how do I tell the size of a pseudoterm window?

[Guy Harris]

>1) How is a program able to tell the size of a pseudoterm's window.

Many systems have adopted the 4.3BSD TIOCGWINSZ "ioctl"; the kernel
stores the screen size along with the tty modes.  SunOS 3.2 and later
has it, and I'd suspect many of the other "started with BSD" UNIX
systems do as well.  (Some may have stuck at 4.2BSD level, though, and
haven't picked it up.) SunOS releases prior to 3.2 had TIOCGSIZE, which
was a Sun invention which antedated 4.3BSD (for better or worse,
Berkeley didn't pick Sun's version up).  (Yes, I know it's not in the
3.x documentation, but it is there, along with "stty size", "stty rows",
and "stty cols".)

>   Is there a termcap entry which specifies that the screen can be
>   resized? (The termcap man pages dont mention it).

No, and that's why the "termcap" man pages don't mention it.  Instead,
you have:

	1) the Sun mechanism, wherein the "termcap" library code does a
	   TIOCGSIZE or TIOCGWINSZ, and if the sizes it gets back are
	   non-zero, it ignores the ones in the "termcap" entry and uses
	   the ones from the "ioctl";

	2) the 4.3BSD mechanism, where this is left up to the code that
	   uses the "termcap" library routines; "curses" does it for
	   you, but programs that don't use "curses", such as "vi", have
	   to be modified to do the "ioctl" and override the values in
	   the "termcap" entry.

(In SunOS the S5 "curses"/"terminfo" library takes approach 1), so
applications don't have to know about it.)

>   How does a program get 'signalled' when a resize occurs (ie. like vi
>   doing a redraw after a resize) ?

If the window size for a pseudo-tty is changed, a SIGWINCH signal is
delivered to the process group for that pseudo-tty.

>2) How can the termcap be modified automatically when a 'resize' occurs, when
>   not in a program (ie at the shell prompt)?  This would be useful so that
>   the next 'ls' or other such commands would output an appropriate number
>   of columns.

With difficulty, at best.  The 4.3BSD scheme, and the SunOS scheme which
preceded it, render this unnecessary, since software that cares how big
the screen is tries the "ioctl" first and, only if it returns zero for
the sizes, uses the value from the "termcap" entry.

>I am looking for a general case solution which should be able to be
>applied to various windowing systems.  Does this involve a special
>tty/pty driver?

The technique described above requires:

	1) a tty/pty driver which supports TIOCGWINSZ/TIOCSWINSZ or
	   equivalents (such as the older SunOS TIOCGSIZE/TIOCSSIZE);

	2) a SIGWINCH signal, and some way for it to be delivered when
	   the window size is changed (if you do the 4.3BSD TIOCSWINSZ
	   "ioctl", which sets the sizes, on either the master or slave
	   side, the tty driver delivers a SIGWINCH if the sizes are
	   actually being changed).

4.3BSD has all this, as does SunOS and, as indicated above, many other
"started with BSD" systems.  Various System V releases, and other UNIX
variants, may have picked it up as well.