linux-kernel - Re: [PATCH 1/2] Add a manpage for watch

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Message-ID: <348448.1598291641@warthog.procyon.org.uk>
Date:   Mon, 24 Aug 2020 18:54:01 +0100
From:   David Howells <dhowells@...hat.com>
To:     me@...boeckel.net
Cc:     dhowells@...hat.com, mtk.manpages@...il.com,
        torvalds@...ux-foundation.org, keyrings@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-man@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Add a manpage for watch_queue(7)

Ben Boeckel <me@...boeckel.net> wrote:

> > One loss message.  I set a flag on the last slot in the pipe ring to say that
> > message loss occurred, but there's insufficient space to store a counter
> > without making the slot larger (and I really don't want to do that).
> > 
> > Note that every slot in the pipe ring has such a flag, so you could,
> > theoretically, get a loss message after every normal message that you read
> > out.
> 
> Ah, so a "you lost something" is just a flag on the next event that does
> make it into the queue? I read it as a whole message existed indicating
> that data was lost. Not sure of the best wording here.

No.  That flag is internal.  It causes read() to fabricate a message and
insert it into the user buffer after the flagged message has been copied over.

> > bit 0 is 2^0 in this case.  I'm not sure how better to describe it.
> 
> OK, so the bits are in native-endian order in the enclosing bytes. But C
> just doesn't have a set ABI for bitfields (AFAIK), so I guess it's
> "whatever GCC does" in practice?

Hard to say - powerpc and s390 have bit 0 as the MSB:-/

But "& (1 << 0)" gets you 2^0, whatever the CPU book says.

David