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From: chris@umcp-cs.UUCP (Chris Torek)
Newsgroups: net.unix-wizards
Subject: Re: tty select feature
Message-ID: <3926@umcp-cs.UUCP>
Date: Sat, 18-Oct-86 03:20:06 EDT
Article-I.D.: umcp-cs.3926
Posted: Sat Oct 18 03:20:06 1986
Date-Received: Sat, 18-Oct-86 23:26:04 EDT
References: <4569@brl-smoke.ARPA>
Reply-To: chris@umcp-cs.UUCP (Chris Torek)
Organization: University of Maryland, Dept. of Computer Sci.
Lines: 41

In article <4569@brl-smoke.ARPA> art@ACC.arpa writes:
>...  Most of the terminal device drivers point the cdevsw entries for
>the select system call to ttselect(), which contains the following line:

>	register struct tty *tp = &cdevsw[major(dev)].d_ttys[minor(dev)];

>This code assumes that the minor device number can be used as an index into
>the driver's tty structure array.  My understanding is that ONLY THE DRIVER
>should interpret the meaning and format of the minor device number.

The interpretation inside ttselect() is a code-saving device.  The
driver can override this easily.  (There should be an equivalent
routine that can be called with a `struct tty *'; as it is a driver
has to play games with `dev', then call ttselect().  But as distributed
all of the drivers could use ttselect() directly, which makes this
other routine unnecessary.  Writing, e.g.,

	ttselect(dev)
		dev_t dev;
	{

		return (tty_select(&cdevsw[major(dev)].d_ttys[minor(dev)]));
	}

	tty_select(tp)
		register struct tty *tp;
	{
		...

would be cleaner, but slower; ergo, it was not done.)

>Our driver defines minor numbers which have more structure than
>that and caused select to break.

It was your option to put `ttselect' into conf.c. . . .

(It *would* be nice if ttselect's assumption were mentioned in conf.c.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1516)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris@umcp-cs		ARPA:	chris@mimsy.umd.edu