Aucbvax.4204
fa.unix-wizards
utzoo!decvax!ucbvax!network
Sat Oct  3 20:53:27 1981
at Berkeley
Subject: Re:  process interlocking problems in unix
Via:  Ucb-C70.ArpaNet; 3 Oct 81 17:43-PDT

In-real-life: Steven M. Bellovin
Location: University of North Carolina at Chapel Hill
Phone: (919) 966-3305

If you have the multiplexor (or Bill Joy's new stuff) available, you
can implement a locking process which runs the multiplexor channel.
Processes wanting to lock on a resource open the lock 'file' and write
a line that looks something like

	LOCK resource

and then do a read.  When it's free, the mpx driver writes back to them
giving permission; others will be held pending a RELEASE line being
written (or a close on the channel).  There could also be a TEST line
and maybe a non-blocking LOCK, which would return error status if it
couldn't grant the request.