| PTY(4) | Device Drivers Manual | PTY(4) | 
pty —
pseudo-device pty
pty driver provides support for a device-pair termed
  a pseudo terminal. A pseudo terminal is a pair of character
  devices, a master device and a slave
  device. The slave device provides to a process an interface identical to that
  described in tty(4). However,
  whereas all other devices which provide the interface described in
  tty(4) have a hardware device of
  some sort behind them, the slave device has, instead, another process
  manipulating it through the master half of the pseudo terminal. That is,
  anything written on the master device is given to the slave device as input
  and anything written on the slave device is presented as input on the master
  device.
Pseudo terminal pairs are allocated on as-needed basis, maximum number of them is controlled via kern.maxptys sysctl (defaults to 992).
The following ioctl(2) calls apply only to pseudo terminals:
TIOCEXTTIOCPKT_IOCTL packets. External processing is
      enabled by specifying (by reference) a nonzero int
      parameter and disabled by specifying (by reference) a zero
      int parameter.
    TIOCEXT is reset to its default
        (disabled) when the slave closes the pty.
TIOCSTOP^S’). Takes no parameter.TIOCSTARTTIOCSTOP or by typing
      ‘^S’). Takes no parameter.TIOCPKTTIOCPKT_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_FLUSHREADTIOCPKT_FLUSHWRITETIOCPKT_STOP^S’.TIOCPKT_STARTTIOCPKT_DOSTOP^S’ and
          t_startc is
          ‘^Q’.TIOCPKT_NOSTOP^S/^Q’.
        While this mode is in use, the presence of control status information to be read from the master side may be detected by a select(2) for exceptional conditions.
This mode is used by
            rlogin(1) and
            rlogind(8) 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.
TIOCPKT_IOCTLpty is the new
          termios(4) structure.
          The master side of the pty can also use
          tcgetattr(3) to read
          the new termios(4)
          structure.
        The master will not read packets with the bit
            TIOCPKT_IOCTL set until it has activated
            “external processing” using
            TIOCEXT.
This is used by telnetd(8) to implement TELNET "line mode" - it allows the telnetd(8) to detect tty(4) state changes by the slave, and negotiate the appropriate TELNET protocol equivalents with the remote peer.
TIOCUCNTLTIOCPKT. The TIOCUCNTL and
      TIOCPKT modes are mutually exclusive. This mode is
      enabled from the master side of a pseudo terminal by specifying (by
      reference) a nonzero int parameter and disabled by
      specifying (by reference) a zero int parameter. Each
      subsequent read(2) from the
      master side will return data written on the slave part of the pseudo
      terminal preceded by a zero byte, or a single byte reflecting a user
      control operation on the slave side. A user control command consists of a
      special ioctl(2) 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) from the
      master side. The ioctl(2)
      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) for
      exceptional conditions.TIOCREMOTETIOCPKT. This mode causes input to the pseudo
      terminal to be flow controlled and not input edited (regardless of the
      terminal mode). Each write to the control terminal produces a record
      boundary for the process reading the terminal. In normal usage, a write of
      data is like the data typed as a line on the terminal; a write of 0 bytes
      is like typing an end-of-file character.
      TIOCREMOTE can be used when doing remote line
      editing in a window manager, or whenever flow controlled input is
      required.pty driver appeared in
  4.2BSD.
| November 30, 2013 | NetBSD 10.0 |