HOWTO: Detect File System Changes under Windows NT

• Microsoft Visual FoxPro for Windows, version 6.0

SUMMARY

Windows NT provides the FindFirstChangeNotification() API function that allows application programs to detect when changes occur in the file system. It can detect when a file or directory is added, changed, deleted, and when a file's size, attributes, or security descriptors change.

NOTE: This code is somewhat complex, and engineers may require some time to familiarize themselves with the contents and concepts of the example.

MORE INFORMATION

1. Create a directory C:\TEST if a directory by that name does not already exist, or modify the variable lcPathSpec in the sample code to point to a directory of your choice.



2. Save and execute the following code. The code appears to 'hang' Visual FoxPro when it executes the call to the WaitForSingleObject() function. This line causes the application to enter a very processor-efficient wait state:

   
         DECLARE LONG WaitForSingleObject IN kernel32 ;
            INTEGER hHandle, ;
            INTEGER dwMilliseconds
   
         DECLARE LONG FindFirstChangeNotification IN kernel32 ;
            STRING lpPathName, ;
            INTEGER bWatchSubtree, ;
            INTEGER dwNotifyFilter AS LONG)
   
         DECLARE LONG FindNextChangeNotification IN kernel32 ;
            INTEGER hChangeHandle
   
         DECLARE LONG GetLastError IN kernel32
         DECLARE LONG FindCloseChangeNotification IN kernel32 ;
            INTEGER hChangeHandle
   
         * Only the second constant is demonstrated in this
         * example; the others are listed for convenience.
   
         * A file has been created, renamed or deleted.
         #DEFINE FILE_NOTIFY_CHANGE_FILE_NAME 0x01
   
         * Directory has been created, renamed or deleted.
         #DEFINE FILE_NOTIFY_CHANGE_DIR_NAME 0x02
   
         * A file's attribute (archive, read-only) has been changed.
         #DEFINE FILE_NOTIFY_CHANGE_ATTRIBUTES 0x04
   
         * A file's size has changed.
         #DEFINE FILE_NOTIFY_CHANGE_SIZE 0x08
   
         * A file's last write data has changed.
         #DEFINE FILE_NOTIFY_CHANGE_LAST_WRITE 0x10
   
         * A security descriptor for a directory or file has been changed.
         #DEFINE FILE_NOTIFY_CHANGE_SECURITY 0x100
   
         * Look for any changes - bitwise OR these values together.
         #DEFINE FILE_NOTIFY_ALL = FILE_NOTIFY_CHANGE_ATTRIBUTES BITOR ;
            FILE_NOTIFY_CHANGE_DIR_NAME BITOR ;
            FILE_NOTIFY_CHANGE_FILE_NAME BITOR ;
            FILE_NOTIFY_CHANGE_LAST_WRITE BITOR ;
            FILE_NOTIFY_CHANGE_SECURITY BITOR ;
            FILE_NOTIFY_CHANGE_SIZE
   
         #DEFINE INVALID_HANDLE_VALUE -1
         #DEFINE INFINITE 2147483647
         #DEFINE WAIT_FAILED 2147483647
         #DEFINE True -1
         #DEFINE False 0
   
         lcPathSpec = "C:\TEST"
         lbWatchSubtree = True
   
         * Watch for a directory added, changed or deleted beneath the
         * specified PathSpec.
         lihNotify = FindFirstChangeNotification(lcPathSpec, lbWatchSubtree, ;
            FILE_NOTIFY_CHANGE_DIR_NAME)
   
         IF lihNotify <= INVALID_HANDLE_VALUE
            MESSAGEBOX ("FindFirstChangeNotification failed. Error code: " + ;
               LTRIM(STR(GetLastError())))
            RETURN
         ELSE
         * Uncomment for debugging purposes.
         *   MessageBox ("FindFirstChangfeNotification instantiation ;
         *       succeeded. Handle is: " + ltrim(str(lihNotify)))
         ENDIF
   
         * Now, go into a wait state.
         * This is going to look like a hang, since the IDE is all in
         * one thread.
         liWaitReturn = WaitForSingleObject(lihNotify, INFINITE)
         IF liWaitReturn >= WAIT_FAILED
             MESSAGEBOX ("WaitForSingleObject failed. Error is: " + ;
                LTRIM(STR(GetLastError())))
            RETURN
         ELSE
             MESSAGEBOX ("Directory change in " + lcPathSpec + " occurred")
         ENDIF
   
         liWaitReturn = FindCloseChangeNotification(lihNotify) 




3. In the Windows NT Explorer, create a new directory underneath C:\TEST, using File/New Folder. As soon as the directory is created, a message box appears in the Visual FoxPro application notifying you of the directory change.

REFERENCES

"Advanced Windows", Jeffrey Richter, Microsoft Press, pages 664-687, Third Edition 1997 ISBN: 1-57231-548-2

Additional query words: kbVFp600 kbAPI

Keywords :
Version : WINDOWS:6.0
Platform : WINDOWS
Issue type : kbhowto