pty - Pseudo terminal driver
pseudo-device pty [count] options RPTY
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. STREAMSbased
ptys use the options RPTY line in the kernel configuration
file, while clist-based ptys use the pseudodevice,
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 [Toc] [Back]
The operating system can make only 3162 STREAMS-based
ptys, but can support up to 8192. To create additional
ptys, do the following: 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. Increase the value of the following
attributes for the proc subsystem as follows: maxusers --
4096 max-thread -- 3*maxusers max-task -- 3*maxusers maxproc-per-user
-- 3*maxusers
Use either the dxkerneltuner application or
sysconfig -r command to make these changes. 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 [Toc] [Back]
The following ioctl calls apply only to pseudo terminals:
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. Stops output to a terminal (for example,
like entering [Ctrl-S]). Takes no parameter.
Restarts output (stopped by TIOCSTOP or by typing [CtrlS]).
Takes no parameter. 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: Whenever the
read queue for the terminal is flushed. Whenever the
write queue for the terminal is flushed. Whenever output
to the terminal is stopped by [Ctrl-S]. Whenever output
to the terminal is restarted. Whenever t_stopc is [CtrlS]
and t_startc is [Ctrl-Q]. 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 [CtrlS/Ctrl-Q]
flow-controlled remote login with proper
back-flushing of output; it can be used by other
similar programs. 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. 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-ofFile
character. The TIOCREMOTE mode can be used
when doing remote line editing in a window manager,
or whenever flow controlled input is required.
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.
Master pseudo terminals Slave pseudo terminals System V
master cloning device BSD master cloning device SVR4 slave
pseudo terminal
openpty(3)
System Administration
pty(7)
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