Path: utzoo!attcan!uunet!husc6!cmcl2!nrl-cmf!ames!killer!netsys!cucstud!c3pe!c3engr!charles
From: charles@c3engr.UUCP (Charles Green)
Newsgroups: comp.unix.wizards
Subject: Re: I need a SysV select()
Summary: Multiplexor processes writing a shared pipe read by main process
Message-ID: <2643@c3engr.UUCP>
Date: 21 May 88 02:30:52 GMT
References: <9738@sol.ARPA> <7904@brl-smoke.ARPA> <11536@mimsy.UUCP>
Reply-To: charles@c3engr.UUCP (Charles Green)
Organization: C3, Inc. Engineering Division, Herndon, VA 22070-5201
Lines: 33

In article <11536@mimsy.UUCP> jds@mimsy.umd.edu (James da Silva) writes:
>>In article <9738@sol.ARPA> jpayne@cs.rochester.edu (Jonathan Payne) writes:
>>>I want to get input
>>>from either the keyboard or from some other file description.  I don't
>>>want to sit around polling the two, because that would be ridiculous.

>The natural way to handle this in System V is to use two processes, each of
>which is waiting on one file descriptor.  You can then use shared memory
>and/or signals, semaphores or message queues to get the data around, to taste.

My favorite solution is to have several small "multiplexor" processes, all
identical but exec()ed with a unique numeric argument, reading their respective
stdins and all writing into the same pipe that's read by the main process.
When one of the multiplexor processes completes a read(), he does a single
write() to the pipe containing his unique ID, a byte count, and the data.

The main process sleeps on a read() of the pipe of a size sufficient to pick up
the "header" (ID and byte count).  When he gets one, he issues another read()
to pick up the data.

This scheme works because a write() to a pipe is atomic; data from the other
multiplexor processes won't be interspersed with it (as opposed to TTYs...)
However, you have to be able to accept data from all "channels" simultaneously;
you can't selectively read data from one channel and not another.

	fd >----> [mux] >--
			   \
	fd >----> [mux] >---+--> (pipe) >--> [main process]
			   /
	fd >----> [mux] >--
-- 
#ident	"@(#).signature	1.5 :~charles/s..signature 04/27/88 21:40:43"
charles@c3pe.UUCP				charles_green%c3gate@c3pe