Re: GNU 'head' bug with respect to file position after 'head' exits

[Jim Meyering]

Thanks, Paul!
I've applied that patch.

Paul Eggert <address@hidden> wrote:
>> Date: Fri, 8 Feb 2002 10:37:55 -0800
>> From: Jim Hill <address@hidden>
>
>> I'll note that neither GNU's head nor FreeBSD's will work properly with
>> David Korn's example: GNU head uses `read(fd,buff,BUFSIZE)` and doesn't
>> lseek at the end; and FreeBSD's uses a getc-putc loop and doesn't
>> fflush() at the end.
>
> Here's a patch to GNU head to fix that bug.
>
> Note to textutils maintainer: David Korn's example:
> `(head -n 5 >/dev/null;cat) < file'
> would successfully behead the first 5 lines of file.  See:
>
> http://www.opengroup.org/sophocles/show_mail.tpl?source=L&listname=austin-group-l&id=3609
>
>
> 2002-02-08  Paul Eggert  <address@hidden>
>
>       * src/head.c (head_lines): If we have read too much data, try
>       to seek back to the position we would have gotten to had we
>       been reading one byte at a time.  POSIX currently doesn't
>       require this, but it's easy to do and some software relies on
>       it.
>
> --- src/head.c-orig   Sat Dec  1 09:29:26 2001
> +++ src/head.c        Fri Feb  8 14:29:20 2002
> @@ -175,7 +175,16 @@ head_lines (const char *filename, int fd
>       break;
>        while (bytes_to_write < bytes_read)
>       if (buffer[bytes_to_write++] == '\n' && --lines_to_write == 0)
> -       break;
> +       {
> +         if (lseek (fd, bytes_to_write - bytes_read, SEEK_CUR) < 0)
> +           {
> +             int e = errno;
> +             struct stat st;
> +             if (fstat (fd, &st) != 0 || S_ISREG (st.st_mode))
> +               error (0, e, _("lseek error"));
> +           }
> +         break;
> +       }
>        if (fwrite (buffer, 1, bytes_to_write, stdout) == 0)
>       error (EXIT_FAILURE, errno, _("write error"));
>      }
>