Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!mips!zorba!dtynan
From: allbery@NCoast.ORG (Brandon S. Allbery KB8JRR)
Newsgroups: comp.unix
Subject: Re: curses standout()
Message-ID: <3653@zorba.Tynan.COM>
Date: 4 Jul 90 00:57:30 GMT
References: <3610@zorba.Tynan.COM>
Sender: dtynan@zorba.Tynan.COM
Reply-To: uunet!ncoast.ORG!allbery (Brandon S. Allbery KB8JRR)
Followup-To: comp.unix
Organization: North Coast Public Access UN*X, Cleveland, OH
Lines: 61
Approved: dtynan@zorba.Tynan.COM

As quoted from <3610@zorba.Tynan.COM> by raza@cs.heriot-watt.ac.uk:
+---------------
| the problem is that when i try and use the curses function standout(), it
| simply doesn't work! (this function is used with standend() to make text
| appear in reverse video).
| i am using a TELEVIDEO model 910+, 920c and 910g type terminal, the problem 
| is that the function DOES work with a vt100 type terminal.
+---------------

The Televideo 910/920 series terminal is possessed of an affliction known as
"attribute spaces", sometimes called "magic cookies".  This means that the
start-reverse-video and end-reverse-video sequences take up space on the
terminal display.  Moreover, the sequences are not applied to individual
characters; they're applied to the space on the screen between the standout()
and standend() sequences.  Some terminals enforce an automatic end-reverse-
video at the end of a physical line as well.  Curses was written for ANSI
X3.64 terminals, however; it assumes that attributes are stored as "extra
bits" in a character instead of in "invisible" character positions, and apply
to any characters output between the start-reverse-video and end-reverse-video
sequences regardless of screen position.

This can cause curses to fail in either of two ways:

(1) It can output the start-reverse-video sequence somewhere where it won't
    affect the text you want to show reversed; that is, after the text (with
    respect to the physical position on the terminal screen) or on a different
    line (of the terminal forces end-reverse-video at end of line);

(2) It can output the start-reverse-video sequence, then write over it with
    your message; since the terminal considers the attribute space to be like
    any other character, if something else is printed over it, it is replaced
    by the character.  As a result, the string comes out in normal video.

System V curses attempts to handle this "correctly"; it requires that the
terminfo entry spacify both "xmc#1" to indicate that attributes take up space
on the screen and "!msgr" to specify that attributes apply to screen positions
and not characters.  And it doesn't always get it right.

Alternatives:

(1) Use ANSI X3.64 compatible terminals like the VT100 or the Televideo 970.
    (Sometimes known as "Get a *real* terminal!" by snobs.)

(2) Many terminals of the Televideo persuasion support "protected" characters;
    often, the attribute used to indicate that a character is "protected" can
    be configured at terminal set-up or sometimes from software.  Moreover,
    the "protect" attribute can be set without actually activating write-
    protect mode.  (The usual escape sequences are <ESC>) to begin protect
    mode and <ESC>( to end it.)  If you can set the "protect attribute" to
    reversed and change the termcap/terminfo to use the start/end protect mode
    escape sequences, reverse video will work from curses: protect mode acts
    like ANSI X3.64 attributes, not like "attribute spaces".  (This is espec-
    ially useful on Wyse Technologies' terminals that lack ANSI compatibility;
    and the WY-75 actually uses this trick internally to make <ESC>[?m (where
    "?" is any number except 0) do ANSI-compliant reverse video.)

++Brandon
-- 
Me: Brandon S. Allbery			VHF: KB8JRR on 224.50 (Lake County, OH)
Internet: allbery@NCoast.ORG		Delphi: ALLBERY
uunet!cwjcc.cwru.edu!ncoast!allbery	America OnLine: BrandonA or KB8JRR