pty - pseudo-terminal driver
The pty driver provides support for a pair of devices collectively known as a pseudo-terminal. The two devices comprising a pseudo-terminal are known as a controller and a slave. The slave device distinguishes 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 structure and by the IGNBRK, IGNPAR, PARMRK, or INPCK flags in the c_iflag word of the termios structure, as these functions apply only to asynchronous serial ports. All other termio(7I) functions must be performed by STREAMS modules pushed atop the driver; when a slave device is opened, the ldterm(7M) and ttcompat(7M) STREAMS modules are automatically 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-terminal.
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
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-terminal, 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 unconsumed 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:
#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 */
rlogin(1), rlogind(1M), ldterm(7M), termio(7I), ttcompat(7M),
It is apparently not possible to send an EOT by writing zero bytes in TIOCREMOTE mode.
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Created 1996-2022 by Maxim Chirkov
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