signals are not IPC!

[ignatz at ihuxx.UUCP]

	One of my pet peeves is people who try to use a wrench as a hammer
	and then complain because it doesn't do a good job.  Signals were
	never intended as a general interprocess-communication scheme, and
	anyone who tries to use them as such deserves what he/she gets!
	
	Signals are basically a way of killing a process, with some minor and
	less-successful arrangements grafted on to permit such a process to
	clean up first.  Trying to patch them up into a general IPC scheme
	is foolish -- it would be much better to take the time and effort to
	figure out what is really wanted for the job, and then do it right as
	a completely separate facility.  And the last thing we need at the
	software level is to re-create the full ugliness of hardware interrupts!
	This will merely force the application-level programmers to re-invent
	all the solutions to the problems interrupts cause.  A far superior way
	to proceed would be to provide some relatively safe, relatively simple,
	relatively clean facility like message-passing.
	
	I agree that the lack of a general IPC mechanism is a major wart of
	Unix.  And the signal mechanism could stand some improving to make it
	into a better and more foolproof way of terminating processes while
	allowing them to clean up.  But trying to pervert the same mechanism
	to meet both needs is a dubious approach, and complaining because
	standard Unix signals don't do both jobs is just plain stupid.
	--
					Henry Spencer
					U of Toronto
					{allegra,ihnp4,linus,decvax}!utzoo!henry

Now, mind that it's rather late, and I may well miss some details; but I'll
attempt to be reasonably complete in the following statements.
Yes, you're quite right...signals were never meant to be a general-purpose
IPC mechanism.  However, they were meant to be more than just 'cleanup'.  Look
at 'The UNIX Time-Sharing System', The Bell System Technical Journal, July-August
1978 (commonly known as the 'UNIX BSTJ'); on page 1925, section VII (Traps), Ritchie
and Thomson explicitly make the point that the signal mechanism is intended to
allow the programmer to handle just the type of situations that are handled
with TRAPS.  Yes, Virginia, signals were intended to be trap-handler type
mechanisms.

It is quite true that signals should not have the horrendous windows that they
do; and they are NOT IPC mechanisms.  But they *are* intended to handle program
exceptions.

				Ta,

				Dave Ihnat
				ihuxx!ignatz