linux-kernel - Re: dup2() vs dup3() inconsistency when

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Date:	Fri, 10 Oct 2008 13:59:30 +0200
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	7eggert@....de
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"Ulrich Drepper" <drepper@...hat.com>,
	"Al Viro" <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: dup2() vs dup3() inconsistency when

On Fri, Oct 10, 2008 at 1:42 PM, Bodo Eggert <7eggert@....de> wrote:
> Michael Kerrisk <mtk.manpages@...glemail.com> wrote:
>
>> Well, as long as we are fixing the dup3() interface in the way that Al
>> and Ulrich have suggested, what about another fix:
>>
>> give an error if newfd is already open, thus forcing the user to do an
>> explicit close
>>
>> ?
>>
>> This silent close in dup2() is an implementation blemish.  Why not eliminate
>> it?
>
> I think it might be usefull:
> Thread B does some logging to fd 42
> Thread A switches the logfile by creating a new file, writing a header and
> then does dup3(fd, 42, O_WRONLY|O_APPEND|O_CLOEXEC); close(fd);

I don't know the details of the kernel locks here, so perhaps this is
a naive question: but, as things stand is there not the potential for
some nasty race if one thread is writing to fd 42 at the same time as
another thread does a dup2(fd, 42)?

> (Off cause this is not yet implemented, O_RDONLY would give some problems,
>  O_CLOEXEC alone might be better done while open()ing the file, ... but you
>  get the idea.)
>
>
> BTW: I think dup3(fd, -1, flags) should use the file descriptor dup() would
> return. Or should there be a dupf(fd, flags) syscall instead?

If one did this, maybe it would be better to have an extra flag that
said: "use the first free file descriptor >= newfd", thus giving the
more general functionality like fcntl(F_DUPFD).

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
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