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Host netplay integration notes

Status: active · 2026-07-23

recomp-ui owns presentation and the universal UDP port policy. The game owns lobby transport, soft-return, and rematch reboot. This page lists contracts that bite every snesrecomp (and sibling) title wiring MotK-style netplay through RecompLauncherCNetplayCallbacks.

For the engine-side checklist see snesrecomp docs/RECOMP_NET.md → "Soft-return rematch checklist".

join() guest bind

int (*join)(void* ctx, const char* lobby_id, const char* password,
            char* guest_bind /* in/out, capacity >= 64 */);

Before every join() call, the ImGui backend fills guest_bind via launcher_udp_prepare_guest_bind() (prefer UDP 7778, then +1..+31 → 0.0.0.0:<port>). Hosts must advertise that bind on the lobby join.

Path Host behavior
Online / lobby-server join Pass guest_bind through to the lobby client
LAN file-registry (lan:…) May ignore guest_bind
Engine fallback snesrecomp snes_lobby_join still rewrites NULL/empty/host:0

Never rewrite a prepared bind to :0 — the server would publish peer_ip:0 and LAN session start rejects it.

Host create uses the same helper family (launcher_udp_port.*): LAN keeps the exact UI port; online prefers 7777..+31.

Soft-return launcher and SDL_Quit

launcher_platform_close() (SDL2 and SDL3) ends with SDL_Quit() so the game gets a clean slate (GL attributes reset, no leftover launcher window).

That is intentional. Side effect: all SDL subsystems — including audio — are torn down when the waiting room closes for a rematch.

Symptom

After Escape / peer leave → lobby → Play:

audio outputs: -1 device(s)
Audio subsystem is not initialized
Failed to open audio device

(or a silent hang if the host treats audio open as fatal).

Host fix

On the rematch session_reboot path (after recomp_launcher_run_window returns with a new netplay_launch), re-init before creating the game window or opening audio:

if (!SDL_WasInit(SDL_INIT_VIDEO) || !SDL_WasInit(SDL_INIT_AUDIO) ||
    !SDL_WasInit(SDL_INIT_GAMECONTROLLER)) {
  if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_GAMECONTROLLER) != 0)
    return 1;
}

First boot usually already called SDL_Init once; the WasInit guard avoids stacking unnecessary init refcounts.

Do not remove SDL_Quit() from the platform close without a coordinated host change — games rely on a clean shutdown between launcher and gameplay.

Resume room fields

When soft-returning after a match, set on RecompLauncherCGameInfo:

  • resume_netplay_room = 1
  • resume_netplay_endpoint — optional LAN endpoint string for the header

Clear launch-pending / re-arm ready in the game’s lobby callbacks before showing the waiting room again (snes_lobby_clear_launch_pending, snes_lobby_set_ready, etc.).

Peer disconnect UX

recomp-ui does not own the in-game disconnect dialog. Hosts should treat mid-match peer loss like a local quit: soft-return to the lobby without a blocking SDL_ShowSimpleMessageBox. Reserve modals for connect-timeout / firewall guidance before the session starts.