SUMMARY: CURSES: magic cookie glitch chars

[Robert Felps]

The question posted was,

    Can anyone shed some light on 'magic cookie glitch characters'?

And the answers are ...


Nick Sayer writes,

    It is my understanding that "magic cookie" terminals, like the
    TVI-925, are such because the characters that start and end the
    standout mode(s) actually take space on the CRT. That is, they
    are like special versions of the space character. If you over-print
    them, their special meaning disappears, thus the standout is turned off
    for the data immediately following. This causes all sorts of hell,
    but that's the way it is.


Bob Weissman writes,

    I haven't read this book, but I do know that certain extremely
    brain-damaged terminals such as the TeleVideo 900 series have this
    problem.

    Changing character attributes on the screen takes up a character
    location.  E.g., if you want to underline only part of a word,
    as in
    	underline
    	  ----
    you find you can't do it because changing the attributes causes
    an extra character space to get used and you end up with
    	un derl ine
    	   ----
    instead, or maybe
    	un derl ine
    	   -----

    So some modern termcap/curses combinations have special ways of
    getting around this.  Sun's termcap, for example, has a "ug" capability,
    which is described as
    	ug	num	number of garbage chars left by us or ue (default 0)
    so that you can at least do whole-word underlining without extra spaces
    appearing.


Andy Behrens writes,

    Some terminals highlight characters by storing the highlighting mode
    (inverse video, dim, blinking, etc.) of each position on the screen.
    For these terminals, you set the mode and then write the characters to
    the screen.

    Other terminals (mostly older ones) store mode information on the
    screen -- it takes up a character position.  The "character" occupying
    this position is called a magic cookie; I don't know why.  The mode
    change affects all the characters on the screen until the next magic
    cookie.  The advantages of this system are that you can highlight a
    block of text without having to redraw it -- just rewrite the magic
    cookies at the start and end.  Also, you only need 7 or 8 bits for each
    character, instead of 12 (8 databits + 4 mode bits for 4 different
    types of highlighting).  That seems like a small difference now, but
    back when memory was expensive, it was a significant savings.  On the
    down side, you have to use up a character position on the screen every
    time you change modes.

    The VT100, "ansi" terminals, and most other modern terminals, store
    mode information with each character.  The widely emulated Televideo
    925 is an example of the "magic cookie" type of terminal.


Phong Vo writes,

    Actually, the right characterization is that a magic cookie is a mark
    on the screen which alters the current attributes. There are two major
    families of terminals, HP26xx and Televideo, that have this
    (mis)feature.  For the HPs, the marks have zero width (i.e., it does
    not take up one space on the screen) while for the TVIs, the mark is
    1-char width. These cookie critters make life interesting for software
    like curses. The curses that I wrote, SysVr3.1 or later, handles the
    HPs correctly and does the best it can for the TVIs.


Thanks to all of you for the explanations.


Robert Felps                                felps at convex.com
Convex Computer Corp                        OS System Specialist
3000 Waterview Parkway                      Tech. Assistant Ctr
Richardson, Tx.  75083                      1(800) 952-0379