linux-kernel - Re: [PATCH] nextfd(2)

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Date:	Tue, 10 Apr 2012 20:09:56 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
	drepper@...il.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] nextfd(2)

On Fri, Apr 6, 2012 at 12:23 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 04/06/2012 02:54 AM, Alexey Dobriyan wrote:
>>
>> Without proc knowledge about fdtable is gathered linearly and still unreliable.
>> With nextfd(2), even procful environments could lose several failure branches.
>> And they can keep old dumb fd++ or smart /proc/self/fd loops for a change.
>>
>
> Incidentally, if we were to create a system call for this -- which I so
> far see no reason for -- I would make it return a select-style bitmask
> of file descriptors in use, not a "next fd" which would require a system
> call per iteration.

I know the reason. fcntl(F_NEXT) is one of a proposal of next SUS enhancement.

  http://austingroupbugs.net/view.php?id=149

nextfd() has a semantics of F_NEXT.

Next, why shoundn't we implement fcntl(F_NEXT) in our kernel? I think
we have two reason.

1) As linus pointed out, linux specific "flags" argument may be useful.
2) The name of F_NEXT is not fixed yet. another url of the austin says
it is FD_NEXT.
     So, we can't choose right name yet. Moreover, A meanings of 3rd
argument of F_NEXT
     haven't been fixed.


I dont think following #ifdef is insane. but glibc also can provide
correct F_NEXT when next SUS is published.

#ifdef FOO
#define NEXTFD(fd) nextfd(fd, flags)
#else
#define NEXTFD(fd) fcntl(fd, F_NEXT, O_FDWR)
#endif


In short, kernel developer don't care any standard at all. but
application programmer usually care it.
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