File-level filesystem change notification in Mac OS X

Question

I want my code to be notified when any file under (either directly or indirectly) a given directory is modified. By "modified", I mead I want my code to be notified whenever a file's contents are altered, it's renamed, or it's deleted; or if a new file is added. For my application, there can be thousands of files.

I looked as FSEvents, but its Technology Overview says, in part:

The important point to take away is that the granularity of notifications is at a directory level. It tells you only that something in the directory has changed, but does not tell you what changed.

It also says:

The file system events API is also not designed for finding out when a particular file changes. For such purposes, the kqueues mechanism is more appropriate.

However, in order to use kqueue on a given file, one has to open the file to obtain a file descriptor. It's impractical to manage thousands of file descriptors (and would probably exceed the maximum allowable number of open file descriptors anyway).

Curiously, under Windows, I can use the ReadDirectoryChangesW() function and it does precisely what I want.

So how can one do what I want under Mac OS X? Or, asked another way: how would one go about writing the equivalent of ReadDirectoryChangesW() for Mac OS X in user-space (and do so very efficiently)?

Answer 1

I don't believe there's a specific API for what you're looking for. Apple provides sample code for a similar problem called Watcher. It is not what you are looking for, but it is about the best you can do at this point. You have to take snapshots of the directory, and rescan it when you find out something changed. Modification time is the best thing to check of course, if you can trust modification time.

You are probably correct that trying to register for an unbounded number of kqueues would likely be unworkable.

Answer 2

You might want to check out man fs_usage, though it's not specific to a directory and requires root privileges.