man7/pty.7

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  25 .TH PTY 7 2015-07-23 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 pty \- pseudoterminal interfaces
  28 .SH DESCRIPTION
  29 A pseudoterminal (sometimes abbreviated "pty")
  30 is a pair of virtual character devices that
  31 provide a bidirectional communication channel.
  32 One end of the channel is called the
  33 .IR master ;
  34 the other end is called the
  35 .IR slave .
  36 The slave end of the pseudoterminal provides an interface
  37 that behaves exactly like a classical terminal.
  38 A process that expects to be connected to a terminal,
  39 can open the slave end of a pseudoterminal and
  40 then be driven by a program that has opened the master end.
  41 Anything that is written on the master end is provided to the process
  42 on the slave end as though it was input typed on a terminal.
  43 For example, writing the interrupt character (usually control-C)
  44 to the master device would cause an interrupt signal
  45 .RB ( SIGINT )
  46 to be generated for the foreground process group
  47 that is connected to the slave.
  48 Conversely, anything that is written to the slave end of the
  49 pseudoterminal can be read by the process that is connected to
  50 the master end.
  51 Pseudoterminals are used by applications such as network login services
  52 .RB ( ssh "(1), " rlogin "(1), " telnet (1)),
  53 terminal emulators,
  54 .BR script (1),
  55 .BR screen (1),
  56 and
  57 .BR expect (1).
  58
  59 Data flow between master and slave is handled asynchronously,
  60 much like data flow with a physical terminal.
  61 Data written to the slave will be available at the master promptly,
  62 but may not be available immediately.
  63 Similarly, there may be a small processing delay between
  64 a write to the master, and the effect being visible at the slave.
  65
  66 Historically, two pseudoterminal APIs have evolved: BSD and System V.
  67 SUSv1 standardized a pseudoterminal API based on the System V API,
  68 and this API should be employed in all new programs that use
  69 pseudoterminals.
  70
  71 Linux provides both BSD-style and (standardized) System V-style
  72 pseudoterminals.
  73 System V-style terminals are commonly called UNIX 98 pseudoterminals
  74 on Linux systems.
  75 Since kernel 2.6.4, BSD-style pseudoterminals are considered deprecated
  76 (they can be disabled when configuring the kernel);
  77 UNIX 98 pseudoterminals should be used in new applications.
  78 .SS UNIX 98 pseudoterminals
  79 An unused UNIX 98 pseudoterminal master is opened by calling
  80 .BR posix_openpt (3).
  81 (This function opens the master clone device,
  82 .IR /dev/ptmx ;
  83 see
  84 .BR pts (4).)
  85 After performing any program-specific initializations,
  86 changing the ownership and permissions of the slave device using
  87 .BR grantpt (3),
  88 and unlocking the slave using
  89 .BR unlockpt (3)),
  90 the corresponding slave device can be opened by passing
  91 the name returned by
  92 .BR ptsname (3)
  93 in a call to
  94 .BR open (2).
  95
  96 The Linux kernel imposes a limit on the number of available
  97 UNIX 98 pseudoterminals.
  98 In kernels up to and including 2.6.3, this limit is configured
  99 at kernel compilation time
 100 .RB ( CONFIG_UNIX98_PTYS ),
 101 and the permitted number of pseudoterminals can be up to 2048,
 102 with a default setting of 256.
 103 Since kernel 2.6.4, the limit is dynamically adjustable via
 104 .IR /proc/sys/kernel/pty/max ,
 105 and a corresponding file,
 106 .IR /proc/sys/kernel/pty/nr ,
 107 indicates how many pseudoterminals are currently in use.
 108 For further details on these two files, see
 109 .BR proc (5).
 110 .SS BSD pseudoterminals
 111 BSD-style pseudoterminals are provided as precreated pairs, with
 112 names of the form
 113 .I /dev/ptyXY
 114 (master) and
 115 .I /dev/ttyXY
 116 (slave),
 117 where X is a letter from the 16-character set [p-za-e],
 118 and Y is a letter from the 16-character set [0-9a-f].
 119 (The precise range of letters in these two sets varies across UNIX
 120 implementations.)
 121 For example,
 122 .I /dev/ptyp1
 123 and
 124 .I /dev/ttyp1
 125 constitute a BSD pseudoterminal pair.
 126 A process finds an unused pseudoterminal pair by trying to
 127 .BR open (2)
 128 each pseudoterminal master until an open succeeds.
 129 The corresponding pseudoterminal slave (substitute "tty"
 130 for "pty" in the name of the master) can then be opened.
 131 .SH FILES
 132 .I /dev/ptmx
 133 (UNIX 98 master clone device)
 134 .br
 135 .I /dev/pts/*
 136 (UNIX 98 slave devices)
 137 .br
 138 .I /dev/pty[p-za-e][0-9a-f]
 139 (BSD master devices)
 140 .br
 141 .I /dev/tty[p-za-e][0-9a-f]
 142 (BSD slave devices)
 143 .SH NOTES
 144 A description of the
 145 .B TIOCPKT
 146 .BR ioctl (2),
 147 which controls packet mode operation, can be found in
 148 .BR tty_ioctl (4).
 149
 150 The BSD
 151 .BR ioctl (2)
 152 operations
 153 .BR TIOCSTOP ,
 154 .BR TIOCSTART ,
 155 .BR TIOCUCNTL ,
 156 and
 157 .BR TIOCREMOTE
 158 have not been implemented under Linux.
 159 .SH SEE ALSO
 160 .BR select (2),
 161 .BR setsid (2),
 162 .BR forkpty (3),
 163 .BR openpty (3),
 164 .BR termios (3),
 165 .BR pts (4),
 166 .BR tty (4),
 167 .BR tty_ioctl (4)