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From: raveling@vaxb.isi.edu (Paul Raveling)
Newsgroups: comp.os.misc
Subject: Re: Asynchronous I/O (was generalized bigotry)
Message-ID: <7304@venera.isi.edu>
Date: 18 Jan 89 18:46:43 GMT
References: <19@xenlink.UUCP> <225800108@uxe.cso.uiuc.edu> <2075@scolex> <KARL.89Jan16142115@triceratops.cis.ohio-state.edu> <2766@ficc.uu.net>
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Reply-To: raveling@vaxb.isi.edu (Paul Raveling)
Organization: USC-Information Sciences Institute
Lines: 57

In article <2766@ficc.uu.net> peter@ficc.uu.net (Peter da Silva) writes:
>
>Perhaps a standard set of real-time calls can be decided on. A set more
>suited to a wider variety of operating systems. Something like:
>
>	token = start_read(fd, buffer, nbytes, timeout);
>	...
>	nread = check_io(token);
>	...
>	nread = wait_io(token);

	Here's a C rendition of what we did in several systems
	starting in 1972 that directly addresses this; I'd still
	recommend something like:


	#define	ANYEVENT	   0
	#define	READ_EVENT_ID	   xxx	/* integer values to uniquely	  */
	#define	TIMEOUT_EVENT_ID   xxx	/*	identify events		  */
	struct event_data {...} evdata;	/*  Generic event data structure  */
	...
	...
	ARead (file, &buffer, nbytes, READ_EVENT_ID);
	SetTimer (timeout, TIMEOUT_EVENT_ID);
	...
	...
	evdata = WaitFor (ANYEVENT);
	/*
	    Or if you prefer,	WaitFor (READ_EVENT_ID)
	    or			WaitFor (READ_EVENT_ID "|" TIMEOUT_EVENT_ID)
	    or			CheckFor (whatever)  [don't block]
	*/

	switch ( evdata.event_id )
		{
		case READ_EVENT_ID:
			completion_status = evdata.status;
			byte_count 	  = evdata.byte_count;
			...
		case TIMEOUT_EVENT_ID:
			...
		}
	


	Communicating event data was the basis for scheduling processes
	in these systems; this proved to be an extremely general mechanism
	that made it easy to program a LARGE variety of functions using
	cooperating processes.  It was also quite fast:  In communicating
	small amounts of info, such as i/o request or i/o completion
	parameters, and doing the consequent context switch, the 1975
	system (EPOS) was about 10-12 times faster than UNIX V6.


---------------------
Paul Raveling
Raveling@vaxb.isi.edu