new_pair(3X) Library calls new_pair(3X)

alloc_pair, find_pair, free_pair - dynamically allocate curses color pairs

#include <curses.h>
int alloc_pair(int fg, int bg);
int find_pair(int fg, int bg);
int free_pair(int pair);

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.

The number of colors may be related to the number of possible color pairs for a given terminal, or it may not:

  • While almost all terminals allow setting the color attributes 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.
  • 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 combinations of colors which are displayed, it will be inefficient.
  • 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:
COLORS * COLORS
•
Terminals which support default colors distinct from “ANSI colors” add to the possible combinations, producing this total:
( COLORS + 1 ) * ( COLORS + 1 )
•
An application might use up to a few dozen color pairs to implement a predefined color scheme.
Beyond that lies in the realm of programs using the foreground and background colors for “ASCII art” (or some other non-textual application).
Also beyond those few dozen pairs, the required size for a table to represent the combinations grows rapidly with an increasing number of colors.
These functions allow a developer to let the screen library manage color pairs.

The alloc_pair function accepts parameters for foreground and background color, and checks if that color combination is already associated with a color pair.

  • If the combination already exists, alloc_pair returns the existing pair.
  • If the combination does not exist, alloc_pair allocates a new color pair and returns that.
  • If the table fills up, alloc_pair discards the least-recently allocated entry using free_pair and allocates a new color pair.

All of the color pairs are allocated from a table of possible color pairs. The size of the table is determined by the terminfo pairs capability. The table is shared with init_pair; in fact alloc_pair calls init_pair after updating the ncurses library's fast index to the colors versus color pairs.

The find_pair 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.

Marks the given color pair as unused, i.e., like color pair 0.

The alloc_pair function returns a color pair number in the range 1 through COLOR_PAIRS-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.

The find_pair function returns a color pair number if the given color combination has been associated with a color pair, or -1 if not.

Likewise, free_pair returns OK unless it encounters an error updating the fast index or if no such color pair is in use.

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.

Thomas Dickey

curs_color(3X)

2024-03-16 ncurses 6.5