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DLLCall with Null

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I'm trying to scroll a control or window programmatically using the Win32 API, specifically ScrollWindow or ScrollWindowEx. I've had some success with getting the scrollbar to move, but the client area seems to scroll and immediately jump back. The ScrollWindow call returns 0, indicating it failed.

The goal is to be able to state what % of the way down to scroll a window. I can get a scrollbar position with no problem, but I want to be able to set it and have the associated control respond to the new position. I've tried using ints, shorts, and longs to no avail, so I'm thinking the problem lies in being able to pass a null parameter.

How do you pass a null value through DLLCall? Or could someone tell me what I'm doing wrong?

    $WM_VSCROLL = 277
    $SBS_HORZ = 0
    $SBS_VERT = 1
    $NULL = ""; null-terminated string
    $SW_ERASE = 4
    $SW_SMOOTHSCROLL = 16; might be 8

    $bVal = DllCall("user32.dll", "int", "SetScrollPos", "hwnd", $ctrlHandle, "int", $SBS_VERT, "int", $relPos, "int", 1)
    $xVal = DllCall("user32.dll", "int", "PostMessageA", "hwnd", $ctrlHandle, "int", $WM_VSCROLL, "int", $SB_THUMBPOSITION + 65536 * $relPos, "int", 0)

    $RECT = DllStructCreate("int;int;int;int")
    $aryCtrlPos = ControlGetPos("C:\Document", "", "Scintilla1")
    DllStructSetData($RECT, 1, $aryCtrlPos[0])
    DllStructSetData($RECT, 2, $aryCtrlPos[1])
    DllStructSetData($RECT, 3, $aryCtrlPos[2])
    DllStructSetData($RECT, 4, $aryCtrlPos[3])
    $ptrRECT = DllStructGetPtr($RECT)
    $mVal = DllCall("user32.dll", "int", "ScrollWindow", "hwnd", $ctrlHandle, "short", $horzDiff, "short", $vertDiff, "ptr", "", "ptr", $ptrRECT); returns 0 = FAIL
    $mVal = DllCall("user32.dll", "int", "ScrollWindowEx", "hwnd", $ctrlHandle,  "short", $horzDiff, "short", $vertDiff, "ptr", $NULL, "ptr", $NULL, "hwnd", $NULL, "ptr", $NULL, "short",  $SW_ERASE + $SW_INVALIDATE + $SW_SCROLLCHILDREN); returns 0 = FAIL
    $nVal = DllCall("user32.dll", "int", "UpdateWindow", "hwnd", $ctrlHandle)


    $scrollInfo = 0;Release the memory
    $RECT = 0

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