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rather than the first You can simply use the alternate structure and initialization function if you require this capability You can use the conditional compilation values PUSHPARAM_NONORMAL or PUSHPARAM_NOSTACKRETURN to eliminate capabilities you aren't using in your project
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Visual Basic functions defined with the Declare statement are always assumed to be _stdcall All functions you code are also generated using this calling convention However, there are many exported functions in various DLLs that use the alternate _cdecl calling convention The code provided with the book provides a CDECLFunctionDelegatorbas file that is similar to the FunctionDelegatorbas file, but which calls CDECL functions This section looks at the provided functions, but not the assembly code implementation Before you get overly excited about the prospects of calling _cdecl functions, please consider the amount of assembly code required to make these calls You must run significantly more than the six bytes of assembly code required to squeeze the this pointer In fact, for a small function, the call overhead could be more than the cost of the call itself You should call _cdecl functions with discretion In addition to the function pointer, the cdecl delegator functions also require a StackSize value In a cdecl function call, the called function does not pop the parameters from the stack Parameter popping is left to the calling function In the case of a cdecl delegator, the calling function is provided by the asm code in the delegator, so the delegator must know the stack size The two cdecl delegator functions are called InitCDECLDelegator and NewCDECLDelegator They act like the corresponding InitDelegator and NewDelegator functions, except they have an extra StackSize parameter The CDECLFunctionDelegator passed into InitCDECLDelegator is also the same as the FunctionDelegator, except it has a StackSize member Using these two functions, you can call a GetProcAddress-retrieved cdecl function pointer However, this is only half the story because you can't use AddressOf directly to provide a cdecl callback function The InitCDECLThunk function solves the second half of the cdecl problem InitCDECLThunk takes a CDECLThunk structure, a function pointer, and a stack size After the function
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ADVANCED VISUAL BASIC 6
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returns, the pfn field of the CDECLThunk variables contains the function pointer If you want to reuse the CDECLThunk structure with a different function pointer and stacksize, you can update the code with the UpdateCDECLThunk function With these functions at your disposal, you are ready to call a cdecl function and provide a cdecl callback function You can use the same callback functions you used with the VB-defined Quicksort earlier in the chapter, but this time let the qsort function in msvcrtDII do all the work Since qsort is now a function pointer, you need a typelib definition to enable the compiler to call the cdecl delegator Here's the ODL for the sort function, described as stdcall, not cdecl CRTQuickSort function uses InitCDECLDelegator and InitCDECLThunk, so this function works without using heap-allocated memory [uuid(C9750742-4659-IID3-AB5C-D41203C10000), odl] interface ICallQSort : IUnknown { void QSort([in] long pBase, [in] long Number, [in] long Size, [in] long pfCompare); } Public Sub CRTQuickSort( _ ByVal pBase As Long, _ ByVal Number As Long, _ ByVal Size As Long, _ ByVal pfCompare As Long) Dim pQSort As ICallQSort Dim CompareThunk As CDECLThunk Dim Delegator As CDECLFunctionDelegator Dim pCRT As Long Dim pfnQSort As Long pCRT = LoadLibrary("msvcrtDll") pfnQSort = GetProcAddress(pCRT, "qsort")
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'4 long parameters =16 byte stack size
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Set pQSort = InitCDECLDelegator( _ Delegator, pfnQSort, 16) '2 long parameters = 8 byte stack size InitCDECLThunk CompareThunk, pfCompare, 8
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pQSortQSort pBase, Number, Size, CompareThunkpfn FreeLibrary pCRT End Sub If you were to use this function in real code, you would most likely encapsulate it in a class to cache the module handle for msvcrtDII and the address of qsort Note that if you compile VB to fast code with array bounds checking disabled, the VB Quicksort performs about 35 percent faster than the CRTQuickSort version because of the cdecl thunking overhead Of course, if the CRTQuickSort or the next cdecl function you want to use is fast enough for your needs, then you can skip coding a replacement function in VB altogether, printf, anyone
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