LangLock

A lightweight, native Windows background utility that intercepts the Caps Lock key to switch the system input language.

Why LangLock?

Traditional keyboard remapping solutions that emulate virtual keystrokes (via SendInput) can be problematic:

• Anti-cheat detection — many games detect and block keystroke emulation
• Modifier key conflicts — ghost key presses and inconsistent behavior during fast typing

LangLock takes a different approach: it sends WM_INPUTLANGCHANGEREQUEST directly to the window instead of emulating keystrokes. This is the same mechanism Windows uses internally when you press Alt+Shift.

Anti-Cheat Considerations

LangLock uses a low-level keyboard hook (WH_KEYBOARD_LL) — a standard Windows API also used by many legitimate programs (input methods, screenshot tools, accessibility software). While hooks are generally safe, some aggressive anti-cheat systems may flag any software using them. LangLock minimizes risk by:

• Not emulating keystrokes — no SendInput calls
• Ignoring injected events — filters out LLKHF_INJECTED flag
• Using standard Windows APIs — only PostMessageW with documented messages

Comparison with Alternatives

Feature	LangLock	capslang	CapsWitch
Switch method	WM_INPUTLANGCHANGEREQUEST	WM_INPUTLANGCHANGEREQUEST	SendInput (Win+Space)
Keystroke emulation	No	No	Yes
Shift+Caps Lock = Caps	✓ (optional)	✓	✓ (via SendInput)
Tray icon	✓	✗	✓
Works with admin apps	✓	✓	✗
Filter injected keys	✓	✗	✗
Installer	Inno Setup	PowerShell script	MSIX (self-signed)
License	MIT	GPLv3	MIT

How It Works

1. Low-level keyboard hook (WH_KEYBOARD_LL) intercepts Caps Lock
2. Swallows the keypress completely (returns 1 from hook)
3. Sends WM_INPUTLANGCHANGEREQUEST directly to the foreground window
4. No virtual keystrokes are ever generated

This approach minimizes anti-cheat risk because it never simulates keyboard input — see Anti-Cheat Considerations for details.

Features

• System tray icon with context menu
• Run on startup option (uses Task Scheduler for elevated privileges)
• Shift+Caps Lock for regular Caps Lock behavior (optional, toggle in tray menu)
• Hide tray icon to minimize clutter (relaunch to restore)
• Single instance enforcement with IPC
• Zero dependencies — single portable executable

Limitations

Login Screen

LangLock does not work on the Windows login screen (password entry). This is a deliberate limitation:

• The login screen runs in a separate Windows session (Session 0)
• Keyboard hooks are session-specific and cannot cross session boundaries
• Implementing this would require a kernel-mode driver or Windows Service with complex cross-session communication

Workaround: Use the standard Windows language switcher (Win+Space or Alt+Shift) on the login screen.

Installation

Installer (Recommended)

Download the latest langlock-setup-*.exe from Releases.

The installer:

• Requires administrator privileges
• Allows choosing installation folder (default: C:\Program Files\LangLock)
• Optionally adds to Windows startup
• Optionally enables portable mode (settings stored in installation folder)
• Creates Start Menu shortcuts
• Removes the scheduled task on uninstall

Portable

Download langlock.exe from Releases and run it directly.

Usage

1. Run LangLock
2. Press Caps Lock to switch input language
3. Right-click the tray icon for options:
   • Run on startup — Enables auto-start on login
   • Shift+Caps Lock = regular Caps Lock — When enabled, pressing Shift+Caps Lock toggles Caps Lock as usual
   • Hide tray icon — Hides the tray icon (relaunch to restore)
   • Exit — Closes LangLock

Building from Source

Prerequisites

• Rust (stable, MSVC toolchain)
• Windows 10/11
• Inno Setup 6 (for building installer)

Build

git clone https://github.com/risenxxx/langlock.git
cd langlock
cargo build --release

The binary will be at target/release/langlock.exe.

Build Installer

# Requires Inno Setup 6 installed
iscc /DMyAppVersion="0.2.0" installer/langlock.iss

Technical Details

Why Task Scheduler for Startup?

LangLock uses Windows Task Scheduler instead of the registry Run key because:

1. UIPI (User Interface Privilege Isolation) — Low-level keyboard hooks need elevated privileges to intercept keystrokes in admin windows and games
2. UAC-free startup — Tasks with "Run with highest privileges" don't trigger UAC prompts on every boot
3. Works with games — Games running as Administrator receive the hook properly

Hook Architecture

Caps Lock pressed
       ↓
WH_KEYBOARD_LL hook
       ↓
Check: VK_CAPITAL && not injected?
       ↓ Yes
Check: Shift held && feature enabled?
       ↓ Yes → Pass through (normal Caps Lock)
       ↓ No
PostMessage(hwnd, WM_INPUTLANGCHANGEREQUEST, INPUTLANGCHANGE_FORWARD, 0)
       ↓
Return 1 (swallow keypress)

License

MIT License — see LICENSE for details.

Contributing

Contributions are welcome! Please feel free to submit issues and pull requests.