GNU bug report logs - #11631
Head command does not position file pointer correctly for negative line count

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Anoop Sharma <sendtoanoop <at> gmail.com>
Subject: bug#11631: closed (Re: bug#11631: Head command does not position
 file pointer correctly for negative line count)
Date: Tue, 05 Jun 2012 20:38:02 +0000

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Your bug report

#11631: Head command does not position file pointer correctly for negative line count

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 11631 <at> debbugs.gnu.org.

-- 
11631: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=11631
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Jim Meyering <jim <at> meyering.net>
To: Pádraig Brady <P <at> draigBrady.com>
Cc: 11631-done <at> debbugs.gnu.org
Subject: Re: bug#11631: Head command does not position file pointer correctly
	for negative line count
Date: Tue, 05 Jun 2012 22:35:13 +0200

Jim Meyering wrote:
> Thanks, and thanks for the review.  Pushed.

And with this message, I've closed the issue.

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From: Anoop Sharma <sendtoanoop <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: Head command does not position file pointer correctly for negative
	line count
Date: Tue, 5 Jun 2012 15:07:19 +0530

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Head command does not position file pointer correctly for negative line
count. Here is a demonstration of the problem.

Step 1 - Create a file with 10 lines in it.
$ yes "ABC" | head -c 40 >ip.txt
$

Step 2 - If head behaves correctly, then 2 lines should get printed after
"------------" but nothing gets printed!
$ (head -n -2; echo "------------------------"; cat) <ip.txt
ABC
ABC
ABC
ABC
ABC
ABC
ABC
ABC
------------------------
$

Step 3 - Another try fails. If head behaves correctly, then 8 lines should
get printed after "------------" but nothing gets printed!
$ (head -n -8; echo "------------------------"; cat) <ip.txt
ABC
ABC
------------------------
$



/*****************************************************************************************************************************/
Possible cause of the defect -> Following snippet is copied from head.c
(Function - elide_tail_lines_seekable ). Perhaps, there should be a lseek
after fwrite there...:

              /* Output the initial portion of the buffer
                 in which we found the desired newline byte.
                 Don't bother testing for failure for such a small amount.
                 Any failure will be detected upon close.  */
              fwrite (buffer, 1, n + 1, stdout);
/*****************************************************************************************************************************/

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