man > pty(7)

pty(7)                            Miscellaneous Information Manual                            pty(7)

NAME
       pty - pseudoterminal interfaces

DESCRIPTION
       A  pseudoterminal  (sometimes  abbreviated "pty") 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 pseudoterminal provides an interface that behaves exactly like a classi‐
       cal  terminal.   A process that expects to be connected to a terminal, can open the slave end
       of a pseudoterminal 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 con‐
       trol-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  writ‐
       ten  to  the  slave end of the pseudoterminal can be read by the process that is connected to
       the master end.

       Data flow between master and slave is handled asynchronously, much  like  data  flow  with  a
       physical  terminal.   Data written to the slave will be available at the master promptly, but
       may not be available immediately.  Similarly, there may be a small processing delay between a
       write to the master, and the effect being visible at the slave.

       Historically, two pseudoterminal APIs have evolved: BSD and System V.  SUSv1  standardized  a
       pseudoterminal API based on the System V API, and this API should be employed in all new pro‐
       grams that use pseudoterminals.

       Linux  provides  both BSD-style and (standardized) System V-style pseudoterminals.  System V-
       style terminals are commonly called UNIX 98 pseudoterminals on Linux systems.

       Since Linux 2.6.4, BSD-style pseudoterminals are considered deprecated: support can  be  dis‐
       abled  when  building  the kernel by disabling the CONFIG_LEGACY_PTYS option.  (Starting with
       Linux 2.6.30, that option is disabled by default in the mainline kernel.)  UNIX 98 pseudoter‐
       minals should be used in new applications.

   UNIX 98 pseudoterminals
       An unused UNIX 98 pseudoterminal 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 pseudoterminals.  Up to
       and including Linux 2.6.3,  this  limit  is  configured  at  kernel  compilation  time  (CON‐
       FIG_UNIX98_PTYS),  and  the permitted number of pseudoterminals can be up to 2048, with a de‐
       fault  setting  of  256.   Since  Linux  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 pseudoterminals are currently in use.  For further  details  on  these  two  files,  see
       proc(5).

   BSD pseudoterminals
       BSD-style pseudoterminals 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  pseudoterminal  pair.   A  process  finds  an unused pseudoterminal pair by trying to
       open(2) each pseudoterminal master until an open succeeds.  The corresponding  pseudoterminal
       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
       Pseudoterminals  are  used by applications such as network login services (ssh(1), rlogin(1),
       telnet(1)), terminal emulators such as xterm(1), script(1), screen(1), tmux(1),  unbuffer(1),
       and expect(1).

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

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

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

Linux man-pages 6.7                          2023-11-19                                       pty(7)