netdev - Re: some question about "/sys/class/net/<iface>/operstate"

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Date:   Wed, 11 Nov 2020 12:34:51 -0800
From:   Erik Kline <ek@...n.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        杜英杰 <leondyj@....edu.cn>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: some question about "/sys/class/net/<iface>/operstate"

On Wed, Nov 11, 2020 at 11:55 AM Stephen Hemminger
<stephen@...workplumber.org> wrote:
>
> On Wed, 11 Nov 2020 11:02:14 -0800
> Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> > On Tue, Nov 10, 2020 at 8:32 PM 杜英杰 <leondyj@....edu.cn> wrote:
> > >
> > > I want to use inotify to monitor /sys/class/net//operstate  to detect status of a iface in real time.
> > > when I ifdown &amp;&amp; ifup eth3, the content of operstate changed, but the file's Modify time didn't change.
> > > I don't know the reason, is there any file which can be monitored by inotify to get iface status in real time?
> > > Much appreciation for any advice!
> >
> > You need to listen to netdev netlink messages for changes like
> > this. These messages are generated in real-time.
>
> The /sys and /proc are pseudo-filesystems. The file modify time and inotify do not work
> as expected on these files. Cong is right you need to use netlink for this.

Related question: could/should modify time and/or inotify be made to
work?  I genuinely don't know if that would even be possible (separate
from "desirable").