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Tip of the Day
Language: VB4/32,VB5,VB6
Expertise: Advanced
Aug 21, 1999

Use Currency instead of LARGE_INTEGER values

A few API calls require that you pass one or more LARGE_INTEGER arguments. These are 64-bit integer values, defined as follows:


Type LARGE_INTEGER
    lowpart As Long     ' lower 32-bit value
    highpart As Long    ' higher 32-bit value
End Type
Unfortunately, working with LARGE_INTEGER values in VB isn't easy, because you have to perform a lot of multiplications and divisions. A better solution is to pass the API function a Currency variable. Currency values are 64-bit integers, but when you print their values, VB scales them by a factor of 10,000 - to account for the 4 decimal digits that all Currency have.

As an example of this technique, let's see how you can retrieve the number of free bytes on the Windows versions that support partitions with more than 2G. In this case, you must call the GetDiskFreeSpaceEx API function, whose Declare is:


Private Declare Function GetDiskFreeSpaceEx Lib "kernel32" Alias _
    "GetDiskFreeSpaceExA" (ByVal lpDirectoryName As String, _
    lpFreeBytesAvailableToCaller As LARGE_INTEGER, _
    lpTotalNumberOfBytes As LARGE_INTEGER, lpTotalNumberOfFreeBytes As _
    LARGE_INTEGER) As Long
To apply the trick, you must define an aliased Declare, as follows:

Private Declare Function GetDiskFreeSpaceEx2 Lib "kernel32" Alias _
    "GetDiskFreeSpaceExA" (ByVal lpDirectoryName As String, _
    lpFreeBytesAvailableToCaller As Currency, lpTotalNumberOfBytes As Currency, _
    lpTotalNumberOfFreeBytes As Currency) As Long
and then call it using this code:


Dim lpFreeBytesAvailableToCaller As Currency
Dim lpTotalNumberOfBytes As Currency
Dim lpTotalNumberOfFreeBytes As Currency

GetDiskFreeBytesEx2 "C:\", lpFreeBytesAvailableToCaller, lpTotalNumberOfBytes, _
    lpTotalNumberOfFreeBytes
Debug.Print "Free Bytes Available to Caller = " & lpFreeBytesAvailableToCaller
Debug.Print "Total Number of Bytes = " & lpTotalNumberOfBytes
Debug.Print "Total Number of Free Bytes = " & lpTotalNumberOfFreeBytes
UPDATE: The original example had a few typo, that have been fixed now. Thanks to Kenneth Ives for spotting the problem.
Francesco Balena
 
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