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ReadDirectoryChangesW, if you want it asynch use the correct flag

by mandel on January 17th, 2012

I have had a nice riddle given to me by Tim-Erwin within the comments of Bitten in the ass by ReadDirectoryChangesW and multithreading which I really appreciate. His problem was with the following code:

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import win32file
import win32con
import win32event
import pywintypes
 
FILE_LIST_DIRECTORY = 0x0001
folder = r"C:some folder to watch"
 
handle = win32file.CreateFile (folder, FILE_LIST_DIRECTORY,
              win32con.FILE_SHARE_READ | win32con.FILE_SHARE_WRITE,
              None, win32con.OPEN_EXISTING,
              win32con.FILE_FLAG_BACKUP_SEMANTICS, None)
buffer = win32file.AllocateReadBuffer(1024)
overlapped = pywintypes.OVERLAPPED()
overlapped.hEvent = win32event.CreateEvent(None, 0, 0, None)
flags = win32con.FILE_NOTIFY_CHANGE_FILE_NAME |
                  win32con.FILE_NOTIFY_CHANGE_DIR_NAME |
                  win32con.FILE_NOTIFY_CHANGE_ATTRIBUTES |
                  win32con.FILE_NOTIFY_CHANGE_SIZE |
                  win32con.FILE_NOTIFY_CHANGE_LAST_WRITE |
                  win32con.FILE_NOTIFY_CHANGE_SECURITY
 
win32file.ReadDirectoryChangesW(handle, buffer, False, flags, overlapped)
print "never reached until a change is done in the folder"
win32event.WaitForSingleObject(overlapped.hEvent, 5000)

As Tim-Erwin said in his comments, the above code was not being blocked in the WaitForSingleObject call but in the ReadDirecotryChangesW one. This is clearly my fault because in the blog post I did not give a very important detail that is needed to get the code working async. In order for the above to work correctly it is imperative that you pass to the CreateFile function the FILE_FLAG_OVERLAPPED flag. That means, that in order to fix the code and be blocked in WaitForSingleObject we have to modify the CreateFile call in the following way:

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handle = win32file.CreateFile (folder, FILE_LIST_DIRECTORY,
              win32con.FILE_SHARE_READ | win32con.FILE_SHARE_WRITE,
              None, win32con.OPEN_EXISTING,
              win32con.FILE_FLAG_BACKUP_SEMANTICS | win32file.FILE_FLAG_OVERLAPPED , None)

That fixes Tim-Erwin problem and should help any other person with the same issue. Sorry for not being explicit on such an important detail Tim-Erwin, please accept my apologies :) .

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