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

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Date:	Fri, 3 Jul 2009 08:43:39 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Amerigo Wang <xiyou.wangcong@...il.com>
Cc:	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 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.

>
>>
>>>
>>> And you snipped too much, how can you return that fd? Using the return value?
>>one RW file descriptor is returned. I have answered this in the first post.
>
> No, you never say *how* it is returned.
return value. save in %eax in i386.

>
>>
>>> Ah! This will probably break the user-space program...
>>>
>>I don't think so. As a skillful programmer, who will trasfter pipe() a
>>NULL pointer? In any way, it is break sth, but not very seriously, and
>>won't affact any right and robust program.
>
> Huh? Isn't the code sample below too common?
>
> if (pipe(...))
>   perror("pipe");
>
> Currently pipe(2) can make sure this is robust.
>
> You are trying to break it...
>

It depends on the argument for pipe, if the argument is NULL, it
breaks the code above.



-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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