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c11b1abf | 1 | .\" Copyright (C) 2005 Michael Kerrisk <mtk.manpages@gmail.com> |
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1ae9f937 | 24 | .\" |
5722c835 | 25 | .TH PTY 7 2015-07-23 "Linux" "Linux Programmer's Manual" |
1ae9f937 | 26 | .SH NAME |
b218b023 | 27 | pty \- pseudoterminal interfaces |
1ae9f937 | 28 | .SH DESCRIPTION |
b218b023 | 29 | A pseudoterminal (sometimes abbreviated "pty") |
614e4812 | 30 | is a pair of virtual character devices that |
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31 | provide a bidirectional communication channel. |
32 | One end of the channel is called the | |
1ae9f937 | 33 | .IR master ; |
c13182ef | 34 | the other end is called the |
1ae9f937 | 35 | .IR slave . |
b218b023 | 36 | The slave end of the pseudoterminal provides an interface |
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37 | that behaves exactly like a classical terminal. |
38 | A process that expects to be connected to a terminal, | |
b218b023 | 39 | can open the slave end of a pseudoterminal and |
1ae9f937 | 40 | then be driven by a program that has opened the master end. |
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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. | |
1ae9f937 | 43 | For example, writing the interrupt character (usually control-C) |
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44 | to the master device would cause an interrupt signal |
45 | .RB ( SIGINT ) | |
c13182ef | 46 | to be generated for the foreground process group |
1ae9f937 | 47 | that is connected to the slave. |
c13182ef | 48 | Conversely, anything that is written to the slave end of the |
b218b023 | 49 | pseudoterminal can be read by the process that is connected to |
1ae9f937 | 50 | the master end. |
40a1b471 | 51 | Pseudoterminals are used by applications such as network login services |
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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 | ||
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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 | |
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64 | a write to the master, and the effect being visible at the slave. |
65 | ||
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66 | Historically, two pseudoterminal APIs have evolved: BSD and System V. |
67 | SUSv1 standardized a pseudoterminal API based on the System V API, | |
c13182ef | 68 | and this API should be employed in all new programs that use |
b218b023 | 69 | pseudoterminals. |
1ae9f937 | 70 | |
d9bfdb9c | 71 | Linux provides both BSD-style and (standardized) System V-style |
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72 | pseudoterminals. |
73 | System V-style terminals are commonly called UNIX 98 pseudoterminals | |
1ae9f937 | 74 | on Linux systems. |
b218b023 | 75 | Since kernel 2.6.4, BSD-style pseudoterminals are considered deprecated |
c13182ef | 76 | (they can be disabled when configuring the kernel); |
b218b023 | 77 | UNIX 98 pseudoterminals should be used in new applications. |
73d8cece | 78 | .SS UNIX 98 pseudoterminals |
b218b023 | 79 | An unused UNIX 98 pseudoterminal master is opened by calling |
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80 | .BR posix_openpt (3). |
81 | (This function opens the master clone device, | |
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82 | .IR /dev/ptmx ; |
83 | see | |
5872bd2f | 84 | .BR pts (4).) |
d9bfdb9c | 85 | After performing any program-specific initializations, |
c13182ef | 86 | changing the ownership and permissions of the slave device using |
1ae9f937 | 87 | .BR grantpt (3), |
c13182ef | 88 | and unlocking the slave using |
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89 | .BR unlockpt (3)), |
90 | the corresponding slave device can be opened by passing | |
c13182ef | 91 | the name returned by |
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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 | |
b218b023 | 97 | UNIX 98 pseudoterminals. |
1ae9f937 | 98 | In kernels up to and including 2.6.3, this limit is configured |
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99 | at kernel compilation time |
100 | .RB ( CONFIG_UNIX98_PTYS ), | |
b218b023 | 101 | and the permitted number of pseudoterminals can be up to 2048, |
1ae9f937 | 102 | with a default setting of 256. |
c13182ef | 103 | Since kernel 2.6.4, the limit is dynamically adjustable via |
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104 | .IR /proc/sys/kernel/pty/max , |
105 | and a corresponding file, | |
106 | .IR /proc/sys/kernel/pty/nr , | |
b218b023 | 107 | indicates how many pseudoterminals are currently in use. |
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108 | For further details on these two files, see |
109 | .BR proc (5). | |
73d8cece | 110 | .SS BSD pseudoterminals |
b218b023 | 111 | BSD-style pseudoterminals are provided as precreated pairs, with |
c13182ef | 112 | names of the form |
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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]. | |
008f1ecc | 119 | (The precise range of letters in these two sets varies across UNIX |
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120 | implementations.) |
121 | For example, | |
122 | .I /dev/ptyp1 | |
123 | and | |
124 | .I /dev/ttyp1 | |
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125 | constitute a BSD pseudoterminal pair. |
126 | A process finds an unused pseudoterminal pair by trying to | |
1ae9f937 | 127 | .BR open (2) |
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128 | each pseudoterminal master until an open succeeds. |
129 | The corresponding pseudoterminal slave (substitute "tty" | |
1ae9f937 | 130 | for "pty" in the name of the master) can then be opened. |
47297adb | 131 | .SH FILES |
c13182ef | 132 | .I /dev/ptmx |
008f1ecc | 133 | (UNIX 98 master clone device) |
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134 | .br |
135 | .I /dev/pts/* | |
008f1ecc | 136 | (UNIX 98 slave devices) |
1ae9f937 | 137 | .br |
c13182ef | 138 | .I /dev/pty[p-za-e][0-9a-f] |
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139 | (BSD master devices) |
140 | .br | |
c13182ef | 141 | .I /dev/tty[p-za-e][0-9a-f] |
1ae9f937 | 142 | (BSD slave devices) |
47297adb | 143 | .SH NOTES |
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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 | ||
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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. | |
47297adb | 159 | .SH SEE ALSO |
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160 | .BR select (2), |
161 | .BR setsid (2), | |
162 | .BR forkpty (3), | |
163 | .BR openpty (3), | |
164 | .BR termios (3), | |
5872bd2f | 165 | .BR pts (4), |
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166 | .BR tty (4), |
167 | .BR tty_ioctl (4) |