Array backed by memory mapped file

Private Type SAFEARRAYBOUND
 cElements As Long
 lLbound As Long
End Type
Private Type SAFEARRAY2D
 cDims As Integer
 fFeatures As Integer
 cbElements As Long
 cLocks As Long
 pvData As Long
 Bounds(0 To 1) As SAFEARRAYBOUND
End Type
' keep it safe, be global
Dim mArray() As Double
Dim tSA As SAFEARRAY2D
Dim hFile As Long
Dim hFileMapping As Long
Dim lpFileBase As Long
Sub Create2DMMArray(Filename As String, ElemSize As Long, n As Long, m As Long)
 With tSA
 .cbElements = ElemSize
 .cDims = 2
 .Bounds(0).lLbound = 0
 .Bounds(0).cElements = m
 .Bounds(1).lLbound = 0
 .Bounds(1).cElements = n
 .fFeatures = &H10 Or &H2 ' FADF_FIXEDSIZE and FADF_STATIC
 .cLocks = 1
 
 GetViewOfFile Filename, ElemSize, n, m
 .pvData = lpFileBase
 End With
 
 If tSA.pvData = 0 Then
 Err.Raise 1243, "Create2DMMArray()", "Memory mapping failed"
 Else
 CopyMemory ByVal VarPtrArray(mArray()), VarPtr(tSA), 4
 End If
 
End Sub
Function GetViewOfFile(Filename As String, ElemSize As Long, n As Long, m As Long) As Long
 hFile = CreateFile(Filename, GENERIC_READ Or GENERIC_WRITE, 0, 0, _
    CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, vbEmpty)
 If hFile = -1 Then Err.Raise Err.LastDllError, "GetViewOfFile()", "Could not open file " & Filename
 
 Dim FileSize As Long
 FileSize = ElemSize * m * n
 hFileMapping = CreateFileMapping(hFile, 0, PAGE_READWRITE, 0, FileSize, vbEmpty)
 lpFileBase = MapViewOfFile(hFileMapping, FILE_MAP_ALL_ACCESS, 0, 0, 0 * FileSize)
 GetViewOfFile = lpFileBase
End Function
Function FreeViewOfFile() As Long
Dim ret As Long
 ' Clear the temporary array descriptor
 ' This may be necessary under NT4.
 CopyMemory ByVal VarPtrArray(mArray), 0&, 4
 
 FreeViewOfFile = UnmapViewOfFile(lpFileBase)
 If FreeViewOfFile = 0 Then Debug.Print "Error: ", Err.LastDllError
' If FreeViewOfFile = 0 Then Err.Raise Err.LastDllError, "FreeViewOfFile()", "Memory unmapping failed"
 ret = CloseHandle(hFileMapping)
 ret = CloseHandle(hFile)
End Function
Function checkMMA()
Dim n As Long, m As Long, i As Long, j As Long
Dim Filename As String, ElemSize As Long
 Filename = "c:\kill.me"
 n = 10 ^ 6: m = 10
 ElemSize = 8 ' size of Double is 8
 
 'Create 2D Array(m,n) of Double,
 Create2DMMArray Filename, ElemSize, n, m
 
 'random acess to our file
 For i = 0 To 1000
 mArray(Rnd * n Mod n, Rnd * m Mod m) = i
 Next i
' close down, destroy array
' this MUST be called
FreeViewOfFile
End Function