PowerBASIC and reference counting

[Dominic Mitchell]

Given this code

FUNCTION frameIOleInPlaceFrame_RemoveMenus(BYVAL pThis AS DWORD, BYVAL hmenuShared AS DWORD) AS LONG

  LOCAL riid            AS GUID
  LOCAL pCIFace         AS CIFace PTR
  LOCAL pCFrame         AS CFrame PTR
  LOCAL ptocd           AS OLECONTAINERDATA PTR
  LOCAL pTenant         AS Tenant PTR
  LOCAL hWndSite        AS DWORD
  LOCAL pUnk            AS DWORD
  LOCAL lParam          AS LONG
  LOCAL lpfnCallback    AS DWORD
  LOCAL hr              AS LONG

  pCIFace = pThis
  pCFrame = @pCIFace.pObject

  ' Tell the top-level window to remove its menus
  IF pCFrame THEN
    hWndSite = GetHostWindow(@pCFrame.m_pIOleIPActiveObject)
    IF IsWindow(hWndSite) THEN
      ptocd = GetWindowLong(hWndSite, 0)
      IF @ptocd.m_lpfnMenu THEN
        lpfnCallback = @ptocd.m_lpfnMenu
        lParam       = @ptocd.m_lParam

        riid = $IID_IUNKNOWN
        pTenant = @ptocd.m_pObject   
        IF pTenant THEN
          hr = IUnknown_QueryInterface(@pTenant.m_pIUnknown, riid, pUnk)
          IF hr = %S_OK THEN  
            ' Call the InsertMenus callback function
            ! push  ebx
            ! push  esi
            ! push  edi
            ! push  lParam
            ! push  %TRUE
            ! push  %NULL
            ! push  %NULL
            ! push  %NULL
            ! push  hmenuShared
            ! push  pUnk
            ! call  lpfnCallback
            ! pop   edi
            ! pop   esi
            ! pop   ebx
            IUnknown_Release pUnk
          END IF
        END IF  
      END IF
    END IF
  END IF

	FUNCTION = %S_OK

END FUNCTION

And this version of the callback

FUNCTION InsertMenusCallback _
  ( _
    BYVAL oObject       AS IUNKNOWN, _                ' [in] IUnknown pointer to active object
    BYVAL hMenuShared   AS DWORD, _                   ' [in] shared menu
    BYVAL ptomgw        AS OLEMENUGROUPWIDTHS PTR, _  ' [out] array indicatiing number of items in each group
    BYVAL phMenuFrame   AS DWORD PTR, _               ' [out] points to the handle of the original frame menu
    BYVAL phMenuWindow  AS DWORD PTR, _               ' [out] points to the handle of the original window menu(MDI)
    BYVAL fRemove       AS LONG, _                    ' [in] true, items are being removed. False, items are being added
    BYVAL lParam        AS LONG _                     ' [in] application-defined value
  ) AS LONG

END FUNCTION

PowerBASIC calls Release on oObject an [in] parameter which, in my opinion, is a violation of the
COM reference counting rules. The result is that the IUnknown pointer to the object, @pTenant.m_pIUnknown,
is no longer valid after frameIOleInPlaceFrame_RemoveMenus returns.

[José Roca]

No, because PB will call AddRef if you pass an object variable, and later Release. This guarantees that the object will be valid during the life of the function. But you are bypassing the mechanism by using assembler. Either declare oObject as DWORD, or call AddRef before the assembler code.

[José Roca]

You could also use this:

LOCAL pUnk AS IUnknown
hr = IUnknown_QueryInterface(@pTenant.m_pIUnknown, riid, BYVAL VARPTR(pUnk))
IF hr = %S_OK THEN  
   InsertMenusCallback(pUnk, hmenuShared, %NULL, %NULL, %NULL, %TRUE, lParam)
   pUnk = NOTHING
END IF

[Dominic Mitchell]

or call AddRef before the assembler code.

Well QueryInterface plus an additional AddRef is not good for code maintenance and could lead to trouble with other compilers.
I usually use this

        riid = $IID_IUNKNOWN
        pTenant = @ptocd.m_pObject   
        IF pTenant THEN
          hr = IUnknown_QueryInterface(@pTenant.m_pIUnknown, riid, pUnk)
          IF hr = %S_OK THEN 
            vObj.vt = %VT_DISPATCH 
            vObj.vd.pdispVal = pUnk
            pv = VARPTR(vObj)            
            ! push  ebx
            ! push  esi
            ! push  edi
            ! push  lParam
            ! push  %TRUE
            ! push  %NULL
            ! push  %NULL
            ! push  %NULL
            ! push  hmenuShared
            ! push  pv
            ! call  lpfnCallback
            ! pop   edi
            ! pop   esi
            ! pop   ebx
            IUnknown_Release pUnk
          END IF
        END IF

and use dword for the object variable.

Anyway, for this particular callback a reference to the object is not of any use, therefore, I have replaced it with the handle of the frame window.

[Dominic Mitchell]

LOCAL pUnk AS IUnknown
hr = IUnknown_QueryInterface(@pTenant.m_pIUnknown, riid, BYVAL VARPTR(pUnk))
IF hr = %S_OK THEN  
   InsertMenusCallback(pUnk, hmenuShared, %NULL, %NULL, %NULL, %TRUE, lParam)
   pUnk = NOTHING
END IF

That is not going to work. The code that calls the callback is in a DLL and InsertMenusCallback is not in the DLL.

[Dominic Mitchell]

Using this

CALL DWORD @ptocd.m_lpfnMenu USING InsertMenusCallback(pUnk, hmenuShared, %NULL, %NULL, %NULL, %TRUE, lParam)

will work.

But it requires a declaration which I don't like doing. Object variables are also not allowed in this code.

Thanks for the info on PowerBASIC's AddRef and Release calls. I was only seeing the release, that is why I thought it violated the rules.
It will come in handy when dealing with events from an OCX and inserting a reference to the object in the events.