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PTY(4) Device Drivers Manual PTY(4)

pty, ptm — pseudo terminal driver

pseudo-device pty [count]

The pty driver provides support for a device-pair termed a . A pseudo terminal is a pair of character devices, a device and a 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 of 992.

The following ioctl(2) calls apply only to pseudo terminals and may only be applied to the pty master:

int *on
If on points to a non-zero integer, enable external processing. Otherwise, disable external processing.

While external processing is enabled, input line editing, character echo, and mapping of control characters to signals are disabled regardless of the terminal's termios(4) settings.

void
Stops output to a terminal (e.g., like typing ‘^S’).
void
Restarts output (stopped by TIOCSTOP or by typing ‘^S’).
int *on
If on points to a non-zero integer, enable packet mode. Otherwise, disable packet mode.

While packet mode is enabled, each subsequent read(2) from the pty master will either return data written to the pty slave 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 is ‘^S’ and is ‘^Q’.
whenever the start and stop characters are not ‘^S/^Q’.
whenever the terminal's termios(4) settings change while external processing is enabled.

Additionally, when the TIOCPKT_IOCTL bit is set, the remainder of the data read from the pty master is a copy of the new termios(4) structure.

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.

int *on
If on points to a non-zero integer, enable 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. 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.

While this mode is in use, any of the TIOCSBRK and TIOCCBRK ioctl requests issued on the slave part of the pseudo terminal will be translated to a TIOCUCNTL_SBRK or TIOCUCNTL_CBRK user command on the master side.

As with TIOCPKT mode, command operations may be detected with a select(2) for exceptional conditions.

int *on
If on points to a non-zero integer, enable 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 controlling 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-zA-Z]
master pseudo terminals
/dev/tty[p-zP-T][0-9a-zA-Z]
slave pseudo terminals
/dev/ptm
pseudo terminal management device

openpty(3), tty(4)

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 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.

October 13, 2022 OpenBSD-7.6