PRB: Win32 GetTempFileName API Differs From 16-bit
ID: Q137034
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The information in this article applies to:
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Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 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 or Windows 98.
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.
STATUS
This behavior is by design.
MORE INFORMATIONSteps 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
- Start the program by pressing the F5 key.
- 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.
Additional query words:
kbAPI kbVBp400 kbVBp kbDSupport kbdsd
Keywords : kbVBp400 kbGrpVB
Version : 4.00
Platform : WINDOWS
Issue type : kbprb
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