C++ Directory Watching - How to detect copy has ended

Question

I have a folder to which files are copied. I want to watch it and process files as soon as they are copied to the directory. I can detect when a file is in the directory, whether through polling (my current implementation) or in some tests using Windows APIs from a few samples I've found online.

The problem is that I detect when the file is first created and its still being copied. This makes my program, that needs to access the file, through errors (because the file is not yet complete). How can I detect not when the copying started but when the copying ended? I'm using C++ on Windows, so the answer may be platform dependent but, if possible, I'd prefer it to be platform agnostic.

Answer 1

You could use either lock files or a special naming convention. The easiest is the latter and would work something like this:

Say you want to move a file named "fileA.txt" When copying it to the destination directory, instead, copy it to "fileA.txt.partial" or something like that. When the copy is complete, rename the file from "fileA.txt.partial" to "fileA.txt". So the appearance of "fileA.txt" is atomic as far as the watching program can see.

The other option as mentioned earlier is lock files. So when you copy a file named "fileA.txt", you first create a file called "fileA.txt.lock". When the copying is done, you simply delete the lock file. When the watching program see "fileA.txt", it should check if "fileA.txt.lock" exists, if it does, it can wait or revisit that file in the future as needed.

Answer 2

You should not be polling. Use FindFirstChangeNotification (http://msdn.microsoft.com/en-us/library/windows/desktop/aa364417%28v=vs.85%29.aspx) to watch a directory for changes.

Then use the Wait functions (http://msdn.microsoft.com/en-us/library/windows/desktop/ms687069%28v=vs.85%29.aspx) to wait on change notifications to happen.

Overview and examples here: http://msdn.microsoft.com/en-us/library/windows/desktop/aa365261%28v=vs.85%29.aspx

I'm not sure how exactly file write completion can be determined. Evan Teran's answer is a good idea.