8.6 Test the Comparative Benefits of Various Optimization Techniques

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:

1. Import the modules basDiskInfo and basToken from 11-12.MDB.

2. To call the functions, use the information in Table 11-12. Each function takes only a single parameter, the drive to be interrogated.

    

   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$(0). (Access provides the vbNullChar constant that's equivalent to Chr$(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.