getting mysterious (to me) EINVAL from inotify_rm_watch

[Felix von Leitner <felix-linuxkernel@fefe.de>]

Hi,

I am trying to add inotify support to my tail implementation (for -F).
This is what happens:

  inotify_init()                          = 4
  inotify_add_watch(4, "/tmp/foo", IN_MODIFY) = 1
  inotify_rm_watch(4, 1)                  = -1 EINVAL (Invalid argument)
  inotify_add_watch(4, "/tmp/foo", IN_MODIFY) = 2

There is also some polling, some reading and some statting going on here, but
those are on other descriptors than 4 so they should not matter).

Can somebody explain the EINVAL I'm getting from inotify_rm_watch to me?

This is a stock kernel 4.5.0.

Thanks,

Felix