A line discipline (LDISC) is a layer in the terminal subsystem in some Unix-like systems. [1] The terminal subsystem consists of three layers: the upper layer to provide the character device interface, the lower hardware driver to communicate with the hardware or pseudo terminal, and the middle line discipline to implement behavior common to terminal devices.
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying or printing data from, a computer or a computing system. The teletype was an example of an early day hardcopy terminal, and predated the use of a computer screen by decades.
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-like application is one that behaves like the corresponding Unix command or shell. There is no standard for defining the term, and some difference of opinion is possible as to the degree to which a given operating system or application is "Unix-like".
The line discipline glues the low level device driver code with the high level generic interface routines (such as read(2), write(2) and ioctl(2)), and is responsible for implementing the semantics associated with the device. [2] The policy is separated from the device driver so that the same serial hardware driver can be used by devices that require different data handling.
In computing, ioctl is a system call for device-specific input/output operations and other operations which cannot be expressed by regular system calls. It takes a parameter specifying a request code; the effect of a call depends completely on the request code. Request codes are often device-specific. For instance, a CD-ROM device driver which can instruct a physical device to eject a disc would provide an ioctl request code to do that. Device-independent request codes are sometimes used to give userspace access to kernel functions which are only used by core system software or still under development.
For example, the standard line discipline processes the data it receives from the hardware driver and from applications writing to the device according to the requirements of a terminal on a Unix-like system. On input, it handles special characters such as the interrupt character (typically Control-C) and the erase and kill characters (typically backspace or delete, and Control-U, respectively) and, on output, it replaces all the LF characters with a CR/LF sequence.
Control-C is a common computer command. It is generated by pressing the C key while holding down the Ctrl key on most computer keyboards.
Backspace is the keyboard key that originally pushed the typewriter carriage one position backwards, and in modern computer systems moves the display cursor one position backwards, deletes the character at that position, and shifts back the text after that position by one position.
The delete key is a key on most computer keyboards which typically is used to delete either the character ahead of or beneath the cursor, or the currently-selected object. The key is sometimes referred to as the "forward delete" key. This is because the backspace key also deletes characters, but to the left of the cursor. On many keyboards, such as most Apple keyboards, the key with the backspace function is also labelled "delete".
A serial port could also be used for a dial-up Internet connection using a serial modem and PPP. In this case, a PPP line discipline would be used; it would accumulate input data from the serial line into PPP input packets, delivering them to the networking stack rather than to the character device, and would transmit packets delivered to it by the networking stack on the serial line.
In computer networking, Point-to-Point Protocol (PPP) is a data link layer communications protocol used to establish a direct connection between two nodes. It connects two routers directly without any host or any other networking device in between. It can provide connection authentication, transmission encryption, and compression.
Some Unix-like systems use STREAMS to implement line disciplines.
In computer networking, STREAMS is the native framework in Unix System V for implementing character device drivers, network protocols, and inter-process communication. In this framework, a stream is a chain of coroutines that pass messages between a program and a device driver. STREAMS originated in Version 8 Research Unix, as Streams.
