[Haskell-cafe] inotify-alike for mac os x?

Bryan O'Sullivan bos at serpentine.com
Fri Dec 4 17:30:10 EST 2009

On Fri, Dec 4, 2009 at 8:39 AM, Svein Ove Aas <svein.ove at aas.no> wrote:

> That said.. you say you have to handle the events "fast". What happens
> if you don't?

If you don't handle events quickly, they're typically thrown away by the
kernel without you ever getting to read them. That is, for instance, what
happens on Linux with inotify. Throwing away events means that your app's
internal mirror of the filesystem state becomes wrong, which is Very Bad for
most applications that care. (i.e. Ross's assertion than nothing bad will
happen is generally not true.)

*However*, with inotify you *also* can't afford to perform a single read
system call per event, because that will cause your "watch the filesystem"
event to soak up most of the system's CPU time. So what you have to do is
select to listen for "there's an event ready to be read", then sleep a
little while, *then* read in the hope that many (but not too many!) events
will have been queued that you can all read at once.

And at that point, you'll be getting a stale notification about a file or
directory that may no longer even exist, or may have changed type. Consider:
I create a file f, write data into it, rename it to g, then create a
directory named f. You wake up 10 milliseconds later, and the first event
you hear about is that a file named f was created.

This is all by way of saying that working with filesystem change
notification interfaces is extremely subtle and tricky, enormously more so
than you'd think on casual inspection. It's very easy to write a program
that uses these interfaces in ways that will make it either generate garbage
or consume huge amounts of CPU, and in fact the common case is to write a
program that does both.
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