File locking is impossible in Windows?

[Roger Upole]

I get

The process cannot access the file because another process has locked a
portion of the file.
        0 file(s) copied.

on both Win2k and WinXP.  (Python 2.4, Pywin32 build 203)
Are you sure the log.txt you're copying to is actually the right one ?  You
should at least get a
prompt to confirm overwrite on the command line.

      Roger



"Pekka Niiranen" <pekka.niiranen at wlanmail.com> wrote in message
news:41c899ba$0$15097$39db0f71 at news.song.fi...
> Hi,
>
> I have used the following example from win32 extensions:
>
> -----SCRIPT STARTS----
>
> import win32file
> import win32con
> import win32security
> import pywintypes
>
> class Flock:
>      def __init__(self,file):
> self.file=file
> secur_att = win32security.SECURITY_ATTRIBUTES()
> secur_att.Initialize()
> self.highbits=-0x7fff0000
> self.hfile=win32file.CreateFile( self.file,\
> win32con.GENERIC_READ|win32con.GENERIC_WRITE,\
> win32con.FILE_SHARE_READ|win32con.FILE_SHARE_WRITE,\
> secur_att, win32con.OPEN_ALWAYS,\
> win32con.FILE_ATTRIBUTE_NORMAL , 0 )
> def lock(self):
> lock_flags=win32con.LOCKFILE_EXCLUSIVE_LOCK|\
> win32con.LOCKFILE_FAIL_IMMEDIATELY
> self.ov=pywintypes.OVERLAPPED()
> win32file.LockFileEx(self.hfile,lock_flags,0,\
> self.highbits,self.ov)
> def unlock(self):
> win32file.UnlockFileEx(self.hfile,0,\
> self.highbits,self.ov)
> self.hfile.Close()
>
> if __name__ == '__main__':
> from time import time, strftime, localtime
> import sys
>
> l=Flock("e:\\\\log.txt")
> print 'calling lock'
> l.lock()
> print "Now locked.  Hit enter to release lock."
> dummy = sys.stdin.readline()
>
> l.unlock()
> print 'now unlocked'
>
> -----SCRIPT ENDS----
>
> If I start one python process from dos window I get message:
>
> E:\>python lockker.py
> calling lock
> Now locked.  Hit enter to release lock.
>
> All well, now if
>
> 1) I start another Dos -shell and run the same command I get:
>
> E:\>python lockker.py
> calling lock
> Traceback (most recent call last):
>   File "lockker.py", line 35, in ?
>     l.lock()
>   File "lockker.py", line 23, in lock
>    win32file.LockFileEx(self.hfile,lock_flags,0,\
>    self.highbits,self.ov)
> pywintypes.error: (33, 'LockFileEx',\
> 'The process cannot access the file because\
> another process has locked a portion of  the file.')
>
> Which is correct.
>
> 2) I try to read the contents of the file from Dos -shell, I get:
> E:\>type log.txt
> The process cannot access the file because another\
> process has locked a portion of the file.
>
> This is correct.
>
> 3) When I open the file into notepad.exe I can edit the screen
> but not write changes to disk. Correct again!
>
> 4) I cannot delete the file from Dos shell or from W2K explorer
> which is correct.
>
> 5) However, I can overwrite the file over with:
> E:\>copy d:\log.txt log.txt
>          1 file(s) copied.
>
> Which is WRONG as is me being able to copy another file over it
> with W2K explorer too.
>
>
> Is there a way around this? How can I stop file being COPIED OVER while
> it is being open? Is this window's feature? Is readlines() operation
> "atomic" enough for me not to worry about these issues?
>
> My python script modifies set of files from a directory one by one.
> I try to lock them all exclusively for the script
> until all are modified. If one of the files gets overwritten
> by another version (by another process) the script may fail.
>
> -pekka-
>
>
>
>
>
>
>




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