pty - pseudo terminal driver
pseudo-device pty [count]
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 to a
process an interface
identical to that described in tty(4). However,
whereas all other
devices which provide the interface described in tty(4)
have a hardware
device of some sort 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.
In configuring, if an optional count is given in the specification, space
for that number of pseudo terminal pairs is preallocated.
If the count
is missing or is less than 2, a default count of 8 is used.
This is not
a hard limit--space for additional pseudo terminal pairs is
allocated on
demand up to the limit imposed by the kern.tty.maxptys
sysctl(8) (992 by
default).
The following ioctl(2) calls apply only to pseudo terminals:
TIOCSTOP Stops output to a terminal (e.g., like typing
`^S'). Takes
no parameter.
TIOCSTART Restarts output (stopped by TIOCSTOP or by typing `^S').
Takes no parameter.
TIOCPKT Enable/disable packet mode. Packet mode is enabled by specifying
(by reference) a non-zero parameter and
disabled by
specifying (by reference) a zero parameter.
When applied to
the master side of a pseudo terminal, each subsequent read(2)
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 a la `^S'.
TIOCPKT_START whenever output to the terminal is
restarted.
TIOCPKT_DOSTOP whenever t_stopc is `^S' and
t_startc is
`^Q'.
TIOCPKT_NOSTOP whenever the start and stop
characters
are not `^S/^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(2) for exceptional
conditions.
This mode is used by rlogin
and rlogind
to implement a remoteechoed, locally
`^S/^Q' flow-controlled remote login with
proper back-flushing of output; it can be
used by other similar programs.
TIOCPKT_IOCTL When this bit is set, the
slave has
changed the termios(4)
structure (TTY
state), and the remainder of
the data
read from the master side of
the pty is a
copy of the new termios(4)
structure.
This is used by telnetd(8)
to implement
TELNET "line mode" - it allows the
telnetd(8) to detect tty(4)
state changes
by the slave, and negotiate
the appropriate
TELNET protocol equivalents with the
remote peer.
TIOCUCNTL Enable/disable a mode that allows a small number
of simple
user ioctl(2) 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(2) 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(2) 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(2) from the master side. The ioctl(2)
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(2) 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 bytes is like
typing an
end-of-file character. TIOCREMOTE can be used
when doing remote
line editing in a window manager, or whenever flow controlled
input is required.
The standard way to allocate pty devices is through
openpty(3), a function
which internally uses a PTMGET ioctl(2) call on the
/dev/ptm device.
The PTMGET command allocates a free pseudo terminal, changes
its ownership
to the caller, revokes the access privileges for all
previous users,
opens the file descriptors for the master and slave devices
and returns
them to the caller in struct ptmget.
struct ptmget {
int cfd;
int sfd;
char cn[16];
char sn[16];
};
The cfd and sfd fields are the file descriptors for the controlling and
slave terminals. The cn and sn fields are the file names of
the controlling
and slave devices.
/dev/pty[p-zP-T][0-9a-f] master pseudo terminals
/dev/tty[p-zP-T][0-9a-f] slave pseudo terminals
/dev/ptm pseudo terminal management device
openpty(3), tty(4), sysctl(8)
The pty driver appeared in 4.2BSD. The /dev/ptm device was
added in
OpenBSD 3.5.
The ptm device will only work on systems where the /dev directory has
been properly populated with pty(4) device nodes following
the naming
convention used in OpenBSD. Since ptm impersonates the super
user for
some operations it needs to perform to complete the allocation of a pseudo
terminal, the /dev directory must also be writeable by
the super user.
OpenBSD 3.6 November 30, 1993
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