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PTY(7)                     Linux Programmer's Manual                    PTY(7)



NAME
       pty - pseudo-terminal interfaces

DESCRIPTION
       A  pseudo-terminal  is  a pair of virtual character devices that provide a bidirectional communication channel.
       One end of the channel is called the master; the other end is called the slave.  The slave end of  the  pseudo-
       terminal  provides  an  interface that behaves exactly like a classical terminal.  A process that expects to be
       connected to a terminal, can open the slave end of a pseudo-terminal and then be driven by a program  that  has
       opened  the master end.  Anything that is written on the master end is provided to the process on the slave end
       as though it was input typed on a terminal.  For example, writing the interrupt character  (usually  control-C)
       to  the master device would cause an interrupt signal (SIGINT) to be generated for the foreground process group
       that is connected to the slave.  Conversely, anything that is written to the slave end of  the  pseudo-terminal
       can be read by the process that is connected to the master end.  Pseudo-terminals are used by applications such
       as network login services  (ssh(1),  rlogin(1),  telnet(1)),  terminal  emulators,  script(1),  screen(1),  and
       expect(1).

       Historically,  two  pseudo-terminal  APIs have evolved: BSD and System V.  SUSv1 standardized a pseudo-terminal
       API based on the System V API, and this API should be employed in all new programs that use pseudo-terminals.

       Linux provides both BSD-style and (standardized) System V-style pseudo-terminals.  System V-style terminals are
       commonly  called Unix 98 pseudo-terminals on Linux systems.  Since kernel 2.6.4, BSD-style pseudo-terminals are
       considered deprecated (they can be disabled when configuring the kernel); Unix 98  pseudo-terminals  should  be
       used in new applications.

   Unix 98 pseudo-terminals
       An unused Unix 98 pseudo-terminal master is opened by calling posix_openpt(3).  (This function opens the master
       clone device, /dev/ptmx; see pts(4).)  After performing any program-specific initializations, changing the own-
       ership  and  permissions  of the slave device using grantpt(3), and unlocking the slave using unlockpt(3)), the
       corresponding slave device can be opened by passing the name returned by ptsname(3) in a call to open(2).

       The Linux kernel imposes a limit on the number of available Unix 98 pseudo-terminals.  In  kernels  up  to  and
       including  2.6.3,  this  limit is configured at kernel compilation time (CONFIG_UNIX98_PTYS), and the permitted
       number of pseudo-terminals can be up to 2048, with a default setting of 256.  Since kernel 2.6.4, the limit  is
       dynamically  adjustable  via /proc/sys/kernel/pty/max, and a corresponding file, /proc/sys/kernel/pty/nr, indi-
       cates how many pseudo-terminals are currently in use.  For further details on these two files, see proc(5).

   BSD pseudo-terminals
       BSD-style pseudo-terminals are provided as pre-created pairs, with names of the form  /dev/ptyXY  (master)  and
       /dev/ttyXY (slave), where X is a letter from the 16-character set [p-za-e], and Y is a letter from the 16-char-
       acter set [0-9a-f].  (The precise range of letters in these two sets varies across Unix implementations.)   For
       example,  /dev/ptyp1  and  /dev/ttyp1 constitute a BSD pseudo-terminal pair.  A process finds an unused pseudo-
       terminal pair by trying to open(2) each pseudo-terminal master  until  an  open  succeeds.   The  corresponding
       pseudo-terminal slave (substitute "tty" for "pty" in the name of the master) can then be opened.

FILES
       /dev/ptmx (Unix 98 master clone device)
       /dev/pts/* (Unix 98 slave devices)
       /dev/pty[p-za-e][0-9a-f] (BSD master devices)
       /dev/tty[p-za-e][0-9a-f] (BSD slave devices)

NOTES
       A description of the TIOCPKT ioctl(2), which controls packet mode operation, can be found in tty_ioctl(4).

       The  BSD  ioctl(2)  operations  TIOCSTOP,  TIOCSTART, TIOCUCNTL, and TIOCREMOTE have not been implemented under
       Linux.

SEE ALSO
       select(2), setsid(2), forkpty(3), openpty(3), termios(3), pts(4), tty(4), tty_ioctl(4)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2005-10-10                            PTY(7)