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

From: Anoop Sharma <[email-masked]>
Date: Tue, 5 Jun 2012 15:07:19 +0530

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);
/*****************************************************************************************************************************/