man7/pty.7

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  22 .TH PTY 7 2005-10-10 "Linux 2.6.14" "Linux Programmer's Manual"
  23 .SH NAME
  24 pty \- pseudo-terminal interfaces
  25 .SH DESCRIPTION
  26 A pseudo-terminal is a pair of virtual character devices that
  27 provide a bidirectional communication channel.
  28 One end of the channel is called the
  29 .IR master ;
  30 the other end is called the
  31 .IR slave .
  32 The slave end of the pseudo-terminal provides an interface
  33 that behaves exactly like a classical terminal.
  34 A process that expects to be connected to a terminal,
  35 can open the slave end of a pseudo-terminal and
  36 then be driven by a program that has opened the master end.
  37 Anything that is written on the master end is provided to the process
  38 on the slave end as though it was input typed on a terminal.
  39 For example, writing the interrupt character (usually control-C)
  40 to the master device would cause an interrupt signal (SIGINT)
  41 to be generated for the foreground process group
  42 that is connected to the slave.
  43 Conversely, anything that is written to the slave end of the
  44 pseudo-terminal can be read by the process that is connected to
  45 the master end.
  46 Pseudo-terminals are used by applications such as network login services
  47 .RB ( ssh "(1), " rlogin "(1), " telnet (1)),
  48 terminal emulators,
  49 .BR script (1),
  50 .BR screen (1),
  51 and
  52 .BR expect (1).
  53
  54 Historically, two pseudo-terminal APIs have evolved: BSD and System V.
  55 SUSv1 standardized a pseudo-terminal API based on the System V API,
  56 and this API should be employed in all new programs that use
  57 pseudo-terminals.
  58
  59 Linux provides both BSD-style and (standardised) System V-style
  60 pseudo-terminals.
  61 System V-style terminals are commonly called Unix 98 pseudo-terminals
  62 on Linux systems.
  63 Since kernel 2.6.4, BSD-style pseudo-terminals are considered deprecated
  64 (they can be disabled when configuring the kernel);
  65 Unix 98 pseudo-terminals should be used in new applications.
  66 .SS "Unix 98 pseudo-terminals"
  67 An unused Unix 98 pseudo-terminal master is opened by calling
  68 .BR posix_openpt (3).
  69 (This function opens the master clone device,
  70 .IR /dev/ptmx ;
  71 see
  72 .BR pts (4).)
  73 After performing any program-specific initialisations,
  74 changing the ownership and permissions of the slave device using
  75 .BR grantpt (3),
  76 and unlocking the slave using
  77 .BR unlockpt (3)),
  78 the corresponding slave device can be opened by passing
  79 the name returned by
  80 .BR ptsname (3)
  81 in a call to
  82 .BR open (2).
  83
  84 The Linux kernel imposes a limit on the number of available
  85 Unix 98 pseudo-terminals.
  86 In kernels up to and including 2.6.3, this limit is configured
  87 at kernel compilation time (CONFIG_UNIX98_PTYS),
  88 and the permitted number of pseudo-terminals can be up to 2048,
  89 with a default setting of 256.
  90 Since kernel 2.6.4, the limit is dynamically adjustable via
  91 .IR /proc/sys/kernel/pty/max ,
  92 and a corresponding file,
  93 .IR /proc/sys/kernel/pty/nr ,
  94 indicates how many pseudo-terminals are currently in use.
  95 For further details on these two files, see
  96 .BR proc (5).
  97 .SS "BSD pseudo-terminals"
  98 BSD-style pseudo-terminals are provided as pre-created pairs, with
  99 names of the form
 100 .I /dev/ptyXY
 101 (master) and
 102 .I /dev/ttyXY
 103 (slave),
 104 where X is a letter from the 16-character set [p-za-e],
 105 and Y is a letter from the 16-character set [0-9a-f].
 106 (The precise range of letters in these two sets varies across Unix
 107 implementations.)
 108 For example,
 109 .I /dev/ptyp1
 110 and
 111 .I /dev/ttyp1
 112 constitute a BSD pseudo-terminal pair.
 113 A process finds an unused pseudo-terminal pair by trying to
 114 .BR open (2)
 115 each pseudo-terminal master until an open succeeds.
 116 The corresponding pseudo-terminal slave (substitute "tty"
 117 for "pty" in the name of the master) can then be opened.
 118 .SH "NOTES"
 119 A description of the
 120 .B TIOCPKT
 121 .BR ioctl (),
 122 which controls packet mode operation, can be found in
 123 .BR tty_ioctl (4).
 124
 125 The  BSD  ioctl()s TIOCSTOP, TIOCSTART, TIOCUCNTL, TIOCREMOTE have
 126 not been implemented under Linux.
 127 .SH "FILES"
 128 .I /dev/ptmx
 129 (Unix 98 master clone device)
 130 .br
 131 .I /dev/pts/*
 132 (Unix 98 slave devices)
 133 .br
 134 .I /dev/pty[p-za-e][0-9a-f]
 135 (BSD master devices)
 136 .br
 137 .I /dev/tty[p-za-e][0-9a-f]
 138 (BSD slave devices)
 139 .I
 140 .SH "SEE ALSO"
 141 .BR select (2),
 142 .BR setsid (2),
 143 .BR forkpty (3),
 144 .BR openpty (3),
 145 .BR termios (3),
 146 .BR pts (4),
 147 .BR tty (4),
 148 .BR tty_ioctl (4)