I fell into the Twitters again. @CarrickDB joked about Unix, Files and Directories:
And that is a case of “Haha, only serious”. Because directories
used to be files, and that was a bad time. Check out the V7 Unix
command. At this point in history we do not have a
syscall, yet, so we need to construct the entire directory in
mknod(2)an inode that has the
S_IFDIRflag set, even if that macro does not even exist yet.
- manually link the entry for the current directory
- manually link the entry for the parent directory
This fragile and broken:
mkdir could be interrupted while
doing that or another program could try to race
mkdir while it
is doing that. In both cases we get directories that are invalid
and dangerous to traverse, because they break crucial
assumptions users make about directories.
This is also before
readdir(2) and friends, so programs like
ls open directories like files
and then make assumptions about the format of dentries on disk.
Specifically, they assume a 16 bit inode number and then a
filename of 14 characters or less and a directory that is an array
of these entries. Unfortunately, time has not been kind to the
assumption of 65535 files or less per partition, and also we
require filenames that are longer than 14 bytes these days.
Finally have a look at the hot mess that the rmdir command is. What could probably go wrong?
Well, Jan Kraetzschmar
reminds us that this kind of non-atomic rmdir can also produce
structures in the filesystem that are disconnected from the main
tree starting at
/. In that case you end up with orphaned, unreachable
inodes that still have a non-zero link count.
fsck should be
able to find them and free them, but of course that would be a
disruptive operation. Making
rmdir system call avoids
all of these problems.
That’s why all of this was fixed in 1984 or so, when BSD
FFS came around and we got long filenames, wider inodes,
readdir as syscalls and many other
What if really everything was a file?
Another decade later, around 1995 or so, we got Plan 9, not from outer space, but from Bell Labs.
It not only brought us Unicode everywhere, but also an exploration of ‘What if really everything was a file?’, including other machines on the network and processes on our machine. From that we get todays procfs in Linux (and in many other modern Unices).
Except that you can’t
rm -rf /proc/1 to shut down the box.
Things that still are not a file, and should be dead
I am not going to mention System V IPC her at all. Not shm, not sem, and not msq. They are abominations that should never have escaped the lab cages they have been conceived in.
mmap, and mmap is good. Or can be, as long as you do
not conflate in memory and on disk representations of data, and
understand the value of MVCC. But that is another story and
should be told another day.