Section: Linux Programmer's Manual (7)
pty - pseudoterminal interfaces
A pseudoterminal (sometimes abbreviated "pty")
is a pair of virtual character devices that
provide a bidirectional communication channel.
One end of the channel is called the
the other end is called the
The slave end of the pseudoterminal provides an interface
that behaves exactly like a classical terminal.
A process that expects to be connected to a terminal,
can open the slave end of a pseudoterminal and
then be driven by a program that has opened the master end.
Anything that is written on the master end is provided to the process
on the slave end as though it was input typed on a terminal.
For example, writing the interrupt character (usually control-C)
to the master device would cause an interrupt signal
to be generated for the foreground process group
that is connected to the slave.
Conversely, anything that is written to the slave end of the
pseudoterminal can be read by the process that is connected to
the master end.
Pseudoterminals are used by applications such as network login services
terminal emulators such as
Data flow between master and slave is handled asynchronously,
much like data flow with a physical terminal.
Data written to the slave will be available at the master promptly,
but may not be available immediately.
Similarly, there may be a small processing delay between
a write to the master, and the effect being visible at the slave.
Historically, two pseudoterminal APIs have evolved: BSD and System V.
SUSv1 standardized a pseudoterminal API based on the System V API,
and this API should be employed in all new programs that use
Linux provides both BSD-style and (standardized) System V-style
System V-style terminals are commonly called UNIX 98 pseudoterminals
on Linux systems.
Since kernel 2.6.4, BSD-style pseudoterminals are considered deprecated
(they can be disabled when configuring the kernel);
UNIX 98 pseudoterminals should be used in new applications.
UNIX 98 pseudoterminals
An unused UNIX 98 pseudoterminal master is opened by calling
(This function opens the master clone device,
After performing any program-specific initializations,
changing the ownership and permissions of the slave device using
and unlocking the slave using
the corresponding slave device can be opened by passing
the name returned by
in a call to
The Linux kernel imposes a limit on the number of available
UNIX 98 pseudoterminals.
In kernels up to and including 2.6.3, this limit is configured
at kernel compilation time
and the permitted number of pseudoterminals can be up to 2048,
with a default setting of 256.
Since kernel 2.6.4, the limit is dynamically adjustable via
and a corresponding file,
indicates how many pseudoterminals are currently in use.
For further details on these two files, see
BSD-style pseudoterminals are provided as precreated pairs, with
names of the form
where X is a letter from the 16-character set [p-za-e],
and Y is a letter from the 16-character set [0-9a-f].
(The precise range of letters in these two sets varies across UNIX
constitute a BSD pseudoterminal pair.
A process finds an unused pseudoterminal pair by trying to
each pseudoterminal master until an open succeeds.
The corresponding pseudoterminal slave (substitute "tty"
for "pty" in the name of the master) can then be opened.
UNIX 98 master clone device
UNIX 98 slave devices
BSD master devices
BSD slave devices
A description of the
which controls packet mode operation, can be found in
have not been implemented under Linux.
This page is part of release 5.02 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at