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* @Id: curs_bkgd.3x,v 1.76 2025/08/23 22:39:20 tom Exp @
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<TITLE>curs_bkgd 3x 2025-08-23 ncurses 6.6 Library calls</TITLE>
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<link rel="author" href="mailto:bug-ncurses@gnu.org">
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<H1 class="no-header">curs_bkgd 3x 2025-08-23 ncurses 6.6 Library calls</H1>
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<PRE>
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<STRONG><A HREF="curs_bkgd.3x.html">curs_bkgd(3x)</A></STRONG> Library calls <STRONG><A HREF="curs_bkgd.3x.html">curs_bkgd(3x)</A></STRONG>
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</PRE><H2><a name="h2-NAME">NAME</a></H2><PRE>
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<STRONG>bkgdset</STRONG>, <STRONG>wbkgdset</STRONG>, <STRONG>bkgd</STRONG>, <STRONG>wbkgd</STRONG>, <STRONG>getbkgd</STRONG> - manipulate background of a
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<EM>curses</EM> window of characters
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</PRE><H2><a name="h2-SYNOPSIS">SYNOPSIS</a></H2><PRE>
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<STRONG>#include</STRONG> <STRONG><curses.h></STRONG>
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<STRONG>int</STRONG> <STRONG>bkgd(chtype</STRONG> <EM>ch</EM><STRONG>);</STRONG>
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<STRONG>int</STRONG> <STRONG>wbkgd(WINDOW</STRONG> <STRONG>*</STRONG><EM>win</EM><STRONG>,</STRONG> <STRONG>chtype</STRONG> <EM>ch</EM><STRONG>);</STRONG>
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<STRONG>void</STRONG> <STRONG>bkgdset(chtype</STRONG> <EM>ch</EM><STRONG>);</STRONG>
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<STRONG>void</STRONG> <STRONG>wbkgdset(WINDOW</STRONG> <STRONG>*</STRONG><EM>win</EM><STRONG>,</STRONG> <STRONG>chtype</STRONG> <EM>ch</EM><STRONG>);</STRONG>
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<STRONG>chtype</STRONG> <STRONG>getbkgd(WINDOW</STRONG> <STRONG>*</STRONG><EM>win</EM><STRONG>);</STRONG>
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</PRE><H2><a name="h2-DESCRIPTION">DESCRIPTION</a></H2><PRE>
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Every <EM>curses</EM> window has a <EM>background</EM> <EM>character</EM> property: in the
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library's non-wide-character configuration, it is a <EM>curses</EM> character
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(<EM>chtype</EM>) that combines a set of attributes (and, if colors are enabled,
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a color pair identifier) with a character code. When erasing (parts
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of) a window, <EM>curses</EM> replaces the erased cells with the background
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character.
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<EM>curses</EM> also uses the background character when writing characters to a
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populated window.
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<STRONG>o</STRONG> The attribute part of the background character combines with all
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non-blank character cells in the window, as populated by the
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<STRONG><A HREF="curs_addch.3x.html">waddch(3x)</A></STRONG> and <STRONG><A HREF="curs_insch.3x.html">winsch(3x)</A></STRONG> families of functions (and those that
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call them).
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<STRONG>o</STRONG> Both the character code and attributes of the background character
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combine with blank character cells in the window.
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The background character's set of attributes becomes a property of the
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character cell and move with it through any scrolling and insert/delete
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line/character operations. To the extent possible on the terminal
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type, <EM>curses</EM> displays the attributes of the background character as the
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graphic rendition of a character cell on the display.
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</PRE><H3><a name="h3-bkgd_wbkgd">bkgd, wbkgd</a></H3><PRE>
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<STRONG>bkgd</STRONG> and <STRONG>wbkgd</STRONG> set the background property of <STRONG>stdscr</STRONG> or the specified
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window and then apply this setting to every character cell in that
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window.
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<STRONG>o</STRONG> The rendition of every character in the window changes to the new
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background rendition.
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<STRONG>o</STRONG> Wherever the former background character appears, it changes to the
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new background character.
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<EM>ncurses</EM> updates the rendition of each character cell by comparing the
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character, non-color attributes, and color pair selection. The library
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applies the following procedure to each cell in the window, whether or
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not it is blank.
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<STRONG>o</STRONG> <EM>ncurses</EM> first compares the cell's character to the previously
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specified background character; if they match, <EM>ncurses</EM> writes the
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new background character to the cell.
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<STRONG>o</STRONG> <EM>ncurses</EM> then checks whether the cell uses color; that is, its color
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pair value is nonzero. If not, it simply replaces the attributes
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and color pair in the cell with those from the new background
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character.
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<STRONG>o</STRONG> If the cell uses color, and its background color matches that of
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the current window background, <EM>ncurses</EM> removes attributes that may
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have come from the current background and adds those from the new
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background. It finishes by setting the cell's background to use
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the new window background color.
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<STRONG>o</STRONG> If the cell uses color, and its background color does not match
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that of the current window background, <EM>ncurses</EM> updates only the
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non-color attributes, first removing those that may have come from
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the current background, and then adding attributes from the new
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background.
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If the new background's character is non-spacing (for example, if it is
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a control character), <EM>ncurses</EM> retains the existing background
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character, except for one special case: <EM>ncurses</EM> treats a background
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character code of zero (0) as a space.
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If the terminal does not support color, or if color has not been
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initialized with <STRONG><A HREF="curs_color.3x.html">start_color(3x)</A></STRONG>, <EM>ncurses</EM> ignores the new background
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character's color pair selection.
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</PRE><H3><a name="h3-bkgdset_wbkgdset">bkgdset, wbkgdset</a></H3><PRE>
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<STRONG>bkgdset</STRONG> and <STRONG>wbkgdset</STRONG> manipulate the background of the applicable
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window, without updating the character cells as <STRONG>bkgd</STRONG> and <STRONG>wbkgd</STRONG> do; only
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future writes reflect the updated background.
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</PRE><H3><a name="h3-getbkgd">getbkgd</a></H3><PRE>
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<STRONG>getbkgd</STRONG> returns the given window's background character, attributes,
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and color pair as a <EM>chtype</EM>.
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</PRE><H2><a name="h2-RETURN-VALUE">RETURN VALUE</a></H2><PRE>
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<STRONG>bkgdset</STRONG> and <STRONG>wbkgdset</STRONG> do not return a value.
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Functions returning an <EM>int</EM> return <STRONG>ERR</STRONG> upon failure and <STRONG>OK</STRONG> upon success.
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In <EM>ncurses</EM>, failure occurs if
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<STRONG>o</STRONG> the <EM>curses</EM> screen has not been initialized, or
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<STRONG>o</STRONG> <EM>win</EM> is <EM>NULL</EM>.
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<STRONG>getbkgd</STRONG>'s return value is as described above.
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</PRE><H2><a name="h2-NOTES">NOTES</a></H2><PRE>
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Unusually, there is no <STRONG>wgetbkgd</STRONG> function; <STRONG>getbkgd</STRONG> behaves as one would
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expect <STRONG>wgetbkgd</STRONG> to, accepting a <EM>WINDOW</EM> pointer argument.
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<STRONG>bkgd</STRONG> and <STRONG>bkgdset</STRONG> may be implemented as macros.
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X/Open Curses mentions that the character part of the background must
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be a single-byte value. <EM>ncurses</EM>, like SVr4 <EM>curses</EM>, checks to ensure
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that it is, and retains the existing background character if the check
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fails.
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</PRE><H2><a name="h2-PORTABILITY">PORTABILITY</a></H2><PRE>
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X/Open Curses Issue 4 describes these functions. It indicates that
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<EM>bkgd</EM>, <EM>wbkgd</EM>, and <EM>getbkgd</EM> return <EM>ERR</EM> on failure (in the case of the
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last, this value is cast to <EM>chtype</EM>), but specifies no error conditions
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for them.
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SVr4 documentation says that <EM>bkgd</EM> and <EM>wbkgd</EM> return <EM>OK</EM> "or a non-
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negative integer if <EM>immedok()</EM> is set", referring to the return value
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from <EM>wrefresh</EM>, which in SVr4 returns a count of characters written to
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the window if its <EM>immedok</EM> property is set; in <EM>ncurses</EM>, it does not.
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Neither X/Open Curses nor the SVr4 manual pages detail how the
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rendition of characters in the window updates when <EM>bkgd</EM> or <EM>wbkgd</EM>
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changes the background character. <EM>ncurses</EM>, like SVr4 <EM>curses</EM>, does not
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(in its non-wide-character configuration) store the background and
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window attribute contributions to each character cell separately.
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</PRE><H2><a name="h2-HISTORY">HISTORY</a></H2><PRE>
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SVr3.1 (1987) introduced these functions.
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</PRE><H2><a name="h2-SEE-ALSO">SEE ALSO</a></H2><PRE>
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<STRONG><A HREF="curs_bkgrnd.3x.html">curs_bkgrnd(3x)</A></STRONG> describes the corresponding functions in the wide
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configuration of <EM>ncurses</EM>.
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<STRONG><A HREF="ncurses.3x.html">curses(3x)</A></STRONG>, <STRONG><A HREF="curs_addch.3x.html">curs_addch(3x)</A></STRONG>, <STRONG><A HREF="curs_attr.3x.html">curs_attr(3x)</A></STRONG>
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ncurses 6.6 2025-08-23 <STRONG><A HREF="curs_bkgd.3x.html">curs_bkgd(3x)</A></STRONG>
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</PRE>
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<div class="nav">
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<ul>
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<li><a href="#h2-NAME">NAME</a></li>
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<li><a href="#h2-SYNOPSIS">SYNOPSIS</a></li>
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<li><a href="#h2-DESCRIPTION">DESCRIPTION</a>
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<ul>
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<li><a href="#h3-bkgd_wbkgd">bkgd, wbkgd</a></li>
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<li><a href="#h3-bkgdset_wbkgdset">bkgdset, wbkgdset</a></li>
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<li><a href="#h3-getbkgd">getbkgd</a></li>
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</ul>
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</li>
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<li><a href="#h2-RETURN-VALUE">RETURN VALUE</a></li>
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<li><a href="#h2-NOTES">NOTES</a></li>
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<li><a href="#h2-PORTABILITY">PORTABILITY</a></li>
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<li><a href="#h2-HISTORY">HISTORY</a></li>
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<li><a href="#h2-SEE-ALSO">SEE ALSO</a></li>
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</ul>
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