GNU bug report logs - #21942
Files with incorrect file sizes

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Message #11 received at 21942 <at> debbugs.gnu.org (full text, mbox):

From: Stephan Müller <fruktopus <at> gmail.com>
To: Jim Meyering <jim <at> meyering.net>
Cc: 21942 <at> debbugs.gnu.org
Subject: Re: [bug-diffutils] bug#21942: Files with incorrect file sizes
Date: Fri, 20 Nov 2015 21:06:54 +0100

Am Fri, 20 Nov 2015 18:28:37 +0100
schrieb Jim Meyering <jim <at> meyering.net>:

> On Tue, Nov 17, 2015 at 12:44 PM, Stephan Müller
> <fruktopus <at> gmail.com> wrote:
> > recently I had to debug weird problem. Finally I figured it out.
> >
> > Virtual file systems like /sys or /proc usually don't care about
> > file sizes. All files have a size of 0. This leads to difficulties
> > as diff sometimes looks for file sizes.
> >
> > Say you do:
> >
> >> $ cp /proc/cmdline my_cmdline
> >> $ diff /proc/cmdline my_cmdline ; echo $?
> >> 0      // ok, files don't differ
> >> $ diff --brief /proc/cmdline my_cmdline
> >> Files /proc/cmdline and mycmdline differ
> >
> > The --brief option triggers a binary compare, as we aren't
> > interested in the actual differences this makes sense. As a first
> > step, file sizes are compared (0 vs ~150) and the files are
> > reported as different.
> 
> thanks for the report.
> What version of diffutils are you using?
> I think this has been fixed for some time.
> I was unable to reproduce with 2.8.1 nor with the latest built from
> git. I.e., I created an empty file and used diff-2.8.1 to compare it
> with the nominally-
> zero-length /proc/cmdline file, and diff did the right thing.
> Also, I ran stat to show st_size of each file is indeed 0:
> 
>   $ : > /tmp/k; /p/p/diffutils-2.8.1/bin/diff /proc/cmdline /tmp/k; \
>      stat --format %s /proc/cmdline /tmp/k
>   1d0
>   < ro root=LABEL=...
>   0
>   0
> 
> In fact, I went ahead and built all available versions and tested them
> like this:
> 
>   $ for i in /p/p/*/bin/diff; do p=diffutils-$i; echo $i; $i
> /proc/cmdline /tmp/k > /dev/null && echo bad; done
>   /p/p/diffutils-2.7/bin/diff
>   /p/p/diffutils-2.8.1/bin/diff
>   /p/p/diffutils-2.8/bin/diff
>   /p/p/diffutils-2.9/bin/diff
>   /p/p/diffutils-3.0/bin/diff
>   /p/p/diffutils-3.1/bin/diff
>   /p/p/diffutils-3.2/bin/diff
>   /p/p/diffutils-3.3/bin/diff

Hi,

I am using v.3.3 of diffutils

$ diff -v 
diff (GNU diffutils) 3.3

but I think you misunderstood the problem. Sorry for being ambiguous. I
am not diffing against an empty file. That works well. The point is
procfs doesn't care about size, but 'normal' file systems do. So for
example on my system I have (after cp /proc/cmdline mycmdline)

$ stat --format %s /proc/cmdline mycmdline
0
140

The result of diffing /proc/cmdline against mycmdline depends on the
--brief flag.

STEPS TO REPRODUCE:

cp /proc/cmdline mycmdline
diff --brief /proc/cmdline mycmdline > /dev/null ; echo ?$
1
diff /proc/cmdline mycmdline ; echo $?
0

EXPECTED RESULT:

cp /proc/cmdline mycmdline
diff --brief /proc/cmdline mycmdline > /dev/null ; echo ?$
0
diff /proc/cmdline mycmdline ; echo $?
0

 ~stephan

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