unix98 ptys

Alex Hornung ahornung at gmail.com
Wed Sep 2 13:16:35 PDT 2009

Hi all,


I've recently commited both the kernel and userland part of
unix98 ptys, so I'm giving a short description of what happened and what to do
if problems arise.


What are unix98 ptys?


Unix98 ptys are just a new standard for ptys. They basically
revolve around /dev/ptmx (to open a master pty) and /dev/pts/XXX, where XXX is
a number, for slaves.

BSD ptys are limited to 256 ptys, due to the chosen
nomenclature. Unix98 ptys are limited to 1000 ptys due to the line length of
utmp (8 bytes), which allows for at most "pts/999" (plus a
terminating '\0'). See utmp(5) man page for more info on this.



What happened?


New userland functions in libc are:

* posix_openpt[1] - Implemented as standard dictates
(basically just opening /dev/ptmx)

* ptsname[2] - Implemented as standard dictates

* grantpt[3] - No-op, as the kernel already creates the pty
dynamically with all the right permissions and ownership (see permissions
further down). Following the standard would add a security risk here, imho.
Every single bit of code I've ever seen using grantpt, only does so to follow
the standard, not expecting it to do anything.

* unlockpt[4] - No-op, as it really has no use whatsoever
with dynamically created ptys.


I've also migrated openpty(3) to first try and use unix98
ptys, and if any part of that fails, to fall back to the old BSD ptys. Old BSD
ptys can still be used as they always have been used; nothing has changed for
them. As a matter of fact some programs, as xterm < 247 and aterm don't even
use openpty(3) and just find a free BSD pty (/dev/pty**), so expect to still
see quite a few /dev/pty** and family.


Permissions (as specified in grantpt standard[3]) are the

For slaves: <real uid>, group "tty", 0620

For master: <real uid>, group "wheel", 0600   but
it doesn't matter, because the master device cannot be opened more than once.
Every time /dev/ptmx is opened, a different master is acquired.





Make sure you have all the commits relating to unix98 ptys.
If something is still wrong, please do:

sysctl kern.pty_debug=1

This will enable some basic debugging for unix98 ptys which
should be enough for most (critical) problems. If you find any other problem,
especially with some program doing "something" weird, don't hesitate
to contact me.




Alex Hornung




[1]: http://www.opengroup.org/onlinepubs/000095399/functions/posix_openpt.html

[2]: http://www.opengroup.org/onlinepubs/000095399/functions/ptsname.html

[3]: http://www.opengroup.org/onlinepubs/000095399/functions/grantpt.html

[4]: http://www.opengroup.org/onlinepubs/000095399/functions/unlockpt.html

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