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.\"***************************************************************************
.\" Copyright 2018-2022,2023 Thomas E. Dickey *
.\" Copyright 2017 Free Software Foundation, Inc. *
.\" *
.\" Permission is hereby granted, free of charge, to any person obtaining a *
.\" copy of this software and associated documentation files (the *
.\" "Software"), to deal in the Software without restriction, including *
.\" without limitation the rights to use, copy, modify, merge, publish, *
.\" distribute, distribute with modifications, sublicense, and/or sell *
.\" copies of the Software, and to permit persons to whom the Software is *
.\" furnished to do so, subject to the following conditions: *
.\" *
.\" The above copyright notice and this permission notice shall be included *
.\" in all copies or substantial portions of the Software. *
.\" *
.\" THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS *
.\" OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF *
.\" MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. *
.\" IN NO EVENT SHALL THE ABOVE COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, *
.\" DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR *
.\" OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR *
.\" THE USE OR OTHER DEALINGS IN THE SOFTWARE. *
.\" *
.\" Except as contained in this notice, the name(s) of the above copyright *
.\" holders shall not be used in advertising or otherwise to promote the *
.\" sale, use or other dealings in this Software without prior written *
.\" authorization. *
.\"***************************************************************************
.\"
.\" Author: Thomas E. Dickey
.\"
.\" $Id: new_pair.3x,v 1.44 2023/12/16 20:32:22 tom Exp $
.TH new_pair 3X 2023-12-16 "ncurses 6.4" "Library calls"
.ie \n(.g \{\
.ds `` \(lq
.ds '' \(rq
.\}
.el \{\
.ie t .ds `` ``
.el .ds `` ""
.ie t .ds '' ''
.el .ds '' ""
.\}
.
.de bP
.ie n .IP \(bu 4
.el .IP \(bu 2
..
.SH NAME
\fB\%alloc_pair\fP,
\fB\%find_pair\fP,
\fB\%free_pair\fP \-
dynamically allocate \fIcurses\fR color pairs
.SH SYNOPSIS
.nf
\fB#include <curses.h>
.PP
\fBint alloc_pair(int \fIfg\fP, int \fIbg\fP);
\fBint find_pair(int \fIfg\fP, int \fIbg\fP);
\fBint free_pair(int \fIpair\fP);
.fi
.SH DESCRIPTION
These functions are an extension to the \fIcurses\fP library.
They permit an application to dynamically allocate a color pair using
the foreground/background colors rather than assign a fixed color pair number,
and return an unused pair to the pool.
.PP
The number of colors may be related to the number of possible color
pairs for a given terminal, or it may not:
.bP
While almost all terminals allow setting the color \fIattributes\fP
independently,
it is unlikely that your terminal allows you to modify the attributes
of a given character cell without rewriting it.
That is, the foreground and background colors are applied as a pair.
.bP
Color pairs are the \fIcurses\fP library's way of managing a color palette
on a terminal.
If the library does not keep track of the \fIcombinations\fP of
colors which are displayed, it will be inefficient.
.IP \(bu 4
For simple terminal emulators
with only a few dozen color combinations,
it is convenient to use the maximum number of combinations
as the limit on color pairs:
.PP
.RS 8
.EX
\fBCOLORS\fI * \fBCOLORS\fR
.EE
.RE
.IP \(bu 4
Terminals which support \fIdefault colors\fP distinct
from \*(``ANSI colors\*(''
add to the possible combinations, producing this total:
.PP
.RS 8
.EX
\fI( \fBCOLORS\fI + 1 ) * ( \fBCOLORS\fI + 1 )\fR
.EE
.RE
.bP
An application might use up to a few dozen color pairs to
implement a predefined color scheme.
.IP
Beyond that lies in the realm of programs using the foreground
and background colors for \*(``ASCII art\*(''
(or some other non-textual application).
.IP
Also beyond those few dozen pairs, the required size for a table
to represent the combinations grows rapidly with an increasing number of colors.
.IP
These functions allow a developer to let the screen library
manage color pairs.
.SS alloc_pair
The \fBalloc_pair\fP function accepts parameters for
foreground and background color, and
checks if that color combination is already associated with a color pair.
.bP
If the combination already exists,
\fBalloc_pair\fP returns the existing pair.
.bP
If the combination does not exist,
\fBalloc_pair\fP allocates a new color pair and returns that.
.bP
If the table fills up, \fBalloc_pair\fP discards the least-recently
allocated entry using \fBfree_pair\fP and allocates a new color pair.
.PP
All of the color pairs are allocated from a table of possible color pairs.
The size of the table is determined by the terminfo \fBpairs\fP capability.
The table is shared with \fBinit_pair\fP;
in fact \fBalloc_pair\fP calls \fBinit_pair\fP after
updating the \fI\%ncurses\fP library's fast index
to the colors versus color pairs.
.SS find_pair
The \fBfind_pair\fP function accepts parameters for
foreground and background color, and
checks if that color combination is already associated with a color pair,
returning the pair number if it has been allocated.
Otherwise it returns \-1.
.SS free_pair
Marks the given color pair as unused,
i.e., like color pair 0.
.SH RETURN VALUE
The \fBalloc_pair\fP function returns a color pair number in the range
1 through \fBCOLOR_PAIRS\fP\-1, unless it encounters an error updating
its fast index to the color pair values, preventing it from allocating
a color pair.
In that case, it returns \-1.
.PP
The \fBfind_pair\fP function returns a color pair number if the
given color combination has been associated with a color pair,
or \-1 if not.
.PP
Likewise, \fBfree_pair\fP returns \fBOK\fP unless it encounters an
error updating the fast index or if no such color pair is in use.
.SH PORTABILITY
These routines are specific to \fI\%ncurses\fP.
They were not supported on
Version 7, BSD or System V implementations.
It is recommended that
any code depending on them be conditioned using \fB\%NCURSES_VERSION\fP.
.SH AUTHORS
Thomas Dickey
.SH SEE ALSO
\fB\%curs_color\fP(3X)
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