1PTY(7) Linux Programmer's Manual PTY(7)
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6 pty - pseudo-terminal interfaces
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9 A pseudo-terminal is a pair of virtual character devices that provide a
10 bidirectional communication channel. One end of the channel is called
11 the master; the other end is called the slave. The slave end of the
12 pseudo-terminal provides an interface that behaves exactly like a clas‐
13 sical terminal. A process that expects to be connected to a terminal,
14 can open the slave end of a pseudo-terminal and then be driven by a
15 program that has opened the master end. Anything that is written on
16 the master end is provided to the process on the slave end as though it
17 was input typed on a terminal. For example, writing the interrupt
18 character (usually control-C) to the master device would cause an
19 interrupt signal (SIGINT) to be generated for the foreground process
20 group that is connected to the slave. Conversely, anything that is
21 written to the slave end of the pseudo-terminal can be read by the
22 process that is connected to the master end. Pseudo-terminals are used
23 by applications such as network login services (ssh(1), rlogin(1), tel‐
24 net(1)), terminal emulators, script(1), screen(1), and expect(1).
25 Historically, two pseudo-terminal APIs have evolved: BSD and System V.
27 SUSv1 standardized a pseudo-terminal API based on the System V API, and
28 this API should be employed in all new programs that use pseudo-termi‐
29 nals.
30 Linux provides both BSD-style and (standardized) System V-style pseudo-
32 terminals. System V-style terminals are commonly called Unix 98
33 pseudo-terminals on Linux systems. Since kernel 2.6.4, BSD-style
34 pseudo-terminals are considered deprecated (they can be disabled when
35 configuring the kernel); Unix 98 pseudo-terminals should be used in new
36 applications.
37 Unix 98 pseudo-terminals
39 An unused Unix 98 pseudo-terminal master is opened by calling
40 posix_openpt(3). (This function opens the master clone device,
41 /dev/ptmx; see pts(4).) After performing any program-specific initial‐
42 izations, changing the ownership and permissions of the slave device
43 using grantpt(3), and unlocking the slave using unlockpt(3)), the cor‐
44 responding slave device can be opened by passing the name returned by
45 ptsname(3) in a call to open(2).
46 The Linux kernel imposes a limit on the number of available Unix 98
48 pseudo-terminals. In kernels up to and including 2.6.3, this limit is
49 configured at kernel compilation time (CONFIG_UNIX98_PTYS), and the
50 permitted number of pseudo-terminals can be up to 2048, with a default
51 setting of 256. Since kernel 2.6.4, the limit is dynamically
52 adjustable via /proc/sys/kernel/pty/max, and a corresponding file,
53 /proc/sys/kernel/pty/nr, indicates how many pseudo-terminals are cur‐
54 rently in use. For further details on these two files, see proc(5).
55 BSD pseudo-terminals
57 BSD-style pseudo-terminals are provided as precreated pairs, with names
58 of the form /dev/ptyXY (master) and /dev/ttyXY (slave), where X is a
59 letter from the 16-character set [p-za-e], and Y is a letter from the
60 16-character set [0-9a-f]. (The precise range of letters in these two
61 sets varies across Unix implementations.) For example, /dev/ptyp1 and
62 /dev/ttyp1 constitute a BSD pseudo-terminal pair. A process finds an
63 unused pseudo-terminal pair by trying to open(2) each pseudo-terminal
64 master until an open succeeds. The corresponding pseudo-terminal slave
65 (substitute "tty" for "pty" in the name of the master) can then be
66 opened.
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69 /dev/ptmx (Unix 98 master clone device)
70 /dev/pts/* (Unix 98 slave devices)
71 /dev/pty[p-za-e][0-9a-f] (BSD master devices)
72 /dev/tty[p-za-e][0-9a-f] (BSD slave devices)
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75 A description of the TIOCPKT ioctl(2), which controls packet mode oper‐
76 ation, can be found in tty_ioctl(4).
77 The BSD ioctl(2) operations TIOCSTOP, TIOCSTART, TIOCUCNTL, and TIOCRE‐
79 MOTE have not been implemented under Linux.
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82 select(2), setsid(2), forkpty(3), openpty(3), termios(3), pts(4),
83 tty(4), tty_ioctl(4)
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86 This page is part of release 3.25 of the Linux man-pages project. A
87 description of the project, and information about reporting bugs, can
88 be found at http://www.kernel.org/doc/man-pages/.
89Linux 2005-10-10 PTY(7)