PTY(4) OpenBSD Programmer's Manual PTY(4)NAME
pty - pseudo terminal driver
SYNOPSIS
pseudo-device pty [count]
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 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 remote-echoed, 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 telnet daemons to
implement TELNET "line mode", allowing
them 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.
FILES
/dev/pty[p-zP-T][0-9a-zA-Z] master pseudo terminals
/dev/tty[p-zP-T][0-9a-zA-Z] slave pseudo terminals
/dev/ptm pseudo terminal management device
SEE ALSOopenpty(3), tty(4), sysctl(8)HISTORY
The pty driver appeared in 4.2BSD. The /dev/ptm device was added in
OpenBSD 3.5.
CAVEATS
The ptm device will only work on systems where the /dev directory has
been properly populated with pty 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 4.9 May 31, 2007 OpenBSD 4.9
