From: Garance A Drosihn <drosih@...>
Date: 2002-03-19T18:07:30+09:00
Subject: Re: dots in Dir.entries

At 4:22 PM +0900 3/19/02, Yukihiro Matsumoto wrote:
>Hi,
>
>Could you summarize pros and cons for filtering
>dot files (. and ..) from Dir.entries and Dir.each,
>please?

Matt Armstrong already replied with a fair summary
of what was said in the earlier thread.  Or at least,
what I saw of that thread.

I am one of those who think the '.' and '..' should
show up in ruby if the operating system routines would
show them.  For those file systems which do have them,
then some scripts use them for a useful optimization
trick.

On a filesystem which does have these, a stat() call
for any directory will always show at least two "links"
to it.  Those links come from the '.' and '..'.  There
is one additional link for each subdirectory (the '..'
of that subdirectory).  Files in the directory do not
increase the link count.  If a script is going thru a
directory looking for all of its sub-directories, it
can keep a count of how many directories it has already
seen (including '.' and '..').  When that count is the
same as the link count, the script will know that it has
found all the sub-directories.  It can therefore skip
reading all the remaining entries in the directory,
because they must be files.

When playing with directories that have hundreds or
thousands of files, this can result in avoiding a huge
amount of unnecessary work.

If ruby always filters out the '.' and '..', then there
is no way for a script to perform this trick, because
there will be no way for the script to know when the
'.' and '..' are processed.  If ruby does not filter out
the '.' and '..', then it is trivial for the script to
do that filtering.

[I think there is at least one more example where it is
useful to explicitly get the '.' and '..', but I do not
remember it at the moment]

I agree that is a little annoying to have to filter those
when you do not want them (which is most of the time), but
there are situations where it is important that they are
not filtered out.  I would not mind if this was a different
method name, but I do think it needs to be available.

-- 
Garance Alistair Drosehn            =   gad@eclipse.acs.rpi.edu
Senior Systems Programmer           or  gad@freebsd.org
Rensselaer Polytechnic Institute    or  drosih@rpi.edu