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pty(7)
NAME
pty - Pseudo terminal driver
SYNOPSIS
pseudo-device pty [count]
options RPTY
DESCRIPTION
The pty driver provides support for a device-pair termed a pseudo terminal.
A pseudo terminal is a pair of character devices, a master device and a
slave device. The slave device provides an interface identical to that
described in the tty(7) reference page. However, whereas all other devices
which provide the interface described in the tty(7) reference page have a
hardware device behind them, the slave device has, instead, another process
manipulating it through the master half of the pseudo terminal. That is,
anything written on the master device is given to the slave device as input
and anything written on the slave device is presented as input on the
master device.
The Tru64 UNIX operating system supports a STREAMS-based and clist-based
implementation of the pty subsystem. The default configuration uses
STREAMS-based ptys. STREAMS-based ptys use the options RPTY line in the
kernel configuration file, while clist-based ptys use the pseudo-device,
pty. By default, 32 pseudo-terminal special device files are created.
Note that you cannot have both types of ptys configured at the same time.
To enhance compatibility, STREAMS-based ptys offers two master pseudo
terminal drivers, the BSD compatible master and the System V compatible
master. The BSD master is a non-STREAMS device which interfaces to the
STREAMS-based slave pty. The System V master is a STREAMS-based device
that also interfaces to the STREAMS-based slave pty. The BSD master is
opened through the cloning device, /dev/ptmx_bsd, and through the master
pty special files, /dev/ptyXX. The System V master is opened only through
the cloning device /dev/ptmx. Currently the BSD master cloning device is
used by the libc routine openpty(3).
You should allocate ptys by using the openpty(3) function, which hides the
pty name space that will change in the next major operating system release.
Increasing the Number of Pseudo Terminals
The operating system can make only 3162 STREAMS-based ptys, but can support
up to 8192. To create additional ptys, do the following:
1. Increase the value of the nptys attribute for the pts subsystem by
using either the dxkerneltuner application or the sysconfig-r*O
command. The value you choose should be the maximum number of ptys
that you want active on your system. See sys_attrs_pts(5) for more
information.
2. Increase the value of the following attributes for the proc subsystem
as follows:
· maxusers -- 4096
· max-thread -- 3*maxusers
· max-task -- 3*maxusers
· max-proc-per-user -- 3*maxusers
Use either the dxkerneltuner application or sysconfig -r command to
make these changes.
3. Create and run (as root) the following script, make_ptys:
#!/bin/sh
#
# This script creates additional System V slave ptys in /dev/pts.
# It assumes that all devices < 3161 have been created
# as part of the BSD/tty device creation.
# If these devices do NOT exist, modify the
# 'start' variable to the first device you want to
# create.
#
start=3162
stop=8191
major=6
cd /dev/pts
i=$start
while :
do
mknod $i c $major $i
chmod 666 $i
[ $i = $stop ] && {
echo "done"
break
}
i=`expr $i + 1`
done
exit 0
To access the additional ptys, you must use the openpty(3) function or open
the System V master cloning device, /dev/ptmx.
Pseudo Terminal ioctl Calls
The following ioctl calls apply only to pseudo terminals:
ISPTM
Returns the dev_t of the master file descriptor. ISPTM is valid only
on the master half of the pseudo terminal, and takes no arguments.
TIOCSTOP
Stops output to a terminal (for example, like entering [Ctrl-S]).
Takes no parameter.
TIOCSTART
Restarts output (stopped by TIOCSTOP or by typing [Ctrl-S]). Takes no
parameter.
TIOCPKT
Enable or disable packet mode. Packet mode is enabled by specifying
(by reference) a nonzero parameter and disabled by specifying (by
reference) a zero parameter. When applied to the master side of a
pseudo terminal, each subsequent read() from the terminal will return
data written on the slave part of the pseudo terminal preceded by a
zero byte (symbolically defined as TIOCPKT_DATA), or a single byte
reflecting control status information. In the latter case, the byte is
an inclusive-OR of zero or more of the bits:
TIOCPKT_FLUSHREAD
Whenever the read queue for the terminal is flushed.
TIOCPKT_FLUSHWRITE
Whenever the write queue for the terminal is flushed.
TIOCPKT_STOP
Whenever output to the terminal is stopped by [Ctrl-S].
TIOCPKT_START
Whenever output to the terminal is restarted.
TIOCPKT_DOSTOP
Whenever t_stopc is [Ctrl-S] and t_startc is [Ctrl-Q].
TIOCPKT_NOSTOP
Whenever the start and stop characters are not [Ctrl-S] and [Ctrl-
Q].
While this mode is in use, the presence of control status
information to be read from the master side may be detected by a
select() for exceptional conditions.
This mode is used by the rlogin and rlogind commands to implement a
remote-echoed, locally [Ctrl-S/Ctrl-Q] flow-controlled remote login
with proper back-flushing of output; it can be used by other
similar programs.
TIOCUCNTL
Enable or disable a mode that allows a small number of simple user
ioctl commands to be passed through the pseudo-terminal, using a
protocol similar to that of TIOCPKT. The TIOCUCNTL and TIOCPKT modes
are mutually exclusive. This mode is enabled from the master side of a
pseudo terminal by specifying (by reference) a nonzero parameter and
disabled by specifying (by reference) a zero parameter. Each
subsequent read() from the master side will return data written on the
slave part of the pseudo terminal preceded by a zero byte, or a single
byte reflecting a user control operation on the slave side. A user
control command consists of a special ioctl operation with no data; the
command is given as UIOCCMD(n), where n is a number in the range 1-255.
The operation value n will be received as a single byte on the next
read() from the master side. The ioctl UIOCCMD(0) is a no-op that may
be used to probe for the existence of this facility. As with TIOCPKT
mode, command operations may be detected with a select() for
exceptional conditions.
TIOCREMOTE
A mode for the master half of a pseudo terminal, independent of
TIOCPKT. This mode causes input to the pseudo terminal to be flow
controlled and not input edited (regardless of the terminal mode).
Each write to the control terminal produces a record boundary for the
process reading the terminal. In normal usage, a write of data is like
the data typed as a line on the terminal; a write of 0 (zero) bytes is
like typing an End-of-File character. The TIOCREMOTE mode can be used
when doing remote line editing in a window manager, or whenever flow
controlled input is required.
UNLKPT
Allows the open of the corresponding slave to succeed when using the
System V master. If UNLKPT is not used in conjunction with the System V
master the open of the corresponding slave will fail with EPERM. This
ioctl takes no arguments.
FILES
/dev/pty[a-c, e-z, A-Z][0-9a-zA-Z]
Master pseudo terminals
/dev/tty[a-c, e-z, A-Z][0-9a-zA-Z]
Slave pseudo terminals
/dev/ptmx
System V master cloning device
/dev/ptmx_bsd
BSD master cloning device
/dev/pts/[0-9999]
SVR4 slave pseudo terminal
SEE ALSO
openpty(3)
System Administration
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