PRB: GetTempFileName API Function May Fail on 32-bit Platforms

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 16-bit and 32-bit, for Windows, version 4.0

SUMMARY

GetTempFileName is an API function used to create a temporary filename. Although this function is supported on both 16-bit and 32-bit platforms, minor differences in its calling conventions exist. Additionally, when used with a 32-bit platform such as Windows 95 or Windows NT, GetTempFileName might fail. This article discusses the function's cross-platform portability and explains the cause and resolution of its failure on 32-bit platforms.

SYMPTOMS

When using the GetTempFileName API function on a 32-bit platform, one of the following symptoms may appear:

• The function passes back a null parameter for the filename and returns 0. This is more likely to happen in Windows NT.
• The function passes back a pathname without a filename and returns 0. This more likely to happen in Windows 95.

CAUSE

Contrary to its usage on 16-bit platforms where only the drive letter is passed, GetTempFileName requires you to pass the drive letter and the pathname when used on 32-bit platforms. Furthermore, the pathname specified must contain existing directories. For example, examine the following function declarations:

• For 16-bit platforms such as Windows 3.1 and Windows for Workgroups:

     Declare Function GetTempFileName Lib "Kernel" (ByVal cDriveLetter as _
        Integer, ByVal lpPrefixString As String, ByVal wUnique As Integer, _
        ByVal lpTempFileName As String) As Integer
  
  

• For 32-bit platforms such as Windows NT and Windows 95:

     Private Declare Function GetTempFileName Lib "kernel32" _
        Alias "GetTempFileNameA" (ByVal lpszPath As String, _
        ByVal lpPrefixString As String, ByVal wUnique As Long, _
        ByVal lpTempFileName As String) As Long
  
  

In 32-bit, however, the first parameter is a string that gives the pathname. So if you were to cease a temporary file in the C drive root directory, the first parameter will be assigned a different value for each case:

• For 16-bit: cDriveLetter = 2 (Integer representation of drive C)
• For 32-bit: lpszPath = "C:\"

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce and Resolve Behavior

1. Create a new project in 32-bit Visual Basic 4.0. Form1 is created by default.

2. Copy the following code into the General Declarations section of Form1:

      Private Declare Function GetTempFileName Lib "kernel32" _
         Alias "GetTempFileNameA" (ByVal lpszPath As String, _
         ByVal lpPrefixString As String, ByVal wUnique As Long, _
         ByVal lpTempFileName As String) As Long
   
      Private Sub Form_Click()
         Dim lpszPath As String
         Dim lpPrefixString As String
         Dim wUnique As Long
         Dim lpTempFileName As String
   
         ' Because the C:\windows directory, probably, doesn't exist,
         ' using this line will cause the problem to occur:
         lpszPath = "C:\windows"     ' Line A
   
         ' To resolve the problem, change the previous line into a comment
         ' and change the following line from a comment into an executed line:
         ' lpszPath = "C:\windows"    ' Line B
   
         lpPrefixString = "shn"
   
         ' If wUnique is nonzero, it will be appended
         ' to the temporary filename. If the parameter is zero,
         ' Windows uses the current system time to create a
         ' number to append to the filename.
         wUnique = 0
   
         ' Initialize variable
         lpTempFileName = Space$(100)
         Print GetTempFileName(lpszPath, lpPrefixString,_
            wUnique, lpTempFileName)
   
         ' If running under Windows 95, use the following line to get
         ' rid of the null character
         ' Print Mid$(lpTempFileName, 1, InStr(lpTempFileName, Chr$(0)) - 1)
         Print lpTempFileName
      End Sub
   
   

3. Start the program by pressing the F5 key.

4. Notice how the problem is reproduced. Change Line A into a comment and make Line B into an executed line. Then repeat step 3. The problem should now disappear.