1PTY(7) Linux Programmer's Manual PTY(7)
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6 pty - pseudoterminal interfaces
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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. The slave end of the pseudoterminal provides an interface
13 that behaves exactly like a classical terminal. A process that expects
14 to be connected to a terminal, can open the slave end of a pseudotermi‐
15 nal and then be driven by a program that has opened the master end.
16 Anything that is written on the master end is provided to the process
17 on the slave end as though it was input typed on a terminal. For exam‐
18 ple, writing the interrupt character (usually control-C) to the master
19 device would cause an interrupt signal (SIGINT) to be generated for the
20 foreground process group that is connected to the slave. Conversely,
21 anything that is written to the slave end of the pseudoterminal can be
22 read by the process that is connected to the master end. Pseudotermi‐
23 nals are used by applications such as network login services (ssh(1),
24 rlogin(1), telnet(1)), terminal emulators such as xterm(1), script(1),
25 screen(1), tmux(1), unbuffer(1), and expect(1).
26 Data flow between master and slave is handled asynchronously, much like
28 data flow with a physical terminal. Data written to the slave will be
29 available at the master promptly, but may not be available immediately.
30 Similarly, there may be a small processing delay between a write to the
31 master, and the effect being visible at the slave.
32 Historically, two pseudoterminal APIs have evolved: BSD and System V.
34 SUSv1 standardized a pseudoterminal API based on the System V API, and
35 this API should be employed in all new programs that use pseudotermi‐
36 nals.
37 Linux provides both BSD-style and (standardized) System V-style pseu‐
39 doterminals. System V-style terminals are commonly called UNIX 98
40 pseudoterminals on Linux systems. Since kernel 2.6.4, BSD-style pseu‐
41 doterminals are considered deprecated (they can be disabled when con‐
42 figuring the kernel); UNIX 98 pseudoterminals should be used in new
43 applications.
44 UNIX 98 pseudoterminals
46 An unused UNIX 98 pseudoterminal master is opened by calling
47 posix_openpt(3). (This function opens the master clone device,
48 /dev/ptmx; see pts(4).) After performing any program-specific initial‐
49 izations, changing the ownership and permissions of the slave device
50 using grantpt(3), and unlocking the slave using unlockpt(3)), the cor‐
51 responding slave device can be opened by passing the name returned by
52 ptsname(3) in a call to open(2).
53 The Linux kernel imposes a limit on the number of available UNIX 98
55 pseudoterminals. In kernels up to and including 2.6.3, this limit is
56 configured at kernel compilation time (CONFIG_UNIX98_PTYS), and the
57 permitted number of pseudoterminals can be up to 2048, with a default
58 setting of 256. Since kernel 2.6.4, the limit is dynamically
59 adjustable via /proc/sys/kernel/pty/max, and a corresponding file,
60 /proc/sys/kernel/pty/nr, indicates how many pseudoterminals are cur‐
61 rently in use. For further details on these two files, see proc(5).
62 BSD pseudoterminals
64 BSD-style pseudoterminals are provided as precreated pairs, with names
65 of the form /dev/ptyXY (master) and /dev/ttyXY (slave), where X is a
66 letter from the 16-character set [p-za-e], and Y is a letter from the
67 16-character set [0-9a-f]. (The precise range of letters in these two
68 sets varies across UNIX implementations.) For example, /dev/ptyp1 and
69 /dev/ttyp1 constitute a BSD pseudoterminal pair. A process finds an
70 unused pseudoterminal pair by trying to open(2) each pseudoterminal
71 master until an open succeeds. The corresponding pseudoterminal slave
72 (substitute "tty" for "pty" in the name of the master) can then be
73 opened.
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76 /dev/ptmx
77 UNIX 98 master clone device
78 /dev/pts/*
80 UNIX 98 slave devices
81 /dev/pty[p-za-e][0-9a-f]
83 BSD master devices
84 /dev/tty[p-za-e][0-9a-f]
86 BSD slave devices
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89 A description of the TIOCPKT ioctl(2), which controls packet mode oper‐
90 ation, can be found in ioctl_tty(2).
91 The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCRE‐
93 MOTE have not been implemented under Linux.
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96 ioctl_tty(2), select(2), setsid(2), forkpty(3), openpty(3), termios(3),
97 pts(4), tty(4)
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100 This page is part of release 4.15 of the Linux man-pages project. A
101 description of the project, information about reporting bugs, and the
102 latest version of this page, can be found at
103 https://www.kernel.org/doc/man-pages/.
104Linux 2017-09-15 PTY(7)