changes, it's possible to embed this information straight in the directory entry and then return this to user level, and that's what developers did; on some systems,
readdir(3)will now return directory entries with an additional
d_typefield that has the directory entry's type.
On Twitter, I recently grumbled about Illumos not having this
The ensuing conversation wound up with me curious about exactly where
d_type came from and how far back it went. The answer turns out to
be a bit surprising due to there being two sides of
On the kernel side,
d_type appears to have shown up in 4.4 BSD.
The 4.4 BSD
struct dirent that has a
d_type field, but the field
isn't documented in either the comments in the file or in the
getdirentries(2) manpage; both of those admit only to the traditional
dirent fields. This 4.4 BSD
d_type was carried through
to things that inherited from 4.4 BSD (Lite), specifically FreeBSD,
but it continued to be undocumented for at least a while.
(In FreeBSD, the most convenient history I can find is here, and the
is present in sys/dirent.h as far back as FreeBSD 2.0, which
seems to be as far as the repo goes for releases.)
d_type appeared in the
manpage in FreeBSD 2.2.0, where the manpage itself claims to have
been updated on May 3rd 1995 (cf).
In FreeBSD, this appears to have been part of merging 4.4 BSD
'Lite2', which seems to have been done in 1997. I stumbled over a
repo of UCB BSD commit history,
and in it the documentation appears in this May 3rd 1995 change,
which at least has the same date. It appears that FreeBSD 2.2.0 was
released some time in 1997, which is when this would have appeared
in an official release.
In Linux, it seems that a dirent structure with a
appeared only just before 2.4.0, which was released at the start
of 2001. Linux took this long because the
d_type field only
appeared in the 64-bit 'large file support' version of the
dirent structure, and so was only return by the new 64-bit
getdents64() system call. This would have been a few years after
FreeBSD officially documented
d_type, and probably many years
after it was actually available if you peeked at the structure
(See here for an overview of where to get ancient Linux kernel history from.)
As far as I can tell,
d_type is present on Linux, FreeBSD,
OpenBSD, NetBSD, Dragonfly BSD, and Darwin (aka MacOS or OS X).
It's not present on Solaris and thus Illumos. As far as other
commercial Unixes go, you're on your own; all the links to manpages
for things like AIX from my old entry on the remaining Unixes appear to have rotted away.
Sidebar: The filesystem also matters on modern Unixes
Even if your Unix supports
d_type in directory entries, it
doesn't mean that it's supported by the filesystem of any specific
directory. As far as I know, every Unix with
d_type support has
support for it in their normal local filesystems, but it's not
guaranteed to be in all filesystems, especially non-Unix ones like
FAT32. Your code should always be prepared to deal with a file type
(Filesystems can implement support for file type information in directory entries in a number of different ways. The actual on disk format of directory entries is filesystem specific.)
It's also possible to have things the other way around, where you have a filesystem with support for file type information in directories that's on a Unix that doesn't support it. There are a number of plausible reasons for this to happen, but they're either obvious or beyond the scope of this entry.
Comments on this page:Written on 25 August 2018.