Extending the UNIX pathname synt

[johnl at ima.UUCP]

The chaosnet implementation for 4.1 was largely my fault (we started
with some pretty reasonable code from IIASA, but it had never been
near real Chaos hardware before, nor interchanged packets with any
other Chaos hosts until we pounded on it at Yale 5 years ago.)  There
was some surprisingly nice kernel hackery to allow multiple opens on the
same I-node for different connections, and to allow the driver to parse
the trailing part of /dev/chaos/kaflooga.

Passing the "kaflooga" to the driver was simplicity itself.  I had but to
add one line to nami() which returned a success status if it ever found
itself looking at a character special file with the sticky bit set.  At
that point, the various pointers in the U. block are all set up so that the
device open routine can retrieve the rest of the name with uchar().  Truly
a great hack, due to the IIASA people.

The multiple channel stuff appeared to be adapted from the V7 MPX file
stuff.  What it did was to pun the seek offset in the kernel's per-open
"file" structure to be a pointer to a per-connection structure in the
device driver set up at open time.  To avoid accidental system crashes,
we implemented a FMPX status bit in the file structure (already defined
in the V7 source; this had clearly been done before.)  This bit disallowed
seek calls, since there was no seek pointer for such files, and fiddled the
close() logic so that it bypassed the usual logic that avoids calling a
device close routine when a special file is multiply open.  This was also
a minor kernel change, less than 10 lines outside the driver, which did
just what we wanted.

I'm not going to claim that this pair of kernel tweaks is a substitute for
a general scheme which allows a process to completely simulate a file,
including processing all of the client's system calls.  But if you want
to put in something like labelled tapes or the chaos stuff in a finite
amount of time, it's not a bad way to go.


John Levine, Javelin Software, Cambridge MA 617-494-1400
{ decvax!cca | think | ihnp4 | cbosgd }!ima!johnl, Levine at YALE.ARPA