man/new_pair.3x

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.\" Copyright 2018-2023,2024 Thomas E. Dickey                                *
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.\" Author: Thomas E. Dickey
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.\" $Id: new_pair.3x,v 1.46 2024/03/16 15:35:01 tom Exp $
.TH new_pair 3X 2024-03-16 "ncurses @NCURSES_MAJOR@.@NCURSES_MINOR@" "Library calls"
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.de bP
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..
.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)