GNU bug report logs - #19825
unified output format, number of consecutive unaffected lines, and POSIX

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Vincent Lefevre <vincent <at> vinc17.net>
To: bug-diffutils <at> gnu.org
Subject: unified output format, number of consecutive unaffected lines, and
 POSIX
Date: Tue, 10 Feb 2015 03:23:48 +0100

Concerning the unified output format, POSIX says in

  http://pubs.opengroup.org/onlinepubs/9699919799/utilities/diff.html

  "If -U n is specified, the output shall contain no more than
  n consecutive unaffected lines; and if the output contains an
  affected line and this line is adjacent to up to n consecutive
  unaffected lines in the corresponding file, the output shall
  contain these unaffected lines. -u shall act like -U3."

but with GNU diff and n = 3, one can get up to 6 consecutive
unaffected lines:

@@ -1,14 +1,14 @@
 1
 2
 3
-4
+4b
 5
 6
 7
 8
 9
 10
-11
+11b
 12
 13
 14

I actually think that the GNU diff behavior is better than POSIX.
And it is quite strange that POSIX specifies something different
because this format comes from GNU diff!

I think that the GNU diff manual should say something about this
difference. Or could this be regarded as a defect in POSIX?

Note: I noticed this after seeing that Subversion's internal diff
behaves again differently, with a behavior between GNU diff and
POSIX: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=777479#17

-- 
Vincent Lefèvre <vincent <at> vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Message #8 received at 19825 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Vincent Lefevre <vincent <at> vinc17.net>, 19825 <at> debbugs.gnu.org
Subject: Re: [bug-diffutils] bug#19825: unified output format,
 number of consecutive unaffected lines, and POSIX
Date: Mon, 09 Feb 2015 22:49:35 -0800

Vincent Lefevre wrote:
> Or could this be regarded as a defect in POSIX?

That's what I'd say, yes.  POSIX was intended to codify GNU diff's behavior 
here, and it seems to have missed the corner case you describe.

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