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Date: Tue, 12 Jul 2016 11:41:40 +0200
From: Joakim Sindholt <>
Subject: Re: nftw miscalculates FTW.base when pathnames end in /

On Sat, May 14, 2016 at 5:01 AM, Rich Felker <> wrote:
> lev.base is computed separately depending on whether or not there's a
> previous history; for the no-history case, it should be computed by
> skipping all trailing slashes, then backing up until the previous
> slash or beginning of string, whichever is hit first. But rather than
> special-casing !h in do_nftw, the top-level nftw function should
> probably set up a fake "-1 level" history structure to pass in. This
> gathers the special top-level logic all in one place.

I had a look at copying the entire path over and adjusting the first
.base accordingly. The problem then became that the root dir would come
out as being named something////// depending on how many slashed I
added. This runs counter to the glibc behavior which also just hacks off
the slashes. In the end I think it's cleaner to just remove trailing
slashes than to have them included in all but the root dir.

As per your recommendation I also set up a fake -1 history level and
removed the special casing for !h, but I'm unsure what default values
should be used for dev and ino.

Then, as we talked about on IRC, I tried to reduce the stack usage of
do_nftw, which was 296 B when all was said and done. By moving a lot of
the context stuff explicitly into nftw it got all the way down to 88 on
clang and 104 on gcc. Depending on a few minor details those two numbers
would switch around. I'm not sure why. In any case it's a third of what
it was and I would consider that a substantial win.
The code isn't exactly beautiful though.

View attachment "0002-optimize-nftw-for-stack-usage.patch" of type "text/x-patch" (4930 bytes)

View attachment "0001-ignore-trailing-slashes-in-nftw-path.patch" of type "text/x-patch" (2036 bytes)

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