Path: utzoo!attcan!uunet!aplcen!samsung!crackers!cpoint!frog!tdh
From: tdh@frog.UUCP (T. Dave Hudson)
Newsgroups: comp.unix.questions
Subject: Re: pseudo-tty conventions
Summary: summary of responses
Message-ID: <15618@frog.UUCP>
Date: 13 Jun 90 01:41:00 GMT
Distribution: comp
Organization: Charles River Data Systems
Lines: 77

(I'm not sure where a discussion of the following belongs, and so am
not redirecting followups.)

I received replies from
	omerzu@quando.quantum.de
	andyb@coat.com
	lamy@cs.utoronto.ca
	brnstnd@stealth.acf.nyu.edu
All of these used different schemes, presumably all of which break
would-be-portable code.

1) Nixdorf TARGON/35-50 w/ Pyramid OS
	Naming convention:
		/dev/[pt]ty[p-z][0-9a-f]
	Comments:
		This only allows for 176 ptys.
2) Sequent, w/ DYNIX
	Naming convention (ordering of a-z vs. A-Z reflects numbering):
		/dev/[pt]ty[p-wP-W][0-9a-zA-Z]
	Allocation convention:
		int getpseudotty(char **slave, char **master)
		(returns master r/w FD or -1)
	Misc:
		1) similar scheme starting with ttyx0 for X.25 logins
		2) ispseudotty(char *ttyname)
		   (with ttyname stripped of "/dev/" prefix)
	Comments:
		This allows for 992 ptys.  I also like the idea of
		encapsulating the allocation of ptys.
3) MIPS and SGI, w/ RISC/os
	Naming convention (slave only):
		/dev/ttyqn
		(n is decimal number)
	Allocation convention:
		1) open /dev/ptc, fstat() it, use minor dev for slave
		2) open /dev/ptcm, open /dev/ptcm[0-9] until matching
		   (st_rdev) major device#, multiplying the last digit
		   by 256 and adding the minor dev# for the slave
	Comments:
		The first scheme allows for 256 ptys, the latter for
		64K-1.  I like saving on almost all of pty* device
		files.  It looks like this breaks SVID's utmp.h.
4) Dan Bernstein's pty program
	Naming convention:
		/dev/[pt]ty[p-za-o][0-9a-f]
		(but with PTY1 and PTY2 ranges in Makefile)
	Allocation convention:
		Dan is writing a UNIX-domain sockets daemon for
		allocating pty descriptors, and would re-write for
		streams.
	Comments:
		This allows for at least 416 ptys.  Dan claims that
		under a /dev/[pt]typ<decimal#> scheme "[m]any
		utilities will die horribly if tty extensions are
		larger than two characters", but I don't see how any
		scheme here wouldn't fail to be portable.

I like the Sequent/DYNIX allocation convention.  The only feature that
it lacks is an overall pty index (such as is used by MIPS and SGI), a
feature I've seen used here under a dumb-terminal windows program used
internally.  This lack can be compensated for by using *stat() to get
the device#, playing some games to avoid a large index.  It is not
necessary to accept the Sequent/DYNIX naming convention, since this
can be hidden.

It looks like it is unnecessary for a pty's name to exceed 10
characters, but the SVID limit in utmp.h is 11 characters anyway, if
a character is reserved for a terminating ASCII NUL.

In an ideal world, I'd have "int getpseudotty(char *slave)", with
"slave" (perhaps pre-filled) a 12 char array to be NUL-terminated, and
possibly avoiding the use of a master-side device entirely.

There seem to be no widely accepted pty conventions.  I'd like to see
some discussion before deciding what conventions to use.

				David Hudson