As far as I remember MacOSX has a better support for monitoring file
system changes than Linux. In fact I remember reading a paper stating
it was unacceptable linux had no proper support for this while mac osx
has it.

You should probably have a look at the following page :
http://www.osxbook.com/software/fslogger/
Describing spotlight : "An in-kernel notification mechanism that can
inform user-space "subscribers" of file system changes as they
happen."
And about fslogger : "fslogger is a user-space program that subscribes
to the same file system event notification mechanism as the Spotlight
metadata server."

You'll probably need to embed C code in ocaml.

As for cross-platform solutions, hum, good luck...


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Adrien Nader