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

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Date:	Thu, 09 Oct 2008 13:43:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ulrich Drepper <drepper@...hat.com>
CC:	mtk.manpages@...il.com, Al Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: dup2() vs dup3() inconsistency when

Ulrich Drepper wrote:
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> 
> H. Peter Anvin wrote:
>> The dup2() behavior comes from the logical consequence of dup2()'s
>> "close on reuse"; one would think it would be logical for dup3() to
>> behave the same way.
> 
> No.  We deliberately decided on this change.  Otherwise, what is the
> result of dup3(fd, fd, O_CLOEXEC)?  There is no reason to use
> dup2(fd,fd), so why the hell somebody wants to defend this is beyond me.
> 

The result of dup3(fd, fd, O_CLOEXEC) is to set the O_CLOEXEC flag on fd.

The behaviour of dup2() is functionally the following:

1. Duplicate the file descriptor from file_table[oldfd].
2. If file_table[newfd] is in use, close it.
3. Install the duplicate file descriptor at file_table[newfd].

Step (2) could be considered a bit dubious, but the behaviour of 
dup2(fd, fd) is a direct consequence of the chosen semantics.

	-hpa
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