Unity Single-Instance Fix

Fixes old Unity games (pre-5.4.0) that fail with "Player is already running" on modern Linux systems.

The Problem

Unity 4.x detects duplicate instances by scanning /proc and comparing executable paths. Chromium/CEF/Electron processes (Signal Desktop, VS Code/Cursor, Slack, Discord, Steam since mid-2023) list /proc/self/exe in their cmdline. When Unity reads this, it resolves it from its own perspective, finds itself, and refuses to start.

Usage

# 1. Build the fix libraries
./build.sh

# 2. Apply to any game directory
./apply.sh /path/to/game/directory

# Or manually:
LD_PRELOAD=/path/to/unity-fix-32.so ./GameBinary

Confirmed Working Games

• Shadowrun Returns / Dragonfall / Hong Kong
• Infinifactory
• Motorsport Manager
• Creeper World 3: Arc Eternal
• Poly Bridge
• The Novelist
• Beatbuddy: Tale of the Guardians
• Agatha Christie – The ABC Murders
• And many other Unity 4.x titles

Steam Launch Options

Set this in Properties > Launch Options:

LD_PRELOAD="/path/to/unity-fix-32.so:${LD_PRELOAD}" %command%

Technical Details

The LD_PRELOAD library intercepts:

• fopen() / fopen64() — opening /proc/* files
• fread() — replacing /proc/self/exe with a fake path
• open() / open64() — direct file I/O to /proc
• read() — replacing /proc/self/exe in read buffers
• readlink() — preventing symlink resolution trick
• close() — avoiding stale fd tracking

Based on analysis by janinko and Eike Sauer: https://ein-eike.de/2025/06/05/fix-for-old-unity-games-for-linux-not-starting-anymore/