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SubjectRe: [PATCH] nextfd(2)
On Fri, Apr 6, 2012 at 12:23 PM, H. Peter Anvin <hpa@zytor.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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