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Date:	Tue, 01 Sep 2009 19:38:44 -0600
From:	Eric Blake <ebb9 AT byu DOT net>
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Subject:	Re: struct dirent.d_reclen
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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
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