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.\" Copyright 2018,2020 Thomas E. Dickey                                     *
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.\" Author: Thomas E. Dickey
.\"
.\" $Id: new_pair.3x,v 1.15 2020/10/17 23:54:50 tom Exp $
.TH new_pair 3X ""
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.SH NAME
\fBalloc_pair\fP,
\fBfind_pair\fP,
\fBfree_pair\fP \- new curses color-pair functions
.SH SYNOPSIS
\fB#include <curses.h>\fP
.sp
\fBint alloc_pair(int \fP\fIfg\fP\fB, int \fP\fIbg\fP\fB);\fP
.br
\fBint find_pair(int \fP\fIfg\fP\fB, int \fP\fIbg\fP\fB);\fP
.br
\fBint free_pair(int \fP\fIpair\fP\fB);\fP
.SH DESCRIPTION
These functions are an extension to the curses 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 curses 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.
.bP
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:
.NS
\fBCOLORS\fP\fI * \fP\fBCOLORS\fP
.NE
.bP
Terminals which support \fIdefault colors\fP distinct
from \*(``ANSI colors\*('' 
add to the possible combinations, producing this total:
.NS
\fI( \fP\fBCOLORS\fP\fI + 1 ) * ( \fP\fBCOLORS\fP\fI + 1 )\fP
.NE
.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 \fIpairs\fP capability.
The table is shared with \fBinit_pair\fP;
in fact \fBalloc_pair\fP calls \fBinit_pair\fP after
updating the ncurses 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
.PP
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 ncurses.
They were not supported on
Version 7, BSD or System V implementations.
It is recommended that
any code depending on them be conditioned using NCURSES_VERSION.
.SH SEE ALSO
\fBcurs_color\fR(3X).
.SH AUTHOR
Thomas Dickey.
