PTY(4)							     Kernel Interfaces Manual							    PTY(4)

NAME

       pty - pseudo terminal driver

SYNOPSIS

       /sys/conf/SYSTEM:
	    NPTY ptys # pseudo-terminals, in groups of 8

DESCRIPTION

       The  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 processes 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.

       In configuring, ptys specifies the number of pseudo terminal pairs are configured.

       The following ioctl 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 master side of a pseudo terminal, each subsequent  read  from  the  terminal  will
	      return  data  written  on the slave 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 master side may be detected  by  a  select
	      for exceptional conditions.

	      This mode is used by rlogin(1C) and rlogind(8C) 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 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 master
	      side of a pseudo terminal by specifying (by reference) a nonzero parameter and disabled by specifying (by reference) a zero  parame-
	      ter.  Each subsequent read 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
	      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 from the master side.	The ioctl 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 for exceptional conditions.

       TIOCREMOTE
	      A  mode  for the master half of a pseudo terminal, independent of TIOCPKT.  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.

FILES

       /dev/pty[p-r][0-9a-f]	master pseudo terminals
       /dev/tty[p-r][0-9a-f]	slave pseudo terminals

DIAGNOSTICS

       None.

4.2 Berkeley Distribution					   May 19, 1986 							    PTY(4)