1PTY(7)                     Linux Programmer's Manual                    PTY(7)
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NAME

6       pty - pseudoterminal interfaces
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DESCRIPTION

9       A  pseudoterminal  (sometimes  abbreviated  "pty") is a pair of virtual
10       character devices that provide a bidirectional  communication  channel.
11       One  end  of  the channel is called the master; the other end is called
12       the slave.
13
14       The slave end of the pseudoterminal provides an interface that  behaves
15       exactly  like  a classical terminal.  A process that expects to be con‐
16       nected to a terminal, can open the slave end of  a  pseudoterminal  and
17       then  be  driven by a program that has opened the master end.  Anything
18       that is written on the master end is provided to  the  process  on  the
19       slave  end  as  though  it was input typed on a terminal.  For example,
20       writing the interrupt character (usually control-C) to the  master  de‐
21       vice  would  cause an interrupt signal (SIGINT) to be generated for the
22       foreground process group that is connected to the  slave.   Conversely,
23       anything  that is written to the slave end of the pseudoterminal can be
24       read by the process that is connected to the master end.
25
26       Data flow between master and slave is handled asynchronously, much like
27       data  flow with a physical terminal.  Data written to the slave will be
28       available at the master promptly, but may not be available immediately.
29       Similarly, there may be a small processing delay between a write to the
30       master, and the effect being visible at the slave.
31
32       Historically, two pseudoterminal APIs have evolved: BSD and  System  V.
33       SUSv1  standardized a pseudoterminal API based on the System V API, and
34       this API should be employed in all new programs that  use  pseudotermi‐
35       nals.
36
37       Linux  provides  both BSD-style and (standardized) System V-style pseu‐
38       doterminals.  System V-style terminals  are  commonly  called  UNIX  98
39       pseudoterminals on Linux systems.
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41       Since  kernel  2.6.4,  BSD-style  pseudoterminals are considered depre‐
42       cated: support can be disabled when building the  kernel  by  disabling
43       the  CONFIG_LEGACY_PTYS  option.  (Starting with Linux 2.6.30, that op‐
44       tion is disabled by default in the mainline  kernel.)   UNIX  98  pseu‐
45       doterminals should be used in new applications.
46
47   UNIX 98 pseudoterminals
48       An   unused   UNIX  98  pseudoterminal  master  is  opened  by  calling
49       posix_openpt(3).   (This  function  opens  the  master  clone   device,
50       /dev/ptmx; see pts(4).)  After performing any program-specific initial‐
51       izations, changing the ownership and permissions of  the  slave  device
52       using  grantpt(3), and unlocking the slave using unlockpt(3)), the cor‐
53       responding slave device can be opened by passing the name  returned  by
54       ptsname(3) in a call to open(2).
55
56       The  Linux  kernel  imposes  a limit on the number of available UNIX 98
57       pseudoterminals.  In kernels up to and including 2.6.3, this  limit  is
58       configured  at  kernel  compilation  time (CONFIG_UNIX98_PTYS), and the
59       permitted number of pseudoterminals can be up to 2048, with  a  default
60       setting  of  256.   Since  kernel  2.6.4,  the limit is dynamically ad‐
61       justable  via  /proc/sys/kernel/pty/max,  and  a  corresponding   file,
62       /proc/sys/kernel/pty/nr,  indicates  how  many pseudoterminals are cur‐
63       rently in use.  For further details on these two files, see proc(5).
64
65   BSD pseudoterminals
66       BSD-style pseudoterminals are provided as precreated pairs, with  names
67       of  the  form  /dev/ptyXY (master) and /dev/ttyXY (slave), where X is a
68       letter from the 16-character set [p-za-e], and Y is a letter  from  the
69       16-character  set [0-9a-f].  (The precise range of letters in these two
70       sets varies across UNIX implementations.)  For example, /dev/ptyp1  and
71       /dev/ttyp1  constitute  a  BSD pseudoterminal pair.  A process finds an
72       unused pseudoterminal pair by trying  to  open(2)  each  pseudoterminal
73       master  until an open succeeds.  The corresponding pseudoterminal slave
74       (substitute "tty" for "pty" in the name of  the  master)  can  then  be
75       opened.
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FILES

78       /dev/ptmx
79              UNIX 98 master clone device
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81       /dev/pts/*
82              UNIX 98 slave devices
83
84       /dev/pty[p-za-e][0-9a-f]
85              BSD master devices
86
87       /dev/tty[p-za-e][0-9a-f]
88              BSD slave devices
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NOTES

91       Pseudoterminals are used by applications such as network login services
92       (ssh(1), rlogin(1), telnet(1)), terminal emulators  such  as  xterm(1),
93       script(1), screen(1), tmux(1), unbuffer(1), and expect(1).
94
95       A description of the TIOCPKT ioctl(2), which controls packet mode oper‐
96       ation, can be found in ioctl_tty(2).
97
98       The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCRE‐
99       MOTE have not been implemented under Linux.
100

SEE ALSO

102       ioctl_tty(2), select(2), setsid(2), forkpty(3), openpty(3), termios(3),
103       pts(4), tty(4)
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COLOPHON

106       This page is part of release 5.10 of the Linux  man-pages  project.   A
107       description  of  the project, information about reporting bugs, and the
108       latest    version    of    this    page,    can     be     found     at
109       https://www.kernel.org/doc/man-pages/.
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113Linux                             2020-08-13                            PTY(7)
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