Windows OLE/COM with jextract code generation

[Jorn Vernee]

Hi Duncan,

Thanks for bringing this up. Virtual calls are an interesting use-case, 
and a while ago we added support for linking virtual calls to CLinker, 
where you get back a method handle that takes an additional leading 
Addressable parameter that represents the function that should be called.

On the jextract front though, there is a relatively low-hanging fruit 
that we could also grab for this. IUnknownVtbl is essentially a struct 
with a bunch of function pointers. A simplified example would be:

     struct Foo {
         void (*cb)(int);
     };

Now maybe you get one of these structs back from a native call, and then 
you want to call 'cb'.

Jextract currently generates getters and setters for those kinds of 
function pointer fields, which would allow you to manually get the 
function address and then manually link a MethodHandle to call it, as 
you've found out. But, we could, in the case of function pointer types, 
also add another kind of accessor to call the function directly, and 
wire up the right MethodHandle behind the scenes.

So, your case could theoretically be turned from:

     Addressable vtFuncPtr = (Addressable) 
Ole32_h.IUnknownVtbl.QueryInterface$VH().get(vtable);
     MethodType mt= MethodType.methodType(int.class, 
MemoryAddress.class, MemoryAddress.class, MemoryAddress.class);
     FunctionDescriptor fd = FunctionDescriptor.of(CLinker.C_LONG, 
CLinker.C_POINTER, CLinker.C_POINTER, CLinker.C_POINTER);
     MethodHandle queryInterfaceMH = 
CLinker.getInstance().downcallHandle(vtFuncPtr, mt, fd);

Into something like:

     MemoryAddress hresult = Ole32_h.IUnknownVtbl.QueryInterface(vtable);

Maurizio has filed an issue for this [1].

Thanks,
Jorn

[1] : https://bugs.openjdk.java.net/browse/JDK-8261906

On 16/02/2021 19:13, Duncan Gittins wrote:
> This may be outside the scope of jextract (too Windows specific), or a 
> dumb question (then I'll send it to the usual place for dumb 
> questions: stackoverflow).
>
> I'm converting my handwritten Panama Java application over to use the 
> corresponding data structures generated by jextract, but can't find 
> definitions which could simplify the MethodHandle lookups for 
> referencing Windows OLE/COM API calls. The jextract output of Ole32.h 
> (which contains only "#include <objbase.h>") used is generated from:
>
>      set "WINKIT=c:\Program Files (x86)\Windows 
> Kits\10\Include\10.0.18362.0"
>      jextract -source -lole32    -t duncan.win -d java -I 
> "%WINKIT%\um" Ole32.h
>
> Retrieving any COM object IUnknown is easy with appropriate values for 
> class+iid guids as follows:
>
>      MemoryAddress comObj = Ole32_h.CoCreateInstance(rclsid.address(), 
> pUnkOuter, dwClsContext, riid.address(), ptrComObj.address());
>
> ... and read the COM object vTable by following IUnknown.lpVtbl 
> pointer to IUnknownVTbl:
>
>     MemorySegment memseg = Ole32_h.IUnknown.ofAddressRestricted(comObj);
>     MemoryAddress pVTableAddr = Ole32_h.IUnknown.lpVtbl$get(memseg);
>     MemorySegment vtable = 
> Ole32_h.IUnknownVtbl.ofAddressRestricted(pVTableAddr);
>
> At this point I need downcallHandle for every COM method pointer in 
> vtable - QueryInterface / AddRef... I have this for "QueryInterface" 
> but is there a way to avoid my hardcoded mt/fd declarations for all 
> each method handle?
>
>     Addressable vtFuncPtr = (Addressable) 
> Ole32_h.IUnknownVtbl.QueryInterface$VH().get(vtable);
>     MethodType mt= MethodType.methodType(int.class, 
> MemoryAddress.class, MemoryAddress.class, MemoryAddress.class);
>     FunctionDescriptor fd = FunctionDescriptor.of(CLinker.C_LONG, 
> CLinker.C_POINTER, CLinker.C_POINTER, CLinker.C_POINTER);
>     MethodHandle queryInterfaceMH = 
> CLinker.getInstance().downcallHandle(vtFuncPtr, mt, fd);
>
> The interface IUnknownVtbl.QueryInterface has support for upcalls (- 
> so maybe I could implement QueryInterface for a COM object in Java), 
> and has "fd" internally as xxx_constants.QueryInterface $FUNC, but no 
> equivalent to "mt" or downcalls of this specific comObj - say as 
> "IUnknownVtbl.QueryInterface$MH(vtFuncPtr)".
>
> Kind regards
>
> Duncan Gittins