Re: bkgd() with character part set to 0

[Thomas Dickey]

On Mon, May 06, 2024 at 01:11:04PM -0400, Bill Gray wrote:
> I just ran into this issue, and am not quite sure how it really ought to be
> handled.
> 
> In a perfect world, I think people would call bkgd() with the character part
> set to a non-zero value, such as
> 
>    bkgd( ' ' | COLOR_PAIR( 0));
> 
> However, looking through both the ncurses and PDCurses* tests, I see a fair
> number of calls where the character bits are left blank.  This appears to
> work, right up to the point where you read a background character using the
> inch() family of functions and then output it using addch(), as shown in the
> code snippet below.
> 
> As the boilerplate text inside it says, on wide-char ncurses, the 0
> background character is read in, then output as ^@.  I noticed the problem
> when building 'ozdemo' (a PDCurses* test program) with ncurses.
> 
> PDCurses* evades this by saying that if you call bkgd( ) (internally,
> wbkgdset( )) with a zero character, it gets remapped to a space :
> 
> https://github.com/Bill-Gray/PDCursesMod/blob/master/pdcurses/bkgd.c#L153
> 
> I'd tend to think that the behavior with a zero character is
> implementation-dependent, and mildly risky.  I expect to set the background
> explicitly to a space in future.  But the conversion of 0 to a space
> (apparently done in 8-bit-char ncurses as well as in all PDCurses*) seems
> like good behavior to me.

The background should fill in a space - in revising wbkgd() and wbkgrnd() in
20181208 (released in 6.2 20200212), I broke that detail and fixed it after
releasing 6.2 

Perhaps your example is another instance of the breakage.
These changes are relevant:

20200301
        + modify wbkgd() and wbkgrnd() to avoid storing a null in the
          background character, because it may be used in cases where the
          corresponding 0x80 is not treated as a null (report by Marc Rechte,
          cf: 20181208).
 
20190817
        + amend 20181208 changes for wbkgd() and wbkgrnd(), fixing a few
          details where it still differed from SVr4.

20181208
        + modify wbkgd() and wbkgrnd() to improve compatibility with SVr4
          curses, changing the way the window rendition is updated when the
          background character is modified (report by Valery Ushakov).

> -- Bill
> 
> #include <curses.h>
> 
> /* Compile with gcc -Wall -Wextra -pedantic -O3 -oz z.c -lncursesw
> (or with -lncurses for narrow-char build) */
> 
> int main( void)
> {
>     chtype c;
> 
>     initscr( );
>     bkgd( COLOR_PAIR( 0));
>     c = mvinch( 1, 1);
>     mvaddch( 1, 1, c);
>     mvprintw( 2, 1, "The above line will show ^@ on wide ncurses builds.");
>     mvprintw( 3, 1, "It should be blank on narrow-char builds and on all");
>     mvprintw( 4, 1, "PDCurses and PDCursesMod builds.  Hit the any key.");
>     getch( );
>     endwin( );
>     return(0);
> }
> 

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

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