PTY(7D)PTY(7D)NAMEpty - pseudo-terminal driver
DESCRIPTION
The pty driver provides support for a pair of devices collectively
known as a pseudo-terminal. The two devices comprising a pseudo-termi‐
nal are known as a controller and a slave. The slave device distin‐
guishes between the B0 baud rate and other baud rates specified in the
c_cflag word of the termios structure, and the CLOCAL flag in that
word. It does not support any of the other termio(7I) device control
functions specified by flags in the c_cflag word of the termios struc‐
ture and by the IGNBRK, IGNPAR, PARMRK, or INPCK flags in the c_iflag
word of the termios structure, as these functions apply only to asyn‐
chronous serial ports. All other termio(7I) functions must be per‐
formed by STREAMS modules pushed atop the driver; when a slave device
is opened, the ldterm(7M) and ttcompat(7M) STREAMS modules are automat‐
ically pushed on top of the stream, providing the standard termio(7I)
interface.
Instead of having a hardware interface and associated hardware that
supports the terminal functions, the functions are implemented by
another process manipulating the controller device of the pseudo-termi‐
nal.
The controller and the slave devices of the pseudo-terminal are tightly
connected. Any data written on the controller device is given to the
slave device as input, as though it had been received from a hardware
interface. Any data written on the slave terminal can be read from the
controller device (rather than being transmitted from a UAR).
By default, 48 pseudo-terminal pairs are configured as follows:
/dev/pty[p-r][0-9a-f] controller devices
/dev/tty[p-r][0-9a-f] slave devices
IOCTLS
The standard set of termio ioctls are supported by the slave device.
None of the bits in the c_cflag word have any effect on the pseudo-ter‐
minal, except that if the baud rate is set to B0, it will appear to the
process on the controller device as if the last process on the slave
device had closed the line; thus, setting the baud rate to B0 has the
effect of ``hanging up'' the pseudo-terminal, just as it has the effect
of ``hanging up'' a real terminal.
There is no notion of ``parity'' on a pseudo-terminal, so none of the
flags in the c_iflag word that control the processing of parity errors
have any effect. Similarly, there is no notion of a ``break'', so none
of the flags that control the processing of breaks, and none of the
ioctls that generate breaks, have any effect.
Input flow control is automatically performed; a process that attempts
to write to the controller device will be blocked if too much uncon‐
sumed data is buffered on the slave device. The input flow control
provided by the IXOFF flag in the c_iflag word is not supported.
The delays specified in the c_oflag word are not supported.
As there are no modems involved in a pseudo-terminal, the ioctls that
return or alter the state of modem control lines are silently ignored.
A few special ioctls are provided on the controller devices of pseudo-
terminals to provide the functionality needed by applications programs
to emulate real hardware interfaces:
TIOCSTOP
The argument is ignored. Output to the pseudo-terminal is
suspended, as if a STOP character had been typed.
TIOCSTART
The argument is ignored. Output to the pseudo-terminal is
restarted, as if a START character had been typed.
TIOCPKT
The argument is a pointer to an int. If the value of the
int is non-zero, packet mode is enabled; if the value of
the int is zero, packet mode is disabled. When a pseudo-
terminal is in packet mode, each subsequent read(2) from
the controller device will return data written on the
slave device 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 using ^S.
TIOCPKT_START
whenever output to the terminal is
restarted.
TIOCPKT_DOSTOP
whenever XON/XOFF flow control is
enabled after being disabled; it is
considered ``enabled'' when the
IXON flag in the c_iflag word is
set, the VSTOP member of the c_cc
array is ^S and the VSTART member
of the c_cc array is ^Q.
TIOCPKT_NOSTOP
whenever XON/XOFF flow control is
disabled after being enabled.
TIOCREMOTE
The argument is a pointer to an int. If the value of the
int is non-zero, remote mode is enabled; if the value of
the int is zero, remote mode is disabled. This mode can
be enabled or disabled independently of packet mode. When
a pseudo-terminal is in remote mode, input to the slave
device of the pseudo-terminal is flow controlled and not
input edited (regardless of the mode the slave side of
the pseudo-terminal). Each write to the controller device
produces a record boundary for the process reading the
slave device. 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 EOF character. Note: this means
that a process writing to a pseudo-terminal controller in
remote mode must keep track of line boundaries, and write
only one line at a time to the controller. If, for exam‐
ple, it were to buffer up several NEWLINE characters and
write them to the controller with one write(), it would
appear to a process reading from the slave as if a single
line containing several NEWLINE characters had been typed
(as if, for example, a user had typed the LNEXT character
before typing all but the last of those NEWLINE charac‐
ters). Remote mode can be used when doing remote line
editing in a window manager, or whenever flow controlled
input is required.
EXAMPLES
#include <fcntl.h>
#include <sys/termios.h>
int fdm fds;
fdm = open("/dev/ptyp0, O_RDWR); /* open master */
fds = open("/dev/ttyp0, O_RDWR); /* open slave */
FILES
/dev/pty[p-z][0-9a-f]
pseudo-terminal controller devices
/dev/tty[p-z][0-9a-f]
pseudo-terminal slave devices
SEE ALSOrlogin(1), rlogind(1M), ldterm(7M), termio(7I), ttcompat(7M),
NOTES
It is apparently not possible to send an EOT by writing zero bytes in
TIOCREMOTE mode.
Aug 8, 1994 PTY(7D)
