How do I Monitor File's Content For Changes

[James Tieman]

That may not even be the correct question, but here's what I'm trying to do.
Another application is writing a device number into a file.

I have a program that reads the file. The information is used to send a turn-on command to a device. The device automatically turns off after a second, so while the first program continues, every two seconds, the second program has to send the turn-on command to the device until the number in the file changes to another number or it becomes nothing.

Back to the question, what is the best way to monitor the file for change?
I hope this makes sense.
Thanks,
Jamie

[Mike Doty]

I've read that monitoring a directory can be unreliable.
I've monitored a file for many years with a timer and just
compare the current buffer with a GET. There are ways to let
the operating system report it to you, but from reading be sure
that you use a technique that updates the file so the operating
system always catches the change.

http://www.powerbasic.com/support/pbforums/showthread.php?t=22489&highlight=monitor+directory

[Ian Cairns]

Shouldn't this line of reasoning work?

STATIC oldFileTime AS FILETIME ' use this to hold last known write time
LOCAL myFileTime AS FILETIME

  IF Exist(<filespec>, fileInfo) THEN
' -- Convert file time from UTC to local time
    FileTimeToLocalFileTime fileInfo.ftLastWriteTime, myFileTime
' Compare the LastWrittenFileTimes
    retVal  = CompareFileTime(myFileTime, oldFileTime)
    SELECT CASE retVal
      CASE 0
'     ~~~~~~
' Equal, do nothing
      CASE 1
'     ~~~~~~
' Newer filetime - process
     END SELECT
  END IF

'===========
FUNCTION Exist(BYVAL FileSpec AS STRING, fileInfo AS WIN32_FIND_DATA) AS LONG
' - Return true (-1) if filespec exists.
  LOCAL hFind AS DWORD

  IF LEN(FileSpec) THEN
    hFind = FindFirstFile(BYCOPY FileSpec, fileInfo)
    IF hFind <> %INVALID_HANDLE_VALUE THEN
      FindClose hFind
      FUNCTION = %TRUE
    ELSE
      FUNCTION = %FALSE
    END IF
  ELSE
    FUNCTION = %FALSE
  END IF

END FUNCTION

[Michael Mattias]

Win32(SDK): Internet Cookie Monitor April 25, 2001

Works for any folder, not just the cookie folder.

>I've read that monitoring a directory can be unreliable.
You must be spending too much time on the Internet.

[Mike Doty]

The cookie monitor code needs to be rewritten for current compilers.
It might contain code he could use.

[James Tieman]

Do all solutions involve a sleep or wait timer loop to keep checking the file, irregardless of whether it's changed or not?

[Mike Doty]

No.
FindFirstChangeNotification
WaitForMultipleObjects

[Mike Doty]

This was discussed before.

http://www.powerbasic.com/support/pbforums/showthread.php?t=36483&highlight=FindFirstChangeNotification

[Mike Doty]

'If you hear a BEEP a change occurred
#DIM ALL
#INCLUDE "win32api.inc"
GLOBAL gWaitInMilliseconds AS LONG
GLOBAL gDirectoryToWatch AS ASCIIZ * 64
FUNCTION PBMAIN AS LONG
  LOCAL hThread???, var&, h&, x&, lResult&
  gWaitInMilliseconds = 3000
  gDirectoryToWatch  = "\keep\email"
  MKDIR gDirectoryToWatch   'create directory
  ERRCLEAR
  THREAD CREATE WatchDirectory(var&) TO hThread???
  SLEEP 50
  THREAD CLOSE hThread TO lResult
  h = FREEFILE
  OPEN gDirectoryToWatch  + "\TEST" FOR APPEND AS #h
  FOR x = 1 TO 5                                         'write something every second
    PRINT  #h, "MORE"
    IF ERR THEN ? STR$(ERR)
    SLEEP 3000
  NEXT
  CLOSE #h
  gWaitInMilliseconds = 0                                'signal thread to end so program ends
  SLEEP 4000  'give thread time to end
END FUNCTION
FUNCTION WatchDirectory (BYVAL x AS LONG) AS LONG
  LOCAL lDir AS DWORD, found AS DWORD
  DO
    lDir = FindFirstChangeNotification(gDirectoryToWatch, 0, %FILE_NOTIFY_CHANGE_SIZE)
    Found = WaitForSingleObject(lDir, gWaitInMilliseconds)
    IF found = 0 THEN
      'directory changed
      BEEP                      'change occurred
    END IF
    FindCloseChangeNotification(lDir)
  LOOP UNTIL gWaitInMilliseconds=0
  ? "Thread done"
END FUNCTION

[Michael Mattias]

>The cookie monitor code needs to be rewritten for current compilers
A.

'Program to trap and diplay changes to Internet cookies
'   Author: Michael Mattias, Racine WI April, 2001  using PB/DLL v 6.0
'   Idea from Greg D. Engle
'   Cookie Icons courtesy Dan Stasinski and Bob Houle
'   [B]Placed in the public domain by the author April 23, 2001[/B]

B. This whine's time has come

[James Tieman]

I believe that gets me what I'm looking for.
Thanks

[Michael Mattias]

Do all solutions involve a sleep or wait timer loop to keep checking the file, irregardless of whether it's changed or not?

More generally, Windows provides far far more efficient methods of waiting than a loop which checks a condition each time thru.

If you are waiting for 'something to happen' there are specific system objects used by the WaitForSingleObject and/or WaitFor multipleObjects and/or MsgWaitforMultipleObjects functions; see those functions in your WInApi reference to locate all the objects supported. And, you can always assign your own "event" on which to wait, as demo'd here:
Terminate Worker Threads Using Windows Events (and Using Thread Local Storage) Demo Dec 23 2005

To wait for a 'time certain' rather than some 'event', here's a demo of using the system object designed for that purpose:
Waitable Timer Object Demo June 2005

MCM