Division causes overflow

[tmighty2]

Hello!

I have the following function to return the size of a file to me.

Optionally, I want to have MB returned instead of KB.

The first thing that I stumbled over was a file length > 2 GB.

To work around this, I found the following hack: lRet = 2147483648# + (2147483648# - Abs(lRet))

However, when I then divide this value (for example: 2438086656) by 1000000, I get an overflow exception.

Can anybody tell me what I'm doing wrong here?

Thank you!

Public Function FileLength(ByVal uPath As String, ByVal uMBInsteadOfByte As Boolean, ByVal uCaller As String) As Currency
On Error GoTo ErrHandler

    Debug.Print "Getting filelen from " & uPath
    Debug.Assert modIO.FileExists(uPath)

    Dim ret As Currency 'We could have used Long here, but if ret < 0, then we need to add a huge number 
    ret = VBA.FileLen(uPath)

    If ret < 0 Then
        'bigger than 2 gb
       ret = 2147483648# + (2147483648# - Abs(ret)) 'Not sure what I'm doing here, I copied it from the internet
    End If
    
    Debug.Assert ret > 0

    If uMBInsteadOfByte Then
        ret = (ret \ 1000000) 'this causes an overflow exception. I don't see why. 
    End If

    FileLength = ret

Exit Function
ErrHandler:
Debug.Print Err.Description
Call WriteLog("#FileLength: " & Err.Description & ", err.number: " & Err.Number & ", Params: '" & uPath & "', caller: " & uCaller)
On Error GoTo -1
Debug.Assert False
End Function

[LaVolpe]

The reason for the error is that integer division is used (\). Use (/) instead

[dilettante]

The FileLen() function doesn't handle huge files.

Assuming there is really any need for this at all, I'd probably choose something like this:

Option Explicit

Private Const GENERIC_READ As Long = &H80000000
Private Const OPEN_ALWAYS = 4

Public Enum SHARE_MODES
    FILE_SHARE_EXCLUSIVE = 0
    FILE_SHARE_READ = 1
    FILE_SHARE_WRITE = 2
    FILE_SHARE_DELETE = 4
    FILE_SHARE_ALL = FILE_SHARE_READ Or FILE_SHARE_WRITE Or FILE_SHARE_DELETE
End Enum
#If False Then
Dim FILE_SHARE_EXCLUSIVE, FILE_SHARE_READ, FILE_SHARE_WRITE, FILE_SHARE_DELETE, FILE_SHARE_ALL
#End If

Private Declare Function CloseHandle Lib "kernel32" ( _
    ByVal hObject As Long) As Long

Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileW" ( _
    ByVal lpFileName As Long, _
    ByVal dwDesiredAccess As Long, _
    ByVal dwShareMode As SHARE_MODES, _
    ByVal lpSecurityAttributes As Long, _
    ByVal dwCreationDisposition As Long, _
    ByVal dwFlagsAndAttributes As Long, _
    ByVal hTemplateFile As Long) As Long

Private Declare Function GetFileSizeEx Lib "kernel32" ( _
    ByVal hFile As Long, _
    ByRef curFileSize As Currency) As Long

Private Sub Form_Load()
    Const TEST_FILE As String = "D:\VHDs\Vista w IE8\Vista SP1 x86 with IE8 2009-Apr.vhd"
    Dim hFile As Long
    Dim curFileSize As Currency
    Dim MBFileSize As Double

    hFile = CreateFile(StrPtr(TEST_FILE), _
                       GENERIC_READ, _
                       FILE_SHARE_ALL, _
                       0&, _
                       OPEN_ALWAYS, _
                       0&, _
                       0&)
    GetFileSizeEx hFile, curFileSize
    CloseHandle hFile
    MBFileSize = CDbl(curFileSize) / (1000000 / 10000)

    AutoRedraw = True
    ScaleLeft = -60
    ScaleTop = -60
    Cls
    Print TEST_FILE
    Print Format$(MBFileSize, "#,##0.000 \M\B")
End Sub

That uses a Currency as a stand-in for a 64-bit integer type. The result is converted to a Double (or you might use a Decimal Variant) and then corrected by 10000 before finally dividing by 1000000.

[dilettante]

You might also want to show more detail:

Print Format$(MBFileSize, "#,##0.000000 \M\B")

[tmighty2]

Thanks both of you very much!