more on file \"attributes\"

[x0705]

Jon Shapiro (> >) asked about semaphores and record locking on UNIX.
Marty Shannon (>) pointed out that System V has semaphores and shared memory,
and that:
> System Vr2 (I'm almost certain) has advisory file locking.  I don't have the
> documentation handy, but it may allow the user to specify file addresses to be
> locked.  While this is not mandatory locking (i.e., no processes will block on
> reads or writes due to a lock), cooperating processes can prevent themselves
> from stepping on each other's data with these locks.

Actually, it's the paging release (System V Rel 2, Vax Ver 2, 3B20 Ver 4,
3B2 Ver ???).  I think mandatory locking is supposed to by in Sys V Rel 3.

> 
> > The first facility [semaphores], I believe, is to be greatly preferred.  Please,
> > arguments about using pseudo devices or files in the file system or
> > pipes/sockets/wombats-carrying-postcards don't wash.  These are
> > neither resource efficient nor portable.  The semaphore facilities
> > necessary are not hard to implement (I have done them myself on other
> > systems), and would help a great deal in solving many problems of
> > record access, which contrary to popular opinion in UNIX land
> > constitutes a great deal of what is done out in the real business world.

The SysVR2v* functions were developed to phase in support for the /usr/group
standard, which was put together by UNIX users out in the "real world".

(Actually, wombats carrying postcards can be quite efficient in a distributed
environment.  They're somewhat more portable than TCP/IP, and continue working
if the power goes down.)

(Trademarks and owners  include: { UNIX, Vax, DEC, 3B**, AT&T-**, Wombat Inc.} )
-- 
## Bill Stewart, AT&T Bell Labs, Holmdel NJ 1-201-949-0705 ihnp4!ho95c!wcs