pty man page on OpenBSD

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PTY(4)			  OpenBSD Programmer's 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, space
     for that number of pseudo terminal pairs is preallocated.	If the count
     is missing or is less than 2, a default count of 8 is used.  This is not
     a hard limit--space for additional pseudo terminal pairs is allocated on
     demand up to the limit imposed by the kern.tty.maxptys sysctl(8) (992 by
     default).

     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 non-zero 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 and rlogind
				     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_IOCTL	     When this bit is set, the slave has
				     changed the termios(4) structure (TTY
				     state), and the remainder of the data
				     read from the master side of the pty is a
				     copy of the new termios(4) structure.

				     This is used by telnet daemons to
				     implement TELNET "line mode", allowing
				     them to detect tty(4) state changes by
				     the slave, and negotiate the appropriate
				     TELNET protocol equivalents with the
				     remote peer.

     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 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.

     The standard way to allocate pty devices is through openpty(3), a
     function which internally uses a PTMGET ioctl(2) call on the /dev/ptm
     device.  The PTMGET command allocates a free pseudo terminal, changes its
     ownership to the caller, revokes the access privileges for all previous
     users, opens the file descriptors for the master and slave devices and
     returns them to the caller in struct ptmget.

	   struct ptmget {
		   int	   cfd;
		   int	   sfd;
		   char	   cn[16];
		   char	   sn[16];
	   };

     The cfd and sfd fields are the file descriptors for the controlling and
     slave terminals.  The cn and sn fields are the file names of the
     controlling and slave devices.

FILES
     /dev/pty[p-zP-T][0-9a-zA-Z]   master pseudo terminals
     /dev/tty[p-zP-T][0-9a-zA-Z]   slave pseudo terminals
     /dev/ptm			   pseudo terminal management device

SEE ALSO
     openpty(3), tty(4), sysctl(8)

HISTORY
     The pty driver appeared in 4.2BSD.	 The /dev/ptm device was added in
     OpenBSD 3.5.

CAVEATS
     The ptm device will only work on systems where the /dev directory has
     been properly populated with pty device nodes following the naming
     convention used in OpenBSD.  Since ptm impersonates the super user for
     some operations it needs to perform to complete the allocation of a
     pseudo terminal, the /dev directory must also be writeable by the super
     user.

OpenBSD 4.9			 May 31, 2007			   OpenBSD 4.9
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