How to fix the type of a function pointer call in the Hex-Rays decompiler?

Question

In a disassembly with a call to a DirectDraw->BltFast function, I encountered the following:

(*(void (__stdcall **)(LPDIRECTDRAWSURFACE7, _DWORD, _DWORD, _DWORD, _DWORD, signed int, int, int, int, int))
((void (__stdcall **)(_DWORD, _DWORD, _DWORD, _DWORD, _DWORD, _DWORD, _DWORD, _DWORD, _DWORD, _DWORD))v17->lpVtbl + 7))(
v17,
0,
0,
*(&g_COREvidSurf + 25),
0,
16,
v75,
v77,
v79,
v81);

The cast has too many arguments. How does one edit the call's cast to fix the disassembly ?

EDIT: After applying a struct to the code:

.text:00540825 030                 cmp     [esp+30h+arg_8], ecx
.text:00540829 030                 mov     edx, g_COREvidSurf+64h
.text:0054082F 030                 mov     eax, g_CoreVidsurf_6
.text:00540834 030                 push    10h
.text:00540836 034                 push    0
.text:00540838 038                 push    edx
.text:00540839 03C                 push    0
.text:0054083B 040                 push    0
.text:00540844 044                 mov     ecx, [eax]
.text:00540846 044                 push    eax
.text:00540847 048                 mov     eax,[ecx+IDirectDrawSurface7Vtbl.BltFast]
.text:0054084A 048                 call    eax

and the decompilation:

v17.lpVtbl = (struct IDirectDrawSurface7::IDirectDrawSurface7Vtbl *)g_surface2;
(*(void (__stdcall **)(struct IDirectDrawSurface7::IDirectDrawSurface7Vtbl *, _DWORD, _DWORD, _DWORD, _DWORD, signed int, int, int, int, int))(*(_DWORD *)v17.lpVtbl + offsetof(IDirectDrawSurface7Vtbl, BltFast)))(
v17.lpVtbl,
0,
0,
*(&g_COREvidSurf + 25),
0,
16,
v75,
v77,
v79,
v81);

The struct used was selected from the standard struct selection in IDA, and if i press 'Y' on the declaration of BltFast in the struct declaration, the call is declared like this:

HRESULT (__stdcall *BltFast)(IDirectDrawSurface7 *This, DWORD, DWORD, LPDIRECTDRAWSURFACE7, LPRECT, DWORD)

which is correct, but as seen above, IDA is still showing too many args.

Answer 1

I would define a vtable struct which has the correct function pointers in it. I assume this is an IDirectDrawSurfaceX? The variable named v17->lpVtbl needs to have a type of IDirectDrawSurfaceX *. Create this structure of function pointers according to the interface definition on MSDN or load it from a standard structures if IDA has imported the DirectX C interface definitions. Set the type of the v17->lpVtbl by pressing y on the definition of lpVtbl in whatever structure type v17 is. Then you may need to force the type of the function pointer call to the type of the interface member used in the vtable. You do this by right clicking the call site and choosing Force call type.