Re: Starange behavior of bkgd when called after bkgdset

[Thomas Dickey]

On Sat, Jun 25, 2022 at 04:27:58PM -0400, Thomas Dickey wrote:
> On Sat, Jun 25, 2022 at 07:18:58PM +0200, Anton Vidovic wrote:
...
> > Compare that to the second situation:
> > 
> > Calling bkgdset('+') sets only the bg char of the single line (because
> > of the "\n" subsequent call to addstr):
> > 
> > call+1+++++++++
> > 
> > and the subsequent call to bkgd('-') only affects that line and the
> > second line and leaves the rest of the screen without any background
> > char:
> > 
> > call-1---------
> > call-2---------

Interestingly enough, Solaris (SVr4) curses does this.
The colors may differ (no bce), but the +'s and -'s match up.

> sure - if I conclude that it's correct.

yes... more documentation is needed, to explain this.

What's happening is due to this check:
https://github.com/ThomasDickey/ncurses-snapshots/blob/6bf016303929e7f49758b7ed27e3b255310b959f/ncurses/base/lib_bkgd.c#L222

                if (CharEq(*cp, old_bkgd)) {
#if USE_WIDEC_SUPPORT
                    if (!narrow) {
                        if (Charable(new_bkgd)) {
                            SetChar2(*cp, CharOf(new_char));
                        } else {
                            SetChar(*cp, L' ', AttrOf(new_char));
                        }
                        memcpy(cp->chars,
                               new_char.chars,
                               CCHARW_MAX * sizeof(cp->chars[0]));
                    } else
#endif
                        SetChar2(*cp, CharOf(new_char));
                }

In the logic which I adapted from Solaris, that comparison against
old_bkgd is checking each cell to determine if the cell used the
same background-character as the old value for the background-character.
If it did not, it leaves that untouched.

Setting the background character to "+" without filling the screen with +'s
produces this special case.

-- 
Thomas E. Dickey <dickey@invisible-island.net>
https://invisible-island.net
ftp://ftp.invisible-island.net

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