Win32 C++ ReadDirectoryChangesW "creation" and "modification" of file difference detect?

Question

Here is the problem: I monitor a directory using Win32 API ReadDirectoryChangesW function. And I need to distinguish between newly created files and modified files. But there are problems... as always :(

Cases:

1. I monitor directory for new/modify (FILE_NOTIFY_CHANGE_FILE_NAME | FILE_NOTIFY_CHANGE_SIZE). Problem: After file creation, new file event + modify file event is triggered. But i need only one. How can I avoid that? When file is modified I get what I want :).
2. I monitor directory only for new file (FILE_NOTIFY_CHANGE_FILE_NAME) - NO PROBLEM.
3. I monitor directory only for modify file (FILE_NOTIFY_CHANGE_SIZE). Problem: When a new file is, modify action is fired along with file creation event. How can I avoid that?

Of course, I implemented some workarounds. But, I want to know if there any elegant way of handling the problems I described.

Thanks in advance!

Answer 1

You should be catching FILE_NOTIFY_CHANGE_LAST_WRITE, not FILE_NOTIFY_CHANGE_SIZE, for a modified file. Files may be modified without the size changing.

You should also keep a queue of changes and the time they happened and only process the queue after there have been no changes in the past 1-2 seconds. Some applications can do very strange things when creating or modifying files, and you'll most likely want to special case for popular applications if you plan on using this code in the wild.

ReadDirectoryChanges isn't one of the friendliest winapi functions. You probably can't get around receiving two events on file creation; I'm not completely sure whether you'll get an extra modify for FILE_NOTIFY_CHANGE_LAST_WRITE on creation, but I think you probably will. Using the queue approach will allow you to easily throw out the extra event if it has the same time stamp as the creation event.