tty select feature

[Chris Torek]

In article <4569 at brl-smoke.ARPA> art at 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)
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CSNet:	chris at umcp-cs		ARPA:	chris at mimsy.umd.edu