CALLING FUNCTION POINTERS in Java

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forward the call, let's look at the stack at the beginning of a COM call The stack pointer is at the top of this list (smaller addresses are listed first) return address this parameter 1 parameter n To call a function pointer with the same parameters, the stack must look like this at the beginning of the function return address parameter 1 parameter n To forward a call to a function pointer, our COM method simply squeezes the this pointer out of the picture and turns control over to the function pointer The assembly code that does this is shown below It is important to note that this code is parameter- and return-value invariant, so you can use it to forward any standard vtable call with a this pointer (which includes all COM calls) to a function pointer with the same parameters // Retrieve the return address and // remove it from the stack (59) pop ecx // Retrieve the this pointer and // remove it from the stack (58) pop eax // Put the return address back on the stack (51) push ecx // Jump to the function pointer at this + 4 (FF 60 04) jmp DWORD PTR [eax + 4] # The bytes generated by this assembly code are 59 58 51 FF 60 04 (in hexadecimal) Now comes the lightweight-object connection In order to make VB call the assembly code, you must construct a COM object that has this byte
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stream in one of its vtable functions I'll use a lightweight COM object with the standard IUnknown functions in the first three vtable slots, and the magic byte stream in the fourth The QueryInterface function will be very trusting on the first call: it assumes that you are requesting an interface that matches the parameters supported by the function pointer All subsequent QueryInterface calls fail Since the actual asm code is only six bytes, I'll pad it with two int 3 instructions (CC CO to make eight bytes It can now be stored in a Currency constant, int 3 corresponds to a break statement, so you'll see a system error dialog if something goes wrong, int 3 is a standard padding instruction; nop (no operation, byte code 90) is the other Since a constant isn't in executable memory running the code at VarPtr(asmconst) causes a crash the data is copied from the constant to a module-level Currency variable The VarPtr of the Currency variable becomes the fourth vtable entry The code below shows the InitDelegator function, which initializes the stack-allocated version of the FunctionDelegator lightweight Code for the heap-allocated lightweight, created with NewDelegator, is included on the book's CD in the FunctionDelegatorbas file You can compile out either version using the FUNCTIONDELEGATOR NOHEAP or FUNCTIONDELEGATOR_NOSTACK conditional compilation values 'The magic number Private Const cDelegateASM As Currency = _ -3 689569180076386215@
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The FunctionDelegator redirection architecture
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'Module-level variables 'The variable to hold the asm bytes Private m_DelegateASM As CurrencyPrivate Type DelegatorVTables 'OKQI vtable in 0 to 3, FailQI vtable in 4 to 7 VTable(7) As Long End Type 'Structure and vtables for a stack allocated Delegator Private m_VTables As DelegatorVTables 'Pointer to success vtable, stack allocation version Private m_pVTableOKQI As Long 'Pointer to failing vtable, stack allocation version Private m_pVTableFailQI As Long 'Type declaration Public Type FunctionDelegator pVTable As Long 'This has to stay at offset 0 pfn As Long 'This has to stay at offset 4 End Type 'Initialize an existing FunctionDelegator structure 'as a delegator object Public Function InitDelegator( _ Delegator As FunctionDelegator, _ Optional ByVal pfn As Long) As IUnknown If m_pVTableOKQI = 0 Then InitVTables With Delegator pVTable = m_pVTableOKQI pfn = pfn End With CopyMemory InitDelegator, VarPtr(Delegator), 4 End Function Private Sub InitVTables() Dim pAddRefRelease As Long With m_VTables VTable(0) = FuncAddr(AddressOf QueryInterfaceOK) VTable(4) = FuncAddr(AddressOf QueryInterfaceFail)
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