Re: Tabs

[Thomas Dickey]

On Mon, Nov 07, 2016 at 06:12:32PM -0500, Chris Delozier wrote:
> Hello,
> 
> Just wanted to confirm that I am now on the current release of 6.0 and
> the issue persists. Apologies for the lackluster subject line.

short: no, there's nothing overlooked

long: It won't work as you expect because

  + the behavior that you're seeing is due to the terminal driver,
    not the terminal.

  + the terminal driver does its work without paying any attention
    to what the terminal's tab-stops are.

  + POSIX doesn't define tab-intervals - see

    
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap11.html#tag_11

    which says

        TABDLY
        Select horizontal-tab delays:

                TAB0
                        Horizontal-tab delay type 0.
                TAB1
                        Horizontal-tab delay type 1.
                TAB2
                        Horizontal-tab delay type 2.
                TAB3
                        Expand tabs to spaces.

    but odds are that every system you'll use assumes eight.  Some write in
    their manual pages that POSIX says it's eight:

    http://man7.org/linux/man-pages/man3/termios.3.html

        TABDLY Horizontal tab delay mask.  Values are TAB0, TAB1, TAB2, TAB3
        (or XTABS).  A value of TAB3, that is, XTABS, expands tabs to
        spaces (with tab stops every eight columns).  [requires
        _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]

  + since it doesn't pay any attention (and there's no POSIX termios
    interface to tell it anything different), the terminal driver uses
    8 for the tab interval.

  + in principle, the terminal driver (for each system...) could be modified
    to do this, making it less general.

  + I had this in mind when writing
  
    
http://superuser.com/questions/1037615/detecting-terminal-tabstop-settings/1037780#1037780

> 
> Thank you.
> 
> On Mon, Nov 7, 2016 at 1:56 PM, Chris Delozier <address@hidden> wrote:
> > Hello,
> >
> > I've set tabs to 4 and am noticing that, when inputting data, if I tab
> > twice it will align correctly both times, but backspacing will go back
> > 8 characters. Is there a setting I'm missing that also handles
> > backspacing of tab character?
> >
> > Steps to reproduce: open terminal, `stty tab0`, `tabs -4`, `cat >>
> > tst0`. Enter a visual character, followed by two tabs (should be 8
> > spaces beyond that character), then press backspace once. It will take
> > you to the original character on the input line.
> >
> > version 5.9 (patch 20140913) Ncurses installed.
> >
> > Thanks for the assist.
> 
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-- 
Thomas E. Dickey <address@hidden>
http://invisible-island.net
ftp://invisible-island.net

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