win32 How to make sure a file is completely written?
justin.donato at gmail.com
Mon May 11 10:12:12 EDT 2009
On May 11, 10:03 am, Tim Golden <m... at timgolden.me.uk> wrote:
> justind wrote:
> > Hello,
> > I'm usinghttp://code.activestate.com/recipes/156178/to watch a
> > folder in windows.
> Wow, that takes me back. There's a bit more info (and a different
> technique) here if you're interested:
> But to come back your question...
> > It's working perfectly, but sometimes when I try to
> > open the file immediately after receiving the event, it's not ready to
> > be opened--if I try to open it with PIL I get "IOError: cannot
> > identify image file" and if I try it with a text file, it's empty.
> > This doesn't happen all the time, just occasionally. I think the
> > problem is that the file isn't completely written because if I make
> > the script sleep for a second, it works every time. But that doesn't
> > seem very elegant or robust.
> > What's the proper way to make sure the file is ready to be read?
> I don't believe there's an easy answer to this question. From the
> filesystem's point of view some process creates a file. So it
> tells you that a file has been created. The filesystem has no
> knowledge of when a file is "complete". It could -- altho' it
> doesn't -- fire an event when a handle on that file is closed,
> but even that wouldn't tell you much: you'd have to be able
> to distinguish between the handle which is behind the data
> you're looking for and a handle from a virus scanner which is
> checking the new file for malware.
> To add difficulty, when Windows initiates a file copy (as opposed
> to creating a new file) it immediately creates the file at its
> full size and then copies data in. Which makes some sense as it
> should avoid some element of fragmentation etc. But it removes
> one of your options: noting the size of an original file and
> comparing it with the size of a copy.
> So I think you're out of luck and you're going to have to
> put a try-except loop in place.
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