X-Recipient: archive-cygwin AT delorie DOT com
X-SWARE-Spam-Status: No, hits=-1.9 required=5.0 	tests=AWL,BAYES_00,SPF_SOFTFAIL
X-Spam-Check-By: sourceware.org
Message-ID: <4A9DCCA4.9070808@byu.net>
Date: Tue, 01 Sep 2009 19:38:44 -0600
From: Eric Blake <ebb9 AT byu DOT net>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.23) Gecko/20090812 Thunderbird/2.0.0.23 Mnenhy/0.7.6.666
MIME-Version: 1.0
To: cygwin AT cygwin DOT com
Subject: Re: struct dirent.d_reclen
References: <loom DOT 20090901T193145-508 AT post DOT gmane DOT org> <20090901233750 DOT GA32145 AT ednor DOT casa DOT cgf DOT cx> <4A9DC5FC DOT 2070202 AT byu DOT net>
In-Reply-To: <4A9DC5FC.2070202@byu.net>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
X-IsSubscribed: yes
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
List-Id: <cygwin.cygwin.com>
List-Subscribe: <mailto:cygwin-subscribe AT cygwin DOT com>
List-Archive: <http://sourceware.org/ml/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-help AT cygwin DOT com>, <http://sourceware.org/ml/#faqs>
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
Delivered-To: mailing list cygwin AT cygwin DOT com

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

According to Eric Blake on 9/1/2009 7:10 PM:
> According to Christopher Faylor on 9/1/2009 5:37 PM:
>> Maybe you mean d_namlen?
> 
> Yes; serves me right for confusing readdir(2) and readdir(3) man pages.

Actually, it looks like Linux has only d_reclen (and that d_namlen is a
documentation bug):

http://www.kernel.org/doc/man-pages/online/pages/man3/readdir.3.html

and that Linus prefers d_reclen over d_namlen from the kernel side of
things (where, in the Linux kernel, d_reclen is always aligned, such that
adjusting d_reclen by offsetof(struct dirent,d_name) can be larger than
strlen(d_name)):

http://lkml.indiana.edu/hypermail/linux/kernel/9506/0033.html

So Linux doesn't provide d_namlen, and coreutils can't optimize for known
lengths on Linux.  But BSD does:

http://www.gsp.com/cgi-bin/man.cgi?section=5&topic=dir

At any rate, coreutils uses a macro _D_EXACT_NAMELEN(dirent), which
evaluates to either d_namlen, a calculation on d_reclen (if d_reclen is
accurate enough*), or a call to strlen() if all else fails; so that it is
portable to whichever the underlying semantics happen to be.

* If d_reclen minimally rounds up to an aligned size, for example if it is
only at most 8 bytes larger than strlen(d_name), then it is still faster
to do a strchr(,0)-d_name from a starting point 8 bytes before where
d_reclen says the record ends, rather than strlen(d_name[0]).

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9 AT byu DOT net
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.9 (Cygwin)
Comment: Public key at home.comcast.net/~ericblake/eblake.gpg
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/

iEYEARECAAYFAkqdzKQACgkQ84KuGfSFAYBtsQCgvYcI8Y7CLJOYxPKIySgwCpJn
dvAAoIVO47y0+F24lGktxBAF6gbj0rlh
=Zctr
-----END PGP SIGNATURE-----

--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple