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 de-
     vice and a slave device.  The slave device provides to a process an in-
     terface identical to that described in tty(4).  However, whereas all oth-
     er devices which provide the interface described in tty(4) have a hard-
     ware device of some sort behind them, the slave device has, instead, an-
     other process manipulating it through the master half of the pseudo ter-
     minal.  That is, anything written on the master device is given to the
     slave device as input and anything written on the slave device is pre-
     sented 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 speci-
                 fying (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 in-
                 clusive-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 imple-
                                     ment TELNET "line mode", allowing them to
                                     detect tty(4) state changes by the slave,
                                     and negotiate the appropriate TELNET pro-
                                     tocol 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 termi-
                 nal, 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 op-
                 eration 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 fa-
                 cility.  As with TIOCPKT mode, command operations may be de-
                 tected 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 re-
                 mote line editing in a window manager, or whenever flow con-
                 trolled input is required.

     The standard way to allocate pty devices is through openpty(3), a func-
     tion which internally uses a PTMGET ioctl(2) call on the /dev/ptm device.
     The PTMGET command allocates a free pseudo terminal, changes its owner-
     ship 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 control-
     ling 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 con-
     vention 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 4.7                      May 31, 2007                                3
