---
name: "pty"
section: "4"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/pty/4"
---

# pty(4)

pseudo terminal driver

## NAME

**pty** – pseudo terminal driver

## SYNOPSIS

**pseudo-device pty** \[*count*\]

## DESCRIPTION

The **pty** driver provides support for a device-pair termed a *pseudo* *terminal*. A pseudo terminal is a pair of character devices, a *primary* device and a *replica* device. The replica device provides to a process an interface identical to that described in [tty(4)](/tty/4.md). However, whereas all other devices which provide the  interface described in [tty(4)](/tty/4.md) have a hardware device of some sort behind them, the replica device has, instead, another process manipulating it through the primary half of the pseudo terminal. That is, anything written on the primary device is given to the replica device as input and anything written on the replica device is presented as input on the primary device.

In configuring, if an optional *count* is given in the specification, that number of pseudo terminal pairs are configured; the default count is 32.

The following  [ioctl(2)](/ioctl/2.md) calls apply only to pseudo terminals:

- TIOCSTOP

  Stops output to a terminal (e.g. like typing ‘^S’). Takes no parameter.

- TIOCSTART

  Restarts output (stopped by TIOCSTOP or by typing ‘^S’). Takes no parameter.

- TIOCPKT

  Enable/disable  *packet* mode. Packet mode is enabled by specifying (by reference) a nonzero parameter and disabled by specifying (by reference) a zero parameter. When applied to the primary side of a pseudo terminal, each subsequent  [read(2)](/read/2.md) from the terminal will return data written on the replica part of the pseudo terminal preceded by a zero byte (symbolically defined as TIOCPKT\_DATA), or a single byte reflecting control status information. In the latter case, the byte is an inclusive-or of zero or more of the bits:

  - TIOCPKT\_FLUSHREAD

    whenever the read queue for the terminal is flushed.

  - TIOCPKT\_FLUSHWRITE

    whenever the write queue for the terminal is flushed.

  - TIOCPKT\_STOP

    whenever output to the terminal is stopped a la ‘^S’.

  - TIOCPKT\_START

    whenever output to the terminal is restarted.

  - TIOCPKT\_DOSTOP

    whenever  *t\_stopc* is ‘^S’ and  *t\_startc* is ‘^Q’.

  - TIOCPKT\_NOSTOP

    whenever the start and stop characters are not ‘^S/^Q’.

    While this mode is in use, the presence of control status information to be read from the primary side may be detected by a [select(2)](/select/2.md) for exceptional conditions.

    This mode is used by [rlogin(1)](/rlogin/1.md) and [rlogind(8)](/rlogind/8.md) to implement a remote-echoed, locally ‘^S/^Q’ flow-controlled remote login with proper back-flushing of output; it can be used by other similar programs.

- TIOCUCNTL

  Enable/disable a mode that allows a small number of simple user [ioctl(2)](/ioctl/2.md) commands to be passed through the pseudo-terminal, using a protocol similar to that of TIOCPKT. The TIOCUCNTL and TIOCPKT modes are mutually exclusive. This mode is enabled from the primary side of a pseudo terminal by specifying (by reference) a nonzero parameter and disabled by specifying (by reference) a zero parameter. Each subsequent  [read(2)](/read/2.md) from the primary side will return data written on the replica part of the pseudo terminal preceded by a zero byte, or a single byte reflecting a user control operation on the replica side. A user control command consists of a special [ioctl(2)](/ioctl/2.md) operation with no data; the command is given as UIOCCMD(n), where *n* is a number in the range 1-255. The operation value *n* will be received as a single byte on the next [read(2)](/read/2.md) from the primary side. The [ioctl(2)](/ioctl/2.md) UIOCCMD(0) is a no-op that may be used to probe for the existence of this facility. As with TIOCPKT mode, command operations may be detected with a [select(2)](/select/2.md) for exceptional conditions.

## FILES

- */dev/pty\[p-sP-S\]\[a-z0-9\]*

  primary pseudo terminals

- */dev/tty\[p-sP-S\]\[a-z0-9\]*

  replica pseudo terminals

## DIAGNOSTICS

None.

## HISTORY

The **pty** driver appeared in 4.2BSD.