writing about things, sometimes.

Talking ’bout 9P: open and walk

Written by Omar Polo on 01 August 2021 while listening to The Riddler” by Nightwish.

The Topen request at first looks weird. Here’s its signature

size[4] Topen tag[2] fid[4] mode[1]

It’s strange, isn’t it? For comparison, here’s the C signature for the open(2) system call:

int	open(const char *path, int flags, ...);

Where is the path in the Topen call?

The description for the Topen request says:

The open request asks the file server to check permissions and prepare a fid for I/O with subsequent read and write messages.

Which implies that to write or read a fid you must open… an existing fid?

This morning (well, actually some days ago since this entry got published later) cage explained the mystery to me: Twalk.

When I first skimmed through the 9P documentation I thought that the walk request was basically a chdir(2), which it is, but also is not!

The Twalk requests allows one to navigate from a fid (usually representing the starting directory) through some path components and reach a destination file that will be associated with a new fid.

So, if I connect to a 9P file server and write something to ~/notes.txt (supposing that my home is on that file server) the 9P session could look like this:

# establish a connection and negotiate the version
→ Tversion <msize> 9P2000
← Rversion <msize> 9P2000

# mount the home
→ Tattach 1 "op" "/home/op"
← Rattach <qid>

# walk from fid #1 (/home/op) to “notes.txt” (a file!)
# and associate to it fid #2
→ Twalk 1 2 "notes.txt"
← Rwalk <qid...>

# prepare fid 2 (/home/op/notes.txt) for
# reading and writing
← Ropen <qid> <iounit>

# read/write fid 2…

# close the fid 2 since it’s no longer used
→ Tclunk 2
← Rclunk

(note that as always this is entirely my speculation from reading the documentation. I never used — sigh! — plan9 nor 9p)

So it actually makes sense for Topen to accept a fid and not a path, and Twalk is a general purpose request that can be used to implement various system calls (dup2, chdir, open…)

Then I started to think why it was like this. I mean, everything is finally starting to make sense in my head, but why the 9P people decided to implement Topen and Twalk this way?

Well, I can’t say for sure, but I’m starting to noticing that a fid is something more than a UNIX file descriptor: it’s both a file descriptor AND a path.

*my mind blows*

Which is actually a pretty clever solution. The client can get new fids by mean of Twalk, which then can be passed to Tremove (for removal), or Tstat/Twstat, or being opened and then written/closed. It’s also probably more efficient than passing string around on every request.

This has also some drawbacks probably. For one, it’s not clear at glance if a fid was prepared for I/O or not. On UNIX there is a clear distinction between a file descriptor (a number that references an object in the kernel) and a path (a mere sequence of bytes NUL-terminated.) But since this is an underlying mechanism, it seems pretty clever. It shouldn’t be too much difficult to map the usual UNIX syscalls on top of 9p.