PTY(4)							   BSD Kernel Interfaces Manual 						    PTY(4)

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 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.

     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) 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(2) 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(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.

     TIOCUCNTL	 Enable/disable a mode that allows a small number of simple user ioctl(2) 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 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.

     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 termi-
		 nal; a write of 0 bytes is like typing an end-of-file character.  TIOCREMOTE can be used when doing remote line editing in a win-
		 dow manager, or whenever flow controlled input is required.

FILES

     /dev/pty[p-sP-S][a-z0-9]	master pseudo terminals
     /dev/tty[p-sP-S][a-z0-9]	slave pseudo terminals

DIAGNOSTICS

     None.

HISTORY

     The pty driver appeared in 4.2BSD.

4.2 Berkeley Distribution					 November 30, 1993					 4.2 Berkeley Distribution