pty(7) - Linux manual page (original) (raw)

pty(7) Miscellaneous Information Manual pty(7)

NAME top

   pty - pseudoterminal interfaces

DESCRIPTION top

   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
   _master_; the other end is called the _slave_.

   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 (**SIGINT**) 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.

   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
   pseudoterminals.

   Linux provides both BSD-style and (standardized) System V-style
   pseudoterminals.  System V-style terminals are commonly called
   UNIX 98 pseudoterminals on Linux systems.

   Since Linux 2.6.4, BSD-style pseudoterminals are considered
   deprecated: support can be disabled when building the kernel by
   disabling the **CONFIG_LEGACY_PTYS** option.  (Starting with Linux
   2.6.30, that option is disabled by default in the mainline
   kernel.)  UNIX 98 pseudoterminals should be used in new
   applications.

UNIX 98 pseudoterminals An unused UNIX 98 pseudoterminal master is opened by calling posix_openpt(3). (This function opens the master clone device, /dev/ptmx; see pts(4).) After performing any program-specific initializations, changing the ownership and permissions of the slave device using grantpt(3), and unlocking the slave using unlockpt(3)), the corresponding slave device can be opened by passing the name returned by ptsname(3) in a call to open(2).

   The Linux kernel imposes a limit on the number of available UNIX
   98 pseudoterminals.  Up to and including Linux 2.6.3, this limit
   is configured at kernel compilation time (**CONFIG_UNIX98_PTYS**), and
   the permitted number of pseudoterminals can be up to 2048, with a
   default setting of 256.  Since Linux 2.6.4, the limit is
   dynamically adjustable via _/proc/sys/kernel/pty/max_, and a
   corresponding file, _/proc/sys/kernel/pty/nr_, indicates how many
   pseudoterminals are currently in use.  For further details on
   these two files, see [proc(5)](../man5/proc.5.html).

BSD pseudoterminals BSD-style pseudoterminals are provided as precreated pairs, with names of the form /dev/ptyXY (master) and /dev/ttyXY (slave), 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 implementations.) For example, /dev/ptyp1 and /dev/ttyp1 constitute a BSD pseudoterminal pair. A process finds an unused pseudoterminal pair by trying to open(2) 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.

FILES top

   _/dev/ptmx_
          UNIX 98 master clone device

   _/dev/pts/*_
          UNIX 98 slave devices

   _/dev/pty[p-za-e][0-9a-f]_
          BSD master devices

   _/dev/tty[p-za-e][0-9a-f]_
          BSD slave devices

NOTES top

   Pseudoterminals are used by applications such as network login
   services ([ssh(1)](../man1/ssh.1.html), **rlogin**(1), **telnet**(1)), terminal emulators such
   as **xterm**(1), [script(1)](../man1/script.1.html), [screen(1)](../man1/screen.1.html), [tmux(1)](../man1/tmux.1.html), **unbuffer**(1), and
   [expect(1)](../man1/expect.1.html).

   A description of the **TIOCPKT ioctl**(2), which controls packet mode
   operation, can be found in [ioctl_tty(2)](../man2/ioctl%5Ftty.2.html).

   The BSD [ioctl(2)](../man2/ioctl.2.html) operations **TIOCSTOP**, **TIOCSTART**, **TIOCUCNTL**, and
   **TIOCREMOTE** have not been implemented under Linux.

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Linux man-pages 6.10 2024-05-02 pty(7)