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Subject: |
Files with incorrect file sizes |
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Date: |
Tue, 17 Nov 2015 12:44:08 +0100 |
Hi there,
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.
I am not sure how to fix this. I.e. are the files different or not? What
is the correct behavior. In any case --brief should have no influence on
the overall result! I see the following options (I prefer the 2nd):
1: Don't use file sizes in comparing. This is rather expensive but
would certainly fix it.
2: add a --use-file-sizes flag which enables decisions
based on file sizes. Without this flag, always look into the files.
And --brief should report, if the files differ because of their data or
size. That is
> $ diff /proc/cmdline my_cmdline ; echo $?
> 0 // ok, files don't differ
> $ diff --brief /proc/cmdline my_cmdline ; echo $0
> 0
> $ diff --brief --use-file-sizes /proc/cmdline my_cmdline
> Files /proc/cmdline and mycmdline differ in file size
3: Leave it at it is and add a warning in the man page. However that
would be very inconvenient.
best, stephan
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Subject: |
Re: [bug-diffutils] bug#21942: Files with incorrect file sizes |
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Date: |
Sun, 6 Dec 2015 11:39:44 -0800 |
On Sun, Dec 6, 2015 at 11:27 AM, Jim Meyering <address@hidden> wrote:
...
> Thank you, but I don't want to have to specify some new option to
> avoid this misbehavior, so will push the attached patch shortly.
> If someone finds a system for which a falsely reported stat.st_size
> is nonzero, we can revisit this.
Pushed with commit log changes and one fewer blank line in NEWS.
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