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

     pty -- pseudo terminal driver

     pseudo-device pty [count]

     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 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

     The following ioctl(2) calls apply only to pseudo terminals and may only
     be applied to the pty master:

     TIOCEXT     Enable/disable external processing.  External processing is
                 enabled by specifying (by reference) a non-zero int parameter
                 and disabled by specifying (by reference) a zero int parame-

                 While external processing is enabled, input line editing,
                 character echo, and mapping of control characters to signals
                 are disabled regardless of the terminal's termios(4) set-

     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 int parameter and disabled by
                 specifying (by reference) a zero int parameter.

                 While packet mode is enabled, each subsequent read(2) from
                 the pty master will either return data written to the pty
                 slave 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

                 TIOCPKT_DOSTOP      whenever t_stopc is '^S' and t_startc is

                 TIOCPKT_NOSTOP      whenever the start and stop characters
                                     are not '^S/^Q'.

                 TIOCPKT_IOCTL       whenever the terminal's termios(4) set-
                                     tings change while external processing is

                                     Additionally, when the TIOCPKT_IOCTL bit
                                     is set, the remainder of the data read
                                     from the pty master is a copy of the new
                                     termios(4) structure.

                 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.

     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
                 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 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.

     /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

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

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

     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.

BSD                             April 26, 2012                             BSD