PRB: Win32 GetTempFileName API Differs From 16-bit (137034)

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 or Windows 98.

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.

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, Windows 95, or Windows 98:

   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

Notice how the first parameter is the only parameter that changes in the declarations. In 16-bit, the first parameter is an integer representing a drive letter. The pathname is specified by the function not by the user (see the Visual Basic 3.0 Windows 3.1 SDK help file for more information).

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:\"

If lpszPath is an invalid path or if it contains a nonexistent directory, GetTempFileName will pass back the lpTempFileName parameter with a null value or with a pathname only. The return value of the function in both cases will be zero.

MORE INFORMATION

Steps to Reproduce and Resolve Behavior

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

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 or Windows 98, use the following line
      ' to get rid of the null character
      ' Print Mid$(lpTempFileName, 1, InStr(lpTempFileName, Chr$(0)) - 1)
      Print lpTempFileName
   End Sub