by Jerry Barnett (2 Submissions)
Category: Files/File Controls/Input/Output
Compatability: Visual Basic 3.0
Difficulty: Intermediate
Date Added: Wed 3rd February 2021
Rating: 
(4 Votes)
Here is a useful function I created to solve a
problem I was having in processing files for
one of my applications.
This function works better than trying to use
the VB 'OPEN' command because it will always
return the correct state of the file (even on a
file that is being FTP'd at the time of the
test.) See remarks in the FileReady function on
it's use, as well as a sample at the end of this
note.
API DeclarationsPublic Const SHARE_EXCLUSIVE = &H0
Public Const INVALID_HANDLE_VALUE = -1
Public Const ERROR_ALREADY_EXISTS = 183&
Public Const OPEN_EXISTING = 3
Public Const FILE_ATTRIBUTE_NORMAL = &H80
Public Const GENERIC_READ = &H80000000
Public Type SECURITY_ATTRIBUTES
nLength As Long
lpSecurityDescriptor As Long
bInheritHandle As Long
End Type
Public Declare Function GetLastError Lib "kernel32" () As Long
Public Declare Function CreateFile _
Lib "kernel32" Alias "CreateFileA" _
(ByVal lpFileName As String, _
ByVal dwDesiredAccess As Long, _
ByVal dwShareMode As Long, _
lpSecurityAttributes As SECURITY_ATTRIBUTES, _
ByVal dwCreationDisposition As Long, _
ByVal dwFlagsAndAttributes As Long, _
ByVal hTemplateFile As Long) As Long
Public Declare Function CloseHandle _
Lib "kernel32" (ByVal hObject As Long) As Long
VBcodersPrivate Function FileReady(strFileName As String) As Boolean
'***********************************************
' * Programmer Name : Jerry Barnett
' * Procedure Name : FileReady
' * Parameters : strFileName As String -
' * Filename to check
' * Returns : TRUE - if the file exists and
' * is not in use
' * by any other process.
' * FALSE - if the file is in use by
' * another process, or does
' * not exist.
'***********************************************
' * Comments : This function checks to
' * see if a file is ready for use. It
' * tries to open the file for
' * exclusive use.
' *
' * NOTE - An example of where this
' * function would be used is as
' * follows:
' * You have an application that needs
' * to process files as they are
' * created in a directory. However
' * since they could be large files
' * you don't want to start
' * processing the file before it
' * is completely copied (or FTP'd)
' * into the directory. This function
' * will determine if the copy or FTP
' * is complete so that you can then
' * open the file for processing.
'************************************
' * The following Constants and
' * Declares must be placed in the
' * Module DECLARES section.
'************************************
' *
' * Public Const SHARE_EXCLUSIVE = &H0
' * Public Const INVALID_HANDLE_VALUE = -1
' * Public Const ERROR_ALREADY_EXISTS = 183&
' * Public Const OPEN_EXISTING = 3
' * Public Const FILE_ATTRIBUTE_NORMAL = &H80
' * Public Const GENERIC_READ = &H80000000
' *
' * Public Type SECURITY_ATTRIBUTES
' * nLength As Long
' * lpSecurityDescriptor As Long
' * bInheritHandle As Long
' * End Type
' *
' * Public Declare Function GetLastError _
' * Lib "kernel32" () As Long
' *
' * Public Declare Function CreateFile Lib _
' * "kernel32" Alias "CreateFileA" _
' * (ByVal lpFileName As String, _
' * ByVal dwDesiredAccess As Long, _
' * ByVal dwShareMode As Long, _
' * pSecurityAttributes As SECURITY_ATTRIBUTES, _
' * ByVal dwCreationDisposition As Long, _
' * ByVal dwFlagsAndAttributes As Long, _
' * ByVal hTemplateFile As Long) As Long
' *
' * Public Declare Function CloseHandle Lib _
' * "kernel32" (ByVal hObject As Long) As Long
' *
'************************************************
Dim lReturnCode As Long
Dim typAtrib As SECURITY_ATTRIBUTES
' Try to open the file for exclusive use
lReturnCode = CreateFile(strFileName, _
GENERIC_READ, _
SHARE_EXCLUSIVE, _
typAtrib, _
OPEN_EXISTING, _
FILE_ATTRIBUTE_NORMAL, 0)
If lReturnCode = INVALID_HANDLE_VALUE Then
' Failed exclusive use of file (File not ready)
FileReady = False
Exit Function ' Exit function
End If
' File exists and is ready, so close the file
lReturnCode = CloseHandle(lReturnCode)
' Return True (File is Ready)
FileReady = True
End Function
'************************************************
' A Sample of how to use this function:
Private Sub Main()
Dim lCount as Long
Dim Const MAXCOUNT = 5 ' Actually this would be in
' the module declares section
Do While Not FileReady("FileToCheckFor.txt") Then
lCount = lCount + 1
' ...... wait some predetermined amount
' of time .....
If lCount = MAXCOUNT Then
Msg "File Not Ready! Maximum try's exceeded!"
End
End If
Loop
Msg "File can now pe processed!"
' .... Do your processing code to work
' with the file.
End Sub
