Detecting file changes on macOS with kqueue • Hacker News Board

Re: Detecting file changes on macOS with kqueue

#11

Post by feznyng » Sun, Mar 29, 2026, 12:31 AM UTC

lzhgusapp wrote: post #3

Nice writeup. I work on native macOS utilities and have dealt with file monitoring quite a bit. kqueue is reliable but the per-file descriptor requirement can get tricky when you need to watch large directory trees. For anyone looking at this space, FSEvents is the higher-level alternative Apple provides. It watches directory-level changes without needing a file descriptor per file, which scales better for broad moni…

Can you treat kqueue as infallible? I've found FSEvents sometimes drops events at high volume (unless I'm misunderstanding how to use it).

Re: Detecting file changes on macOS with kqueue

#12

Post by saagarjha » Sun, Mar 29, 2026, 12:51 AM UTC

lzhgusapp wrote: post #3

Nice writeup. I work on native macOS utilities and have dealt with file monitoring quite a bit. kqueue is reliable but the per-file descriptor requirement can get tricky when you need to watch large directory trees. For anyone looking at this space, FSEvents is the higher-level alternative Apple provides. It watches directory-level changes without needing a file descriptor per file, which scales better for broad moni…

NSFilePresenter is an even higher level API and the one I typically recommend to people since it’s very easy to use

Re: Detecting file changes on macOS with kqueue

#13

Post by chasil » Sun, Mar 29, 2026, 1:21 AM UTC

In Linux, this is done with inotify.

Path units in systemd present inotify, but their use is somewhat constrained.

The incron utility is more flexible.

Re: Detecting file changes on macOS with kqueue

#14

Post by kushalpandya » Sun, Mar 29, 2026, 4:03 AM UTC

This looks interesting and I might give it a try. I did look at FSEvents in the past but due to my needs in macOS app, I took a very niche (and not so fast) approach of running find | ls -lT | shasum spawned in a separate process to reliably tell if a folder has changed.

Re: Detecting file changes on macOS with kqueue

#15

Post by steve1977 » Sun, Mar 29, 2026, 5:40 AM UTC

feznyng wrote: post #11

lzhgusapp wrote: post #3

Nice writeup. I work on native macOS utilities and have dealt with file monitoring quite a bit. kqueue is reliable but the per-file descriptor requirement can get tricky when you need to watch large directory trees. For anyone looking at this space, FSEvents is the higher-level alternative Apple provides. It watches directory-level changes without needing a file descriptor per file, which scales better for broad moni…

Can you treat kqueue as infallible? I've found FSEvents sometimes drops events at high volume (unless I'm misunderstanding how to use it).

That's the exciting thing about macOS development, right? There's always some funny riddles because Apple doesn't care to document stuff for 3rd parties (or at all).

Re: Detecting file changes on macOS with kqueue

#16

Post by jvican » Sun, Mar 29, 2026, 6:08 AM UTC

I personally like and use watchexec [1] to run commands on repeat.

Syntax example: `watchexec -r -e py uv run pytest -xvvs file.py`

[1]: https://github.com/watchexec/watchexec

Re: Detecting file changes on macOS with kqueue

#17

Post by hrmtst93837 » Sun, Mar 29, 2026, 8:22 AM UTC

saagarjha wrote: post #12

lzhgusapp wrote: post #3

Nice writeup. I work on native macOS utilities and have dealt with file monitoring quite a bit. kqueue is reliable but the per-file descriptor requirement can get tricky when you need to watch large directory trees. For anyone looking at this space, FSEvents is the higher-level alternative Apple provides. It watches directory-level changes without needing a file descriptor per file, which scales better for broad moni…

NSFilePresenter is an even higher level API and the one I typically recommend to people since it’s very easy to use

[flagged]

Re: Detecting file changes on macOS with kqueue

#18

Post by hilsdev » Sun, Mar 29, 2026, 5:19 PM UTC

This style/format is really easy to follow along with, great blog writing. Separately I’ve been messing with some filesystem stuff so it’s interesting to me.

Re: Detecting file changes on macOS with kqueue

#19

Post by v3gas » Sun, Apr 05, 2026, 6:05 AM UTC

hilsdev wrote: post #18

This style/format is really easy to follow along with, great blog writing. Separately I’ve been messing with some filesystem stuff so it’s interesting to me.

Thank you!!