HOWTO: Use GetTempFileName API to Create a Unique Temporary File

ID: Q195763

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

SUMMARY

For Visual Basic programmers unfamiliar with the Win32 API, making a file name for a temporary file is often a matter of hard-coding some value and hoping that the name does not conflict with an existing file. Fortunately, there is a better way that requires very little code.

This article demonstrates how to use the GetTempPath and GetTempFileName API functions to construct a unique temporary file name located in the default temp directory of an end user's system.

MORE INFORMATION

GetTempPath

The GetTempPath API function allows you to determine the path location of a system's temporary folder. It takes two parameters: the length of a fixed- length or pre-initialized string that will contain the path name, and the string itself. You must use either a fixed-length string, or a string initialized to a length that you believe will be long enough to contain the path information. This is to guarantee that Visual Basic allocates enough buffer space for Windows to return the information.

GetTempPath returns the length of the path name measured in bytes, or 0 if an error occurs. If the return value is greater than the buffer size you specified, then no path information was written to the string.

The declaration for GetTempPath is provided in the sample below.

GetTempFileName

The GetTempFileName API function is used to create a fully-qualified temporary file name at a given location. The function takes four parameters: the string containing the path for the file, a string containing a prefix used to start the unique file name, a unique number to construct the temp name, and, finally, a fixed-length or pre-initialized string used to return the fully qualified file name. Both the path and prefix strings are required and cannot be empty. The unique number can be 0 (NULL), in which case GetTempFileName creates a unique number based on the current system time.

GetTempFileName returns the unique number used to create the file name, or returns 0 if an error occurs.

The declaration for GetTempFileName is provided in the sample below.

Step-by-Step Example

Create a new Standard EXE project. Form1 is created by default.

Add a CommandButton to Form1.

Add the following code to the General Declarations section of Form1:


       Option Explicit

      Private Declare Function GetTempPath Lib "kernel32" _
         Alias "GetTempPathA" (ByVal nBufferLength As Long, _
         ByVal lpBuffer As String) As Long

      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 Function CreateTempFile(sPrefix As String) As String
         Dim sTmpPath As String * 512
         Dim sTmpName As String * 576
         Dim nRet As Long

         nRet = GetTempPath(512, sTmpPath)
         If (nRet > 0 And nRet < 512) Then
            nRet = GetTempFileName(sTmpPath, sPrefix, 0, sTmpName)
            If nRet <> 0 Then
               CreateTempFile = Left$(sTmpName, _
                  InStr(sTmpName, vbNullChar) - 1)
            End If
         End If
      End Function

      Private Sub Command1_Click()
         Dim sTmpFile As String
         Dim sMsg As String
         Dim hFile As Long

         sTmpFile = CreateTempFile("VBT")
         hFile = FreeFile

         Open sTmpFile For Binary As hFile
            Put #hFile, , "This is a test. 1234"
         Close hFile

         sMsg = "Temp FileName: " & sTmpFile & vbCrLf
         sMsg = sMsg & "File Length: " & FileLen(sTmpFile) & vbCrLf
         sMsg = sMsg & "Time Created: " & _
            Format$(FileDateTime(sTmpFile), "long time") & vbCrLf

         MsgBox sMsg, vbInformation, "TempFile"

         Kill sTmpFile
      End Sub

Press the F5 key to run the project. When you click the CommandButton, a temporary file is created. A message box displays information regarding the temp file. The file is then destroyed using the Kill statement.

REFERENCES

For additional information, please see the following articles in the Microsoft Knowledge Base:

Q190000 : HOWTO: Get Started Programming With the Windows API (LONG)

Q137034 : PRB: Win32 GetTempFileName API Differs from 16-bit

Keywords : kbAPI kbKernBase kbSDKWin32 kbVBp kbVBp500 kbVBp600 kbGrpVB kbCodeSam
Version : WINDOWS:5.0,6.0
Platform : WINDOWS
Issue type : kbhowto