DOOM SDL2 (GPL-2.0-only) — Modernized & Enhanced

Modernized, single-target build of the original Linux DOOM 1.10 sources, plus ongoing enhancements. Video, audio, input, and networking run on SDL2 and SDL2_net; music playback ships with bundled libADLMIDI/libOPNMIDI via CMake FetchContent. The top-level CMakeLists.txt is the source of truth.

If you remember the 90s Linux port: this keeps the gameplay, but drops the period-correct plumbing. No X11 renderer fork, no external sndserver, no BSD sockets sprawl. It is DOOM, but built and run like a modern Linux game.

Features

• SDL2 renderer with logical sizing for aspect preservation (4:3 default, -widescreen, -stretch, or -aspect 4:3|16:9|stretch) and Alt+Enter fullscreen toggle.
• SDL2 audio backend (11025 Hz, 16-bit stereo) with in-process mixer.
• Music backends: ADLMIDI (OPL3) + OPNMIDI (OPN2), fetched at configure time; ALSA sequencer fallback when available.
• Multiplayer: in-game Host/Join menu with lobby roster, LAN discovery, and direct connect (host[:port] + IPv6 [addr]:port).
• Secure net transport: all game packets carry a BLAKE2s MAC keyed by a per-session 128-bit key; content hashing gates mismatched WAD sets.
• Single linuxdoom target built by CMake; no legacy sndserver, SDL1, or X11 backends.

What Changed From The Original Linux 1.10 Port

• Rendering: SDL2 + logical sizing for clean aspect preservation instead of fixed X11-era assumptions.
• Audio: no helper process; SDL2 audio mixer runs in-process.
• Networking: SDL2_net (with a stub fallback); lobby bootstrap, LAN discovery, and basic mismatch defenses.
• Build: one CMake build that fetches optional music deps and produces a single modern binary.
• Enhancements: gameplay/UI/network features beyond the original port (this is no longer “just a port”).

Recommended Repo Layout (Vanilla vs Enhanced)

If you want to keep a strict “original port” tree alongside the enhanced one, use one of these workflows:

• Two branches + worktrees (recommended): keep an unmodified vanilla branch and develop on enhanced. Use git worktree so both trees are checked out at once.
• Two directories in one repo: keep an archived vanilla/ source snapshot (not built), and keep the enhanced tree as the CMake build target.

See UPSTREAM.md for a concrete suggested workflow and naming.

Requirements

• CMake ≥ 3.16
• C compiler (C11) and C++ compiler (C++14) for the music backends
• SDL2 development files
• SDL2_net development files (optional; a stub is built if absent)
• ALSA headers (optional; enables sequencer fallback for music)

Build

cmake -S . -B build
cmake --build build

Artifacts land in build/bin/ (linuxdoom, net_harness).

Run

./run-linuxdoom.sh /path/to/DOOM.WAD [extra DOOM args]
# or
DOOMWADDIR=/path/to/wads ./build/bin/linuxdoom

Convenience env vars: IWAD_PATH, PWAD_PATH for the wrapper.

Common flags:

• -aspect 4:3|16:9|stretch (explicit aspect selection)
• -widescreen (alias for 16:9), -stretch (fill without preserving aspect)
• -fullscreen / -windowed
• Alt+Enter toggles fullscreen at runtime.
• Music backend selection:
  • -music_backend adlmidi|opnmidi|alsa|off
  • -midi adlmidi|opnmidi|alsa|off (alias)
  • -nomusic (disable music)
• Multiplayer:
  • In-game: Main Menu -> Multiplayer (Host/Join/LAN/Direct Connect)
  • CLI bootstrap:
    • Host: -host N (optionally -port P)
    • Join: -connect host[:port]
  • Optional: -player <name> to set your lobby roster name.

Saves/config: by default ~/.doomrc and .dsg save files in the working directory.

Development workflow

• Use the enhanced branch for active work; keep vanilla as a clean baseline.
• Prefer worktrees (git worktree) for parallel checkouts. Use separate build dirs per worktree to avoid CMake cache conflicts (e.g. build-enhanced/).
• After code changes, re-run the build (cmake -S . -B build then cmake --build build).
• There are no automated tests; sanity-check with build/bin/net_harness and a quick run of linuxdoom.

Project status

Core SDL2 modernization is complete. See STATUS.md for current state and TODO.md for the roadmap. AGENTS.md documents repo-specific guidance.

License

• Core source: GNU GPL-2.0-only (see LICENSE.TXT).
• Third-party components and their licenses are listed in THIRD_PARTY_NOTICES.md (SDL2/SDL2_net under zlib, libADLMIDI under LGPL/GPL/MIT, libOPNMIDI under LGPL/GPL/MIT).
• Included lightweight BLAKE2s implementation is public domain/CC0.

Support & contributions

Issues and patches are welcome. Please keep the SDL2-only build contract intact and adhere to the licensing notes above when redistributing binaries. Plain-text debug artifacts (e.g., personal logs) should not be committed for releases.