Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84 +SENDMAIL+2.11; site dcl-cs.UUCP
Path: utzoo!watmath!clyde!bonnie!akgua!gatech!seismo!mcvax!ukc!dcl-cs!stephen
From: stephen@dcl-cs.UUCP (Stephen J. Muir)
Newsgroups: net.unix-wizards
Subject: Re: /dev/null
Message-ID: <808@dcl-cs.UUCP>
Date: Fri, 22-Nov-85 18:12:48 EST
Article-I.D.: dcl-cs.808
Posted: Fri Nov 22 18:12:48 1985
Date-Received: Mon, 25-Nov-85 06:51:04 EST
References: <593@ttrdc.UUCP>
Reply-To: stephen@comp.lancs.ac.uk (Stephen J. Muir)
Organization: Department of Computing at Lancaster University.
Lines: 26

>Seeing that /dev/mem and /dev/kmem are "windows" into the physical memory space
>of the computer, I wonder if /dev/null is implemented as a "window" into a
>nonexistent section of physical memory, or what?

cp /etc/motd /dev/null
panic: segmentation violation

What happens is that all writes map onto the "write" system call (write(2)).

The code (off the top of my head) goes like:

write (filedes, buffer, nbytes)
	char	*buffer
	{ if (major_minor (filedes) == that_of ("/dev/null"))
		return (nbytes);
	  else
		go_do_the_REAL_write (filedes, buffer, nbytes);
	}

So the "write" system call just returns success and does nothing!
-- 
UUCP:	...!seismo!mcvax!ukc!dcl-cs!stephen
DARPA:	stephen%comp.lancs.ac.uk@ucl-cs	| Post: University of Lancaster,
JANET:	stephen@uk.ac.lancs.comp	|	Department of Computing,
Phone:	+44 524 65201 Ext. 4599		|	Bailrigg, Lancaster, UK.
Project:Alvey ECLIPSE Distribution	|	LA1 4YR