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

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Message-ID: <20090703081655.GH5880@cr0.nay.redhat.com>
Date:	Fri, 3 Jul 2009 16:16:55 +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 03:40:34PM +0800, Changli Gao wrote:
>On Fri, Jul 3, 2009 at 3:15 PM, Amerigo Wang<xiyou.wangcong@...il.com> wrote:
>>
>> If saving one fd really helps here, probably you want to
>> save more, you will need a syscall like:
>>
>>  int splice_without_new_fd(int infd, int outfd);
>Do you know sendfile()? Its current implementation is buggy, and will
>be blocked on outfd. Anyway, the above code is just a use case, there
>are other cases sendfile can't cover.

So what? So you should fix it intead of inventing a new pipe() and use
splice(2)...

Wait... if splice(2) doesn't block, what is your point for saving
an fd in your code? You can do:

int splice_two_fd(int fd1, int fd2)
{
	int fds[2];
	pipe(fds);
	splice(fd1, fds[0]);//not block
	splice(fds[1], fd2);//not block
	close(fds[0]); //can be closed soon
	close(fds[1]); //ditto
}

Outside this function no new fd's are used.

>
>>
>> But splice(2) is designed to be as it is. You need to increase
>> your fd limit, instead of saving one by pipe().
>>
>I don't agree with you. We should save resource as much as we can, and
>not work around it.

You are saying splice(2) is wrong? Because it is splice(2) who
needs 3 fd's finally.
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