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 ownership 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, indicates 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 precreated 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-character 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 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2005-10-10								    PTY(7)