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Calling Conventions & Exports

The rule​

Every public entry point is __stdcall (WINAPI) except:

  • MovieMakerMain in MovieMakerCore.dll — __cdecl, undecorated
  • the UXCore resource/registry helpers in Resources.cpp — __cdecl

This is not stylistic: the calling convention determines the export decoration, and export parity with the reference binary is mandatory. A __stdcall int f(int) exports as _f@4; a __cdecl one exports as f. Get it wrong and the DLL's export table no longer matches the original — tools/diff_exports.py will (correctly) fail the change.

Decoration cheat sheet​

Convention32-bit decorationExample
__stdcall (WINAPI)_Name@N (N = bytes of params)_CreateVideoPlayer@4 → exported as CreateVideoPlayer@4
__cdeclName (undecorated)MovieMakerMain
C++ mangled (class methods)?Method@Class@@YG...ZDmxBici / WLXPhotoSqm / WLXPhotoBase exports

How parity is achieved​

1. .def files (most modules)​

Each module carries <Module>.def listing exports with ordinals. Two patterns appear:

LIBRARY WLXVideoTrim
EXPORTS
    CreateAVICopierDirect @1
    CreateVideoCopierFromMediaType @2
    ...
LIBRARY DmxBici
EXPORTS
    ; C++ mangled name aliased to a stable C-style undecorated export
    ?AddStringToDataPoint@BiciWrapper@@YG_NKKPB_W@Z = _BiciWrapper_AddStringToDataPoint@12 @1
    ...

The second pattern keeps the original mangled name (as found in the reference binary) while providing a forwarder to the implementation symbol — both names appear in the export table, exactly as in the original.

2. __declspec(dllexport) (UXCore)​

UXCore exports directly from headers via __declspec(dllexport) on the DirectUI classes (Element, HWNDElement, GPURenderer, ...) rather than a .def file.

3. Ordinals matter​

Aliases carry explicit ordinals (@1, @2, ...). Ordinal-only imports by legacy consumers rely on these numbers, so they are treated as part of the contract.

Checking parity​

# One DLL
python tools/diff_exports.py undecomp\WLXVideoTrim.dll build\bin\Debug\WLXVideoTrim.dll

# Whole directory pair
python tools/diff_exports.py --batch undecomp build\bin\Debug

Run this whenever an export is added, removed, renamed, or its convention changes. See Analysis Tools for the full tool guide.

COM entry points​

COM DLLs (WLMFDS, WLMFReadWrite, WLXMP4Parser, WLXCodecHost, MetadataSys, WLXMediaPublishSubscribe, ...) export the standard quartet — DllCanUnloadNow, DllGetClassObject, DllRegisterServer, DllUnregisterServer — alongside any API exports. Factory behavior is pinned by the contract suite; where the reference parity build returned E_NOTIMPL from a factory, our real implementation wins and the contract pins the real HRESULT (see Stub Design).