257 lines
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257 lines
7.3 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of PTY</TITLE>
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</HEAD><BODY>
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<H1>PTY</H1>
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Section: Linux Programmer's Manual (7)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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pty - pseudoterminal interfaces
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<A NAME="lbAC"> </A>
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<H2>DESCRIPTION</H2>
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A pseudoterminal (sometimes abbreviated "pty")
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is a pair of virtual character devices that
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provide a bidirectional communication channel.
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One end of the channel is called the
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<I>master</I>;
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the other end is called the
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<I>slave</I>.
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The slave end of the pseudoterminal provides an interface
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that behaves exactly like a classical terminal.
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A process that expects to be connected to a terminal,
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can open the slave end of a pseudoterminal and
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then be driven by a program that has opened the master end.
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Anything that is written on the master end is provided to the process
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on the slave end as though it was input typed on a terminal.
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For example, writing the interrupt character (usually control-C)
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to the master device would cause an interrupt signal
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(<B>SIGINT</B>)
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to be generated for the foreground process group
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that is connected to the slave.
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Conversely, anything that is written to the slave end of the
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pseudoterminal can be read by the process that is connected to
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the master end.
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Pseudoterminals are used by applications such as network login services
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(<B><A HREF="/cgi-bin/man/man2html?1+ssh">ssh</A></B>(1), <B><A HREF="/cgi-bin/man/man2html?1+rlogin">rlogin</A></B>(1), <B><A HREF="/cgi-bin/man/man2html?1+telnet">telnet</A></B>(1)),
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terminal emulators such as
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<B><A HREF="/cgi-bin/man/man2html?1+xterm">xterm</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?1+script">script</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?1+screen">screen</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?1+tmux">tmux</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?1+unbuffer">unbuffer</A></B>(1),
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and
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<B><A HREF="/cgi-bin/man/man2html?1+expect">expect</A></B>(1).
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<P>
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Data flow between master and slave is handled asynchronously,
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much like data flow with a physical terminal.
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Data written to the slave will be available at the master promptly,
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but may not be available immediately.
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Similarly, there may be a small processing delay between
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a write to the master, and the effect being visible at the slave.
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<P>
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Historically, two pseudoterminal APIs have evolved: BSD and System V.
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SUSv1 standardized a pseudoterminal API based on the System V API,
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and this API should be employed in all new programs that use
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pseudoterminals.
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<P>
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Linux provides both BSD-style and (standardized) System V-style
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pseudoterminals.
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System V-style terminals are commonly called UNIX 98 pseudoterminals
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on Linux systems.
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Since kernel 2.6.4, BSD-style pseudoterminals are considered deprecated
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(they can be disabled when configuring the kernel);
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UNIX 98 pseudoterminals should be used in new applications.
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<A NAME="lbAD"> </A>
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<H3>UNIX 98 pseudoterminals</H3>
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An unused UNIX 98 pseudoterminal master is opened by calling
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<B><A HREF="/cgi-bin/man/man2html?3+posix_openpt">posix_openpt</A></B>(3).
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(This function opens the master clone device,
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<I>/dev/ptmx</I>;
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see
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<B><A HREF="/cgi-bin/man/man2html?4+pts">pts</A></B>(4).)
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After performing any program-specific initializations,
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changing the ownership and permissions of the slave device using
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<B><A HREF="/cgi-bin/man/man2html?3+grantpt">grantpt</A></B>(3),
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and unlocking the slave using
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<B><A HREF="/cgi-bin/man/man2html?3+unlockpt">unlockpt</A></B>(3)),
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the corresponding slave device can be opened by passing
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the name returned by
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<B><A HREF="/cgi-bin/man/man2html?3+ptsname">ptsname</A></B>(3)
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in a call to
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2).
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<P>
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The Linux kernel imposes a limit on the number of available
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UNIX 98 pseudoterminals.
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In kernels up to and including 2.6.3, this limit is configured
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at kernel compilation time
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(<B>CONFIG_UNIX98_PTYS</B>),
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and the permitted number of pseudoterminals can be up to 2048,
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with a default setting of 256.
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Since kernel 2.6.4, the limit is dynamically adjustable via
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<I>/proc/sys/kernel/pty/max</I>,
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and a corresponding file,
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<I>/proc/sys/kernel/pty/nr</I>,
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indicates how many pseudoterminals are currently in use.
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For further details on these two files, see
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<B><A HREF="/cgi-bin/man/man2html?5+proc">proc</A></B>(5).
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<A NAME="lbAE"> </A>
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<H3>BSD pseudoterminals</H3>
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BSD-style pseudoterminals are provided as precreated pairs, with
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names of the form
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<I>/dev/ptyXY</I>
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(master) and
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<I>/dev/ttyXY</I>
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(slave),
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where X is a letter from the 16-character set [p-za-e],
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and Y is a letter from the 16-character set [0-9a-f].
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(The precise range of letters in these two sets varies across UNIX
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implementations.)
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For example,
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<I>/dev/ptyp1</I>
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and
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<I>/dev/ttyp1</I>
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constitute a BSD pseudoterminal pair.
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A process finds an unused pseudoterminal pair by trying to
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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each pseudoterminal master until an open succeeds.
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The corresponding pseudoterminal slave (substitute "tty"
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for "pty" in the name of the master) can then be opened.
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<A NAME="lbAF"> </A>
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<H2>FILES</H2>
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<DL COMPACT>
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<DT id="1"><I>/dev/ptmx</I>
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<DD>
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UNIX 98 master clone device
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<DT id="2"><I>/dev/pts/*</I>
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<DD>
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UNIX 98 slave devices
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<DT id="3"><I>/dev/pty[p-za-e][0-9a-f]</I>
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<DD>
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BSD master devices
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<DT id="4"><I>/dev/tty[p-za-e][0-9a-f]</I>
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<DD>
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BSD slave devices
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</DL>
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<A NAME="lbAG"> </A>
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<H2>NOTES</H2>
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A description of the
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<B>TIOCPKT</B>
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl">ioctl</A></B>(2),
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which controls packet mode operation, can be found in
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl_tty">ioctl_tty</A></B>(2).
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<P>
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The BSD
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl">ioctl</A></B>(2)
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operations
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<B>TIOCSTOP</B>,
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<B>TIOCSTART</B>,
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<B>TIOCUCNTL</B>,
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and
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<B>TIOCREMOTE</B>
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have not been implemented under Linux.
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<A NAME="lbAH"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl_tty">ioctl_tty</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+select">select</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+setsid">setsid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+forkpty">forkpty</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+openpty">openpty</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+termios">termios</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?4+pts">pts</A></B>(4),
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<B><A HREF="/cgi-bin/man/man2html?4+tty">tty</A></B>(4)
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<A NAME="lbAI"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="5"><A HREF="#lbAB">NAME</A><DD>
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<DT id="6"><A HREF="#lbAC">DESCRIPTION</A><DD>
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<DL>
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<DT id="7"><A HREF="#lbAD">UNIX 98 pseudoterminals</A><DD>
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<DT id="8"><A HREF="#lbAE">BSD pseudoterminals</A><DD>
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</DL>
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<DT id="9"><A HREF="#lbAF">FILES</A><DD>
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<DT id="10"><A HREF="#lbAG">NOTES</A><DD>
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<DT id="11"><A HREF="#lbAH">SEE ALSO</A><DD>
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<DT id="12"><A HREF="#lbAI">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:06:09 GMT, March 31, 2021
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</BODY>
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</HTML>
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