PTY(4) Kernel Interfaces Manual PTY(4)
pty - pseudo terminal driver
NPTY ptys # pseudo-terminals, in groups of 8
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 processes 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, ptys specifies the number of pseudo terminal pairs are configured.
The following ioctl calls apply only to pseudo terminals:
Stops output to a terminal (e.g. like typing ^S). Takes no parameter.
Restarts output (stopped by TIOCSTOP or by typing ^S). Takes no parameter.
Enable/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 a la ^S.
whenever output to the terminal is restarted.
whenever t_stopc is ^S and t_startc is ^Q.
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
for exceptional conditions.
This mode is used by rlogin(1C) and rlogind(8C) 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.
Enable/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 parame-
ter. 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 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.
/dev/pty[p-r][0-9a-f] master pseudo terminals
/dev/tty[p-r][0-9a-f] slave pseudo terminals
4.2 Berkeley Distribution May 19, 1986 PTY(4)