linux-kernel - Re: No NONBLOCK flag for dup3() or epoll

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Date:	Mon, 1 Sep 2008 22:07:30 -0700
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Ulrich Drepper" <drepper@...il.com>
Cc:	"Ulrich Drepper" <drepper@...hat.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Al Viro" <viro@...iv.linux.org.uk>,
	lkml <linux-kernel@...r.kernel.org>,
	"Davide Libenzi" <davidel@...ilserver.org>
Subject: Re: No NONBLOCK flag for dup3() or epoll_create1()?

On Mon, Sep 1, 2008 at 5:34 PM, Ulrich Drepper <drepper@...il.com> wrote:
> On Wed, Aug 20, 2008 at 9:49 AM, Michael Kerrisk
> <mtk.manpages@...glemail.com> wrote:
>> I've been doing some testing (2.6.27-rc3) of your new
>> file-descriptor-creating syscalls that accept flags.  According to
>>
>> http://udrepper.livejournal.com/20407.html
>>
>> all of the system calls should accept a *_NONBLOCK flag.
>
> This was never the intention.

All of the below makes some sense, but is at odds with what you posted
on your blog entry, which documents *_NONBLOCK for both
epoll_create1() and dup3().  Thus, I needed to check with you since
the "intention" stated in the blog post differs from the actual
implementation.

>> For epoll_create1() this certainly appears to be an oversight, since I can't
>> think of a reason not to support *_NONBLOCK.
>
> I can.  There is already a method to not block (zero timeout).  We
> should not provide multiple ways to get the same functionality.

Though, one reason to do so would be consistency with the other
changed APIs.  (There are many cases where *ix API provides more than
one way of getting the same functionality.)

> It
> would require changes in the epoll code as well.  That's not worth it.
>
>
>>  (Furthermore, given a open file description for which O_NONBLOCK is
>> already enabled, it is not possible to make a dup3() call that disables
>> O_NONBLOCK, which is an odd asymmetry in the API.)
>
> That's why I didn't add support for the flag.  It's hardly useful to
> add this support.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
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