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

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Message #8 received at 21942 <at> debbugs.gnu.org (full text, mbox):

From: Jim Meyering <jim <at> meyering.net>
To: Stephan Müller <fruktopus <at> gmail.com>
Cc: 21942 <at> debbugs.gnu.org
Subject: Re: [bug-diffutils] bug#21942: Files with incorrect file sizes
Date: Fri, 20 Nov 2015 18:28:37 +0100

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

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