linux-kernel - Re: getting mysterious (to me) EINVAL from inotify_rm

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Date:	Wed, 11 May 2016 17:09:59 +0200
From:	Felix von Leitner <felix-linuxkernel@...e.de>
To:	Peter Meerwald-Stadler <pmeerw@...erw.net>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: getting mysterious (to me) EINVAL from inotify_rm_watch

Thus spake Peter Meerwald-Stadler (pmeerw@...erw.net):
> > 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.

> #include <stdio.h>
> #include <sys/inotify.h>
> int main() {
> int fd, i, j;
> printf("init %d\n", fd=inotify_init()); // 3
> printf("add %d\n", i=inotify_add_watch(fd, "/tmp/foo", IN_MODIFY)); // 1
> printf("rm %d\n", inotify_rm_watch(fd, i)); // 0
> printf("add %d\n", j=inotify_add_watch(fd, "/tmp/foo", IN_MODIFY)); // 2
> return 0;
> }

> Ubuntu kernel x86_64 4.4.0-21, seems to work here
> so we have to guess what's going on between _add and _rm?

Oh, it turns out to be my fault.
I called close() on the file first, then did inotify_rm_watch.

It was not clear to me from the documentation that that automatically
removes the inotify watch.

Sorry for the noise,

Felix