Shoutbox

JavaScript code:

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function InserColumn(hWnd,iCol,Header)
{
    //Message constants
    var LVM_INSERTCOLUMN = 0x101B;
    var LVCF_TEXT = 0x4;
    var LVCF_WIDTH = 0x2;
       
    var cx= Interop.Allocate(4)//How do I know how much bytes I need?
    cx.WriteWord(0, 50)

    var  pszText=Interop.Allocate((Header.length+2)*2)
    pszText.WriteString(0, Header,true)
    //Create an LVCOLUMN structure
    var LVCOLUMN = Interop.Allocate(32);
    LVCOLUMN.WriteDWord(0,    LVCF_WIDTH); //How do I know the offset?
    LVCOLUMN.WriteDWord(6, cx.DataPtr);  //How do I know the offset?
    LVCOLUMN.WriteDWord(16,    LVCF_TEXT); //How do I know the offset?
    LVCOLUMN.WriteDWord(22, pszText.DataPtr); //How do I know the offset?
   

    var Result = Interop.Call("user32", "SendMessageW", hWnd, LVM_INSERTCOLUMN, iCol, LVCOLUMN);
   
    //Clear the DataBlocs
    cx.Size = 0;
    pszText.size=0;
    LVCOLUMN.Size = 0
   
    //Return the result
    return Result;
}

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as said in the comments how do I know the cx bytes and offset when I create the structure?