Project 3: termios Mode Experimenter

An interactive tool that lets you toggle termios flags in real-time and see how terminal behavior changes (echo, canonical mode, signal characters, etc.).

Quick Reference

Attribute Value
Primary Language C
Alternative Languages Rust, Python
Difficulty Level 1: Beginner (The Tinkerer)
Time Estimate 3-5 days
Knowledge Area termios / TTY Configuration
Tooling termios Explorer
Prerequisites Basic C

What You Will Build

An interactive tool that lets you toggle termios flags in real-time and see how terminal behavior changes (echo, canonical mode, signal characters, etc.).

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • termios structure → Understanding c_iflag, c_oflag, c_cflag, c_lflag
  • Raw mode setup → The exact flags needed for character-at-a-time input
  • Signal characters → How Ctrl+C becomes SIGINT
  • Restoring state → Properly cleaning up on exit
  • Special characters → VMIN, VTIME for non-blocking reads

Key Concepts

Real-World Outcome

$ ./termios_explorer

=== termios Explorer ===

Current mode: CANONICAL (cooked)
  ECHO: ON    ICANON: ON    ISIG: ON    ICRNL: ON

Commands:
  [e] Toggle ECHO      [c] Toggle ICANON (canonical)
  [s] Toggle ISIG      [r] Enter raw mode
  [q] Quit (restore terminal)

Current: ECHO=ON, type something:
> hello    (you see what you type)

[e] Toggling ECHO OFF...

Current: ECHO=OFF, type something:
>          (you type but see nothing... eerie!)

[c] Toggling ICANON OFF (non-canonical mode)...

Now every keypress is immediate:
  Pressed: 'h' (0x68)
  Pressed: 'e' (0x65)
  Pressed: 'l' (0x6c)
  Pressed: Ctrl+C (0x03) -> SIGINT would be sent if ISIG=ON

[r] Entering full raw mode...
  All processing disabled.
  Pressed: UP ARROW = 0x1b 0x5b 0x41 (escape sequence!)
  Pressed: Ctrl+C = 0x03 (no signal, just a byte!)

[q] Restoring terminal and exiting...
$

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: TERMINAL_EMULATOR_DEEP_DIVE_PROJECTS.md
  • “The Linux Programming Interface” by Michael Kerrisk