Recipe 11.12 Retrieve Information About Available Drives

11.12.1 Problem

You'd like to be able to gather specific information about the disk drives in your computer: for example, how large they are, how much space is free, whether they're local or remote, and whether they're removable or not. Access does not provide this information. Is it available using a Windows API function?

11.12.2 Solution

The Windows API provides three functions that you can use to extract information about the drives in your computer: GetLogicalDriveStrings, which returns a string containing a list of all the logical drives; GetDriveType, which returns information about the specified drive; and GetDiskFreeSpace, which returns information about the total and free disk space for a specified drive.

Load and run frmDiskSpace from 11-12.MDB. This form, shown in Figure 11-14, contains a list box with information about all the logical drives in your system. To fill the list box, the example code walks through all the drives returned from a call to GetLogicalDriveStrings, calling the other two functions for each drive.

Figure 11-14. frmDiskSpace shows information about all the installed drives

To use these functions in your own applications, follow these steps:

Table 11-12. The functions in basDiskInfo

Function	Purpose	Return value	Example
acbGetFreeSpace	Retrieve the amount of free space on the specified drive.	Variant (the amount of free disk space, in bytes), or Null if the function failed	lngFree = acbGetFreeSpace("C")
acbGetTotalSpace	Retrieve the total amount of space on the specified drive.	Variant (the amount of total disk space, in bytes), or Null if the function failed	lngTotal = acbGetTotalSpace("C")
acbIsDriveCDROM	Verify that a drive is a CD-ROM.	True if CD-ROM, False otherwise	fCD = acbIsDriveCDROM("D")
acbIsDriveFixed	Verify that a drive is a hard disk.	True if a hard disk, False otherwise	fFixed = acbIsDriveFixed("C")
acbIsDriveLocal	Verify that the specified drive is local.	True if local, False if remote	fLocal = acbIsDriveLocal("C")
acbIsDriveRAMDisk	Verify that a drive is a RAM disk.	True if RAM disk, False otherwise	fRAM = acbIsDriveRAMDisk("F")
acbIsDriveRemote	Verify that the specified drive is a network drive.	True if remote, False if local	fNetwork = acbIsDriveRemote("E")
acbIsDriveRemovable	Verify that the specified drive is for removable media.	True if removable, False otherwise	fRemovable = acbIsDriveRemovable("A")

11.12.3 Discussion

The sample form doesn't actually use any of the acbIs functions listed in Table 11-12; these functions are supplied only for your own applications. Instead, it calls the acbGetDrives function in basDiskInfo, which fills an array of acb_tagDriveInfo structures directly with information about each of the installed drives, physical or logical.

The structure looks like this:

Type acb_tagDriveInfo
   strDrive As String
   varFreeSpace As Variant
   varTotalSpace As Variant
   fRemovable As Boolean
   fFixed As Boolean
   fRemote As Boolean
   fCDROM As Boolean
   fRamDisk As Boolean
End Type

It stores all the information that the sample form displays. The sample form then uses a list-filling callback function to display the information in a list box. (For more information on list-filling callback functions, see Chapter 7.)

The acbGetDrives function starts out by calling the Windows API function GetLogicalDriveStrings. This function returns a string containing all the logical drives on your machine, in this format:

C:0D:0G:0H:0

where the 0s indicate null characters, Chr$(0). (VBA provides the vbNullChar constant that's equivalent to Chr$(0).) The acbGetDrives function loops through this string, using the acbGetToken function in basTokens to pull out the drive names, one at a time, and then gathering information about each. The source code for acbGetDrives is:

Public Function acbGetDrives(astrDrives( ) As acb_tagDriveInfo, _
   fIncludeFloppies As Boolean)
   ' Fill astrDrives( ) with all the available logical drive letters.
   
   Dim strBuffer As String
   Dim intCount As Integer
   Dim intI As Integer
   Dim varTemp As Variant
   Dim lngType As Long

   Const conMaxSpace = 1024

   strBuffer = Space(conMaxSpace)
    
   intCount = GetLogicalDriveStrings(conMaxSpace - 1, strBuffer)
   strBuffer = Left(strBuffer, intCount)
   intI = 1
   intCount = 0
   Do
      varTemp = acbGetToken(strBuffer, vbNullChar, intI)
      If Len(varTemp & "") > 0 Then
         ' The next statement will be true except in the
         ' case where the drive < C and you DON'T want
         ' to include floppies. Then it'll skip the drive.
         If (UCase(Left(varTemp, 1) < "C")) Imp fIncludeFloppies Then
            intCount = intCount + 1
            ' Get the drive name.
            astrDrives(intCount).strDrive = varTemp
            
            ' Get the drive type, and set the flags accordingly.
            lngType = GetDriveType(varTemp)
            Select Case lngType
               Case DRIVE_REMOVABLE
                  astrDrives(intCount).fRemovable = True
               Case DRIVE_FIXED
                  astrDrives(intCount).fFixed = True
               Case DRIVE_REMOTE
                  astrDrives(intCount).fRemote = True
               Case DRIVE_CDROM
                  astrDrives(intCount).fCDROM = True
               Case DRIVE_RAMDISK
                  astrDrives(intCount).fRamDisk = True
            End Select
            
            ' Get the drive space information.
            astrDrives(intCount).varTotalSpace = acbGetTotalSpace(varTemp)
            astrDrives(intCount).varFreeSpace = acbGetFreeSpace(varTemp)
         End If
         intI = intI + 1
      End If
   Loop Until Len(varTemp & "") = 0
   acbGetDrives = intCount
End Function

The acbGetTotalSpace and acbGetFreeSpace functions both call the private GetDiskSpace function, which in turn calls the GetDiskFreeSpace API function. GetDiskSpace takes the four pieces of information returned from GetDiskFreeSpace—sectors per cluster, bytes per sector, free clusters, and total clusters—and returns the calculated value that you've requested:

Private Function GetDiskSpace(ByVal strDrive As String, _
 fTotal As Boolean) As Variant

   ' Input:
   '    strDrive: String representing drive letter
   '    fTotal: True for total space on drive, False for free space on drive
   ' Output:
   '    Free or Total space, if no error. Null, otherwise.
    
   Dim lngSectorsPerCluster As Long
   Dim lngBytesPerSector As Long
   Dim lngFreeClusters As Long
   Dim lngTotalClusters As Long
   
   ' Force the string into the correct format.
   strDrive = Left(strDrive, 1) & ":\"
   If GetDiskFreeSpace(strDrive, lngSectorsPerCluster, lngBytesPerSector, _
    lngFreeClusters, lngTotalClusters) Then
     GetDiskSpace = lngSectorsPerCluster * lngBytesPerSector * IIf(fTotal, _
      lngTotalClusters, lngFreeClusters)
   Else
     GetDiskSpace = Null
   End If
End Function

If you want to dig a bit further, investigate the GetVolumeInformation API function. This function retrieves even more information about the specified drive, including its volume name, serial number, whether or not compression is enabled, the filesystem type (FAT, HPFS, NTFS), and other information about how data is stored on that drive. This information is of less importance to Access developers than to system application developers, so we don't discuss it here.