Scroll to navigation

PTS(4) Device Drivers Manual PTS(4)


ptspseudo-terminal driver


The pts 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.
The following ioctl(2) calls apply only to pseudo-terminals:
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(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:
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 VSTOP is ‘^S’ and VSTART 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(2) for exceptional conditions.
This mode is used by rlogin(1) and rlogind(8) 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.
Obtain device unit number, which can be used to generate the filename of the pseudo-terminal slave device. This ioctl(2) should not be used directly. Instead, the ptsname(3) function should be used.
Determine whether the file descriptor is pointing to a pseudo-terminal master device. This ioctl(2) should not be used directly. It is used to implement routines like grantpt(3).
The maximum number of pseudo-terminals is limited to 1000. It is not possible to use more than 1000 pseudo-terminals, as all software which use utmp(5) will not be able to handle pseudo-terminals with number superior to 999.


The files used by this pseudo-terminals implementation are:
Control device, returns a file descriptor to a new master pseudo-terminal when opened. This device should not be opened directly. It's only available for binary compatibility. New devices should only be allocated with posix_openpt(2) and openpty(3).
Pseudo-terminal slave devices.


The following sysctl(8) variables can be used to modify or monitor pts behavior.
Highest pseudo-terminal unit number to be allocated. Because utmp(5) is restricted to an 8-byte line name size, pts will not create any pseudo-terminals with a unit number above 999 by default. After increasing UT_LINESIZE, this variable can be changed to allow more than 1000 pseudo-terminals to be allocated simultaneously.




grantpt(3), posix_openpt(2), ptsname(3), pty(4), tty(4)


A pseudo-terminal driver appeared in 4.2BSD. In FreeBSD 8.0, it was replaced with the pts driver.
August 20, 2008 Debian