Microsoft KB Archive/172240

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HOWTO: Determine If a File Is Already Open

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Q172240

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The information in this article applies to:


 * Microsoft Win32 Application Programming Interface (API), included with:
 * Microsoft Visual Basic Learning Edition for Windows, version 5.0
 * Microsoft Visual Basic Professional Edition for Windows, version 5.0
 * Microsoft Visual Basic Enterprise Edition for Windows, version 5.0

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SUMMARY
You can open a file multiple times in different applications as long as one application does not have exclusive access to the file. There may be times when your application needs to have exclusive access to a file, and the application may need to determine whether it is already open. This article explains how to determine whether a file is already open. It also contains two versions of a function you can use to detect whether a file is already open.

MORE INFORMATION
The only sure way your application can determine whether it can open a file exclusively is by trying to open the file. If you are able to open the file, your application has exclusive access. Otherwise, it doesn't.

If your application needs to determine whether a file is already open, it should attempt to open the file for exclusive-sharing access. If a sharing violation occurs during the attempt, the file is already open. If the attempt causes a different error, such as access denied or file not found, then your application will not be able to tell whether the file is already open.

The function described here is called IsFileAlreadyOpen, declared as:

  BOOL IsFileAlreadyOpen(char *filename); // In C/C++...   Function IsFileAlreadyOpen(Filename As String) As BOOLEAN // In VB... If this function returns TRUE, then the file is already open. It determines that the file is already open only if it gets ERROR_SHARING_VIOLATION when it attempts to open the file.

It will return FALSE in the following conditions:


 * The file is not already open.
 * The calling application does not have access to the file due to NTFS security.
 * The file does not exist.

You should check if the file exists before calling the function to eliminate the possibility of the latter condition.

Sample Code: C/C++ version of IsFileAlreadyOpen
 BOOL IsFileAlreadyOpen(char *filename) {     HFILE theFile = HFILE_ERROR; DWORD lastErr = NO_ERROR;

// Attempt to open the file exclusively. theFile = _lopen(filename, OF_READ | OF_SHARE_EXCLUSIVE);

if (theFile == HFILE_ERROR) // Save last error... lastErr = GetLastError; else // If the open was successful, close the file. _lclose(theFile);

// Return TRUE if there is a sharing-violation. return ((theFile == HFILE_ERROR) &&             (lastErr == ERROR_SHARING_VIOLATION)); }

Sample Code: Microsoft Visual Basic version of IsFileAlreadyOpen
  ' Declaration for APIs used by our function... Private Declare Function lopen Lib "kernel32" Alias "_lopen" (ByVal  lpPathName As String, ByVal iReadWrite As Long) As Long Private Declare Function GetLastError Lib "kernel32" As Long Private Declare Function lclose Lib "kernel32" Alias "_lclose" (ByVal  hFile As Long) As Long

' Our Function... Function IsFileAlreadyOpen(Filename As String) As Boolean Dim hFile As Long Dim lastErr As Long

' Initialize file handle and error variable. hFile = -1 lastErr = 0

' Open for for read and exclusive sharing. hFile = lopen(Filename, &H10)

' If we couldn't open the file, get the last error. If hFile = -1 Then lastErr = Err.LastDllError ' Make sure we close the file on success. Else lclose (hFile) End If

' Check for sharing violation error. If (hFile = -1) And (lastErr = 32) Then IsFileAlreadyOpen = True Else IsFileAlreadyOpen = False End If

End Function