Line Discipline

This crate implements the terminal line discipline logic for Starnix.

Purpose

The line discipline is responsible for the intermediate processing of characters between the terminal device (e.g., a PTY master or a real serial port) and the reading process (e.g., bash). Its responsibilities include:

• Canonical Mode Processing: Buffering input line-by-line, handling backspace (\x08 or \x7f), line kill (^U), etc.
• Echoing: Echoing typed characters back to the output, potentially transforming them (e.g., echoing ^C for SIGINT).
• Signal Generation: Detecting special control characters (like ^C, ^\, ^Z) and generating corresponding signals (SIGINT, SIGQUIT, SIGSTOP).
• Output Processing: Transforming output characters (e.g., converting \n to \r\n).

Notes on Flow Control

While the LineDiscipline struct maintains IXON and IXOFF flags, automatic input flow control (throttling) via IXOFF (sending VSTOP/VSTART when the input buffer fills/empties) is not implemented. This matches the behavior of Linux PTYs, which only apply throttling logic to hardware serial devices, not pseudo-terminals. PTYs rely on producer-consumer backpressure rather than in-band flow control characters.

Architecture

This logic was extracted from starnix_core to decouple it from the kernel structures and allow for easier testing and potential reusability.

Key components:

• LineDiscipline: The main state struct holding the termios configuration, queues, and cursor state.
• Queue: Manages the flow of data, handling wait buffers (raw data) and read buffers (processed data).
• InputBuffer / OutputBuffer traits: Abstractions for the data sources/sinks to avoid direct dependencies on Starnix kernel buffer types.

Testing

Tests are defined in lib.rs and run as part of the line_discipline_tests package.