freebsd-skq/share/man/man4/pty.4
Ed Schouten bc093719ca Integrate the new MPSAFE TTY layer to the FreeBSD operating system.
The last half year I've been working on a replacement TTY layer for the
FreeBSD kernel. The new TTY layer was designed to improve the following:

- Improved driver model:

  The old TTY layer has a driver model that is not abstract enough to
  make it friendly to use. A good example is the output path, where the
  device drivers directly access the output buffers. This means that an
  in-kernel PPP implementation must always convert network buffers into
  TTY buffers.

  If a PPP implementation would be built on top of the new TTY layer
  (still needs a hooks layer, though), it would allow the PPP
  implementation to directly hand the data to the TTY driver.

- Improved hotplugging:

  With the old TTY layer, it isn't entirely safe to destroy TTY's from
  the system. This implementation has a two-step destructing design,
  where the driver first abandons the TTY. After all threads have left
  the TTY, the TTY layer calls a routine in the driver, which can be
  used to free resources (unit numbers, etc).

  The pts(4) driver also implements this feature, which means
  posix_openpt() will now return PTY's that are created on the fly.

- Improved performance:

  One of the major improvements is the per-TTY mutex, which is expected
  to improve scalability when compared to the old Giant locking.
  Another change is the unbuffered copying to userspace, which is both
  used on TTY device nodes and PTY masters.

Upgrading should be quite straightforward. Unlike previous versions,
existing kernel configuration files do not need to be changed, except
when they reference device drivers that are listed in UPDATING.

Obtained from:		//depot/projects/mpsafetty/...
Approved by:		philip (ex-mentor)
Discussed:		on the lists, at BSDCan, at the DevSummit
Sponsored by:		Snow B.V., the Netherlands
dcons(4) fixed by:	kan
2008-08-20 08:31:58 +00:00

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.\" Copyright (c) 2008 Ed Schouten <ed@FreeBSD.org>
.\" All rights reserved.
.\"
.\" Portions of this software were developed under sponsorship from Snow
.\" B.V., the Netherlands.
.\"
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.\" $FreeBSD$
.\"
.Dd August 20, 2008
.Dt PTY 4
.Os
.Sh NAME
.Nm pty
.Nd BSD-style compatibility pseudo-terminal driver
.Sh SYNOPSIS
.Cd "device pty"
.Sh DESCRIPTION
The
.Nm
driver provides support for the traditional BSD naming scheme that was
used for accessing pseudo-terminals.
When the device
.Pa /dev/ptyXX
is being opened, a new terminal shall be created with the
.Xr pts 4
driver.
A device node for this terminal shall be created, which has the name
.Pa /dev/ttyXX .
.Pp
New code should not try to allocate pseudo-terminals using this
interface.
It is only provided for compatibility with older C libraries
that tried to open such devices when
.Xr posix_openpt 2
was being called.
.Sh FILES
The BSD-style compatibility pseudo-terminal driver uses the following
device names:
.Pp
.Bl -tag -width ".Pa /dev/pty[l-sL-S][0-9a-v]"
.It Pa /dev/pty[l-sL-S][0-9a-v]
Pseudo-terminal master devices.
.It Pa /dev/tty[l-sL-S][0-9a-v]
Pseudo-terminal slave devices.
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr posix_openpt 2 ,
.Xr pts 4 ,
.Xr tty 4
.Sh HISTORY
A
pseudo-terminal driver appeared in
.Bx 4.2 .
.Sh BUGS
Unlike previous implementations, the master slave device nodes are
destroyed when the PTY becomes unused.
A call to
.Xr stat 2
on a nonexistent master device will already cause a new master device
node to be created.
The master device can only be destroyed by opening and closing it.
.Pp
The
.Nm
driver cannot be unloaded, because it cannot determine if it is being
used.