linux-kernel - Re: PROPOSAL: extend pipe() to support NULL argument.

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Message-ID: <20090703015554.GB5880@cr0.nay.redhat.com>
Date:	Fri, 3 Jul 2009 09:55:54 +0800
From:	Amerigo Wang <xiyou.wangcong@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Amerigo Wang <xiyou.wangcong@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PROPOSAL: extend pipe() to support NULL argument.

On Fri, Jul 03, 2009 at 08:43:39AM +0800, Changli Gao wrote:
>On Thu, Jul 2, 2009 at 6:21 PM, Amerigo Wang<xiyou.wangcong@...il.com> wrote:
>> On Thu, Jul 02, 2009 at 06:04:02PM +0800, Changli Gao wrote:
>>>On Thu, Jul 2, 2009 at 5:46 PM, Amerigo Wang<xiyou.wangcong@...il.com> wrote:
>>>>>Are you familiar with splice() and tee()? They both use pipes as kernel buffers.
>>>>
>>>> You are not answering the question, obviously.
>>>When you use pipes as kernel buffer handlers, two fd isn't necessary.
>>>Using one will save half of fd resources. Is it obviously?
>>
>> Not really.. I can't see any reason why you use this method to save
>> fd's... pick read(2)/write(2).
>In order to use splice, you must have pipe first. The common code is:
>
>int fd[2];
>
>pipe(fd);
>splice(infd, NULL, fd[1], NULL, getpagesize(), SPLICE_F_MOVE);
>splice(fd[0], NULL, outfd, NULL, getpagesize(), SPLICE_F_MOVE);
>close(fd[0]);
>close(fd[1]);
>
>if pipe supports NULL argument, and returnes a RW pipe fd in that
>case, the code will become:
>
>int fd;
>
>fd = pipe(NULL);
>splice(infd, NULL, fd, NULL, getpagesize(), SPLICE_F_MOVE);
>splice(fd, NULL, outfd, NULL, getpagesize(), SPLICE_F_MOVE);
>close(fd);
>
>One fd is saved.


What is a pipe in your mind? A pipe has two points, obviously,
you are making it a single point, thus you can't call it a pipe
any more...

I don't like this.

And just as what you showed above, you can close the fd's
after finishing, thus you only save one fd during these
two splice(), not useful...


>>
>> You are trying to break it...
>>
>
>It depends on the argument for pipe, if the argument is NULL, it
>breaks the code above.
>

Sure, malloc(2) can return NULL.

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