Manually Inject Steam Overlay and Associate With an AppID

gistfile1.txt

#include <string>

const wchar_t*
SK_GetSteamDir (void)
{
  static
    wchar_t wszSteamPath [MAX_PATH + 2] = { };

  DWORD     len    =      MAX_PATH;
  LSTATUS   status =
    RegGetValueW ( HKEY_CURRENT_USER,
                     LR"(SOFTWARE\Valve\Steam\)",
                                      L"SteamPath",
                       RRF_RT_REG_SZ,
                         nullptr,
                           wszSteamPath,
                             (LPDWORD)&len );

  if (status == ERROR_SUCCESS)
    return wszSteamPath;

  else
    return nullptr;
}

constexpr int
__stdcall
SK_GetBitness (void)
{
#ifdef _M_AMD64
  return 64;
#endif
  return 32;
}

#define SK_RunLHIfBitness(b,l,r)   SK_GetBitness () == (b) ? (l) : (r)

bool
SK_Steam_LoadOverlay (int32 appid = -1)
{
  static HMODULE hModOverlay = nullptr,
                 hModClient  = nullptr;

  if (hModOverlay != nullptr)
    return true;

  const wchar_t* wszSteamPath =
    SK_GetSteamDir ();

  if (wszSteamPath == nullptr)
    return false;

  if (appid != -1)
  {
    SetEnvironmentVariableA (
      "SteamGameId",
        std::to_string (appid).c_str ()
    );
  }

  wchar_t    wszOverlayDLL [MAX_PATH + 2] = { };
  wchar_t    wszClientDLL  [MAX_PATH + 2] = { };

  lstrcatW ( wszOverlayDLL, wszSteamPath );
  lstrcatW ( wszClientDLL,  wszSteamPath );

  lstrcatW ( wszOverlayDLL,
    SK_RunLHIfBitness ( 64, LR"(\GameOverlayRenderer64.dll)",
                            LR"(\GameOverlayRenderer.dll)"    ) );
  lstrcatW ( wszClientDLL, 
    SK_RunLHIfBitness ( 64, LR"(\steamclient64.dll)",
                            LR"(\steamclient.dll)"    ) );

  hModClient =
    LoadLibraryW (wszClientDLL);

  hModOverlay =
    LoadLibraryW (wszOverlayDLL);

  return hModOverlay != nullptr;
}