curs_color(3X)

NAME
start_color, has_colors, can_change_color, init_pair, init_color, init_extended_pair, init_extended_color, color_content, pair_content, extended_color_content, extended_pair_content, reset_color_pairs, COLOR_PAIR, PAIR_NUMBER, COLORS, COLOR_PAIRS, COLOR_BLACK, COLOR_RED, COLOR_GREEN, COLOR_YELLOW, COLOR_BLUE, COLOR_MAGENTA, COLOR_CYAN, COLOR_WHITE, A_COLOR - manipulate terminal colors with curses

SYNOPSIS
#include <curses.h>

/* variables */
int COLOR_PAIRS;
int COLORS;

int start_color(void);

bool has_colors(void);
bool can_change_color(void);

int init_pair(short pair, short f, short b);
int init_color(short color, short r, short g, short b);
/* extensions */
int init_extended_pair(int pair, int f, int b);
int init_extended_color(int color, int r, int g, int b);

int color_content(short color, short *r, short *g, short *b);
int pair_content(short pair, short *f, short *b);
/* extensions */
int extended_color_content(int color, int *r, int *g, int *b);
int extended_pair_content(int pair, int *f, int *b);

/* extension */
void reset_color_pairs(void);

/* macros */
int COLOR_PAIR(int n);
PAIR_NUMBER(int attr);
COLOR_BLACK
COLOR_RED
COLOR_GREEN
COLOR_YELLOW
COLOR_BLUE
COLOR_MAGENTA
COLOR_CYAN
COLOR_WHITE
A_COLOR

DESCRIPTION

  Overview
curses supports color rendering on terminals with applicable capabilities. Once the library has initialized the terminal, has_colors tells an application whether that terminal type has color capability. If it does, calling start_color enables the feature. (See section “NOTES” below regarding ripoffline(3X).)

When applying colors to a curses window, the library manages them in pairs. A color pair couples a foreground color applied to the visible strokes of a glyph with a background color for the field in the character cell within which the glyph appears. init_pair initializes a color pair identifier, whose value you select, from a pair of color indices, foreground and background. Each index represents a color. curses standardizes and predefines a small set of color indices; see section “CONSTANTS” below. The macro COLOR_PAIR(n) converts a pair n thus intialized to the value required to encode it in a chtype or attr_t. pair_content permits discovery of a color pair's current definition. Configure at least one color pair to use the color feature. color_content extracts the red, green, and blue components of color the using the given index.

can_change_color tells an application whether the terminal permits (re)definition of a color. If it does, you can call init_color to update the specified color index to use red, green, and blue components of your choice.

Subsection “Color Handling” of terminfo(5) describes the capabilities that terminal types use to manage color.

  Rendering
curses combines the following data to render a character cell. Any of them can include color information. curses character attributes, as from waddch(3X) or wadd_wch(3X) window attributes, as from wattrset(3X) or wattr_set(3X) window background character attributes, as from wbkgdset(3X) or wbkgrndset(3X)

Per-character and window attributes are usually set through a function parameter containing attributes and/or a color pair identifier. Some functions, such as wattr_set, use a separate color pair number parameter.

The background character is a special case: it includes a character code, as if it were passed to waddch.

The curses library does the actual work of combining these color pairs in an internal function called from waddch. If the parameter passed to waddch is blank (see waddch(3X) or wadd_wch(3X)), and it uses the special color pair 0, curses next checks the window attribute.
    If the window attribute does not use color pair 0, curses uses the color pair from the window attribute. Otherwise, curses uses the background character.
If the parameter passed to waddch is not blank, or it does not use the special color pair 0, curses prefers the color pair from the parameter, if it is nonzero. Otherwise, it tries the window attribute next, and finally the background character.

Some curses functions such as wprintw(3X) call waddch. Those do not combine its parameter with a color pair. Consequently those calls use only the window attribute or the background character.

CONSTANTS
ISO 6429 and ECMA-48 define eight standard colors (also known as “ANSI” colors). curses.h defines object-like macros COLOR_BLACK, COLOR_RED, COLOR_GREEN, COLOR_YELLOW, COLOR_BLUE, COLOR_MAGENTA, COLOR_CYAN, and COLOR_WHITE accordingly. curses assumes that COLOR_BLACK is the default background color for all terminals. ncurses offers an extension to override that assumption; see assume_default_colors(3X). Some terminals support additional colors that lack standard names.

A_COLOR is a bit mask that, when bitwise “and”-ed with a chtype, extracts its color pair identifier.

VARIABLES

  COLORS
is initialized by start_color to the maximum number of colors the terminal can support.

  COLOR_PAIRS
is initialized by start_color to the maximum number of color pairs the terminal can support. Often, its value is the product COLORS × COLORS, but this is not always true. A few terminals use the HLS color space (see start_color below), ignoring this rule; and terminals supporting a large number of colors are limited to the number of color pairs that a signed short value can represent.

FUNCTIONS

  start_color
The start_color routine requires no arguments. It must be called if the programmer wants to use colors, and before any other color manipulation routine is called. It is good practice to call this routine right after initscr. start_color does this: It initializes two global variables, COLORS and COLOR_PAIRS (respectively defining the maximum number of colors and color pairs the terminal can support). It initializes the special color pair 0 to the default foreground and background colors. No other color pairs are initialized. It restores the colors on the terminal to the values they had when the terminal was just turned on. If the terminal supports the initc (initialize_color) capability, start_color initializes its internal table representing the red, green, and blue components of the color palette.

    The components depend on whether the terminal uses CGA (aka “ANSI”) or HLS (i.e., the hls (hue_lightness_saturation) capability is set). The table is initialized first for eight basic colors (black, red, green, yellow, blue, magenta, cyan, and white), using weights that depend upon the CGA/HLS choice. For “ANSI” colors the weights are 680 or 0 depending on whether the corresponding red, green, or blue component is used or not. That permits using 1000 to represent bold/bright colors. After the initial eight colors (if the terminal supports more than eight colors) the components are initialized using the same pattern, but with weights of 1000. SVr4 uses a similar scheme, but uses 1000 for the components of the initial eight colors.

    start_color does not attempt to set the terminal's color palette to match its built-in table. An application may use init_color to alter the internal table along with the terminal's color.

These limits apply to color values and color pairs. Values outside these limits are not valid, and may result in a runtime error: COLORS corresponds to the terminal database's max_colors capability, (see terminfo(5)). color values are expected to be in the range 0 to COLORS-1, inclusive (including 0 and COLORS-1). a special color value -1 is used in certain extended functions to denote the default color (see use_default_colors(3X)). COLOR_PAIRS corresponds to the terminal database's max_pairs capability, (see terminfo(5)). valid color pair identifiers are in the range 1 to COLOR_PAIRS-1, inclusive. color pair 0 is special; it denotes “no color”.

    Color pair 0 is assumed to be white on black, but is actually whatever the terminal implements before color is initialized. It cannot be modified by the application.

  has_colors
has_colors returns TRUE if the terminal supports colors and FALSE if it does not. initscr(3X) or newterm(3X) must be called first, but start_color need not be. An application might call has_colors to decide whether to use color or a video attribute like A_BOLD to render text.

Color support in curses requires that the terminal type description support the capabilities max_colors (colors), max_pairs (pairs), and any of set_foreground (setf) and set_background (setb); set_a_foreground (setaf) and set_a_background (setab); or set_color_pair (scp).

  can_change_color
The can_change_color routine requires no arguments. It returns TRUE if the terminal supports colors and can change their definitions; other, it returns FALSE. This routine facilitates writing terminal-independent programs.

  init_pair
The init_pair routine changes the definition of a color pair. It takes three arguments: the number of the color pair to be changed, the foreground color number, and the background color number. For portable applications: The first argument must be a valid color pair identifier. If default colors are used (see use_default_colors(3X)) the upper limit is adjusted to allow for extra pairs which use a default color in foreground and/or background. The second and third arguments must be valid color values.

If the color pair was previously initialized, the screen is refreshed and all occurrences of that color pair are changed to the new definition.

As an extension, ncurses allows you to set color pair 0 via the assume_default_colors(3X) routine, or to specify the use of default colors (color number -1) if you first invoke the use_default_colors(3X) routine.

  init_extended_pair
Because init_pair uses signed shorts for its parameters, that limits color pairs and color-values to 32767 on modern hardware. The extension init_extended_pair uses ints for the color pair and color-value, allowing a larger number of colors to be supported.

  init_color
The init_color routine changes the definition of a color. It takes four arguments: the number of the color to be changed followed by three RGB values (for the amounts of red, green, and blue components). The first argument must be a valid color value; default colors are not allowed here. (See start_color above for the default color index.) Each of the last three arguments must be a value in the range 0 through 1000.

When init_color is used, all occurrences of that color on the screen immediately change to the new definition.

  init_extended_color
Because init_color uses signed shorts for its parameters, that limits color-values to 32767 on modern hardware. The extension init_extended_color uses ints for the color value and for the red, green, and blue components, allowing a larger number of color-values to be supported. The first argument must be a valid color value; default colors are not allowed here. (See start_color above for the default color index.) Each of the last three arguments must be a value in the range 0 through 1000.

When init_extended_color is used, all occurrences of that color on the screen immediately change to the new definition.

  color_content
The color_content routine gives programmers a way to find the intensity of the red, green, and blue (RGB) components in a color. It requires four arguments: the color number, and three addresses of shorts for storing the information about the amounts of red, green, and blue components in the given color. The first argument must be a valid color value, i.e., 0 through COLORS-1, inclusive. The values that are stored at the addresses pointed to by the last three arguments are in the range 0 (no component) through 1000 (maximum amount of component), inclusive.

  extended_color_content
Because color_content uses signed shorts for its parameters, that limits color-values and their red, green, and blue components to 32767 on modern hardware. The extension extended_color_content uses ints for the color value and for returning the red, green, and blue components, allowing a larger number of colors to be supported.

  pair_content
The pair_content routine allows programmers to find out what colors a given color pair consists of. It requires three arguments: the color pair number, and two addresses of shorts for storing the foreground and the background color numbers. The first argument must be a valid color value, i.e., in the range 1 through COLOR_PAIRS-1, inclusive. The values that are stored at the addresses pointed to by the second and third arguments are in the range 0 through COLORS, inclusive.

  extended_pair_content
Because pair_content uses signed shorts for its parameters, that limits color pair and color-values to 32767 on modern hardware. The extension extended_pair_content uses ints for the color pair and for returning the foreground and background colors, allowing a larger number of colors to be supported.

  reset_color_pairs
The extension reset_color_pairs tells ncurses to discard all of the color pair information which was set with init_pair. It also touches the current- and standard-screens, allowing an application to switch color palettes rapidly.

  COLOR_PAIR
COLOR_PAIR(n) converts a color pair number to an attribute. Attributes can hold color pairs in the range 0 to 255. If you need a color pair larger than that, you must use functions such as attr_set (which pass the color pair as a separate parameter) rather than the legacy functions such as attrset.

  PAIR_NUMBER
PAIR_NUMBER(attr) extracts the color information from its attr parameter and returns it as a color pair number; it is the inverse operation of COLOR_PAIR.

RETURN VALUE
can_change_color and has_colors return TRUE or FALSE. The other functions return OK on success and ERR on failure.

In ncurses, functions returning an int recognize several error conditions. All return ERR if the screen has not been initialized; see initscr(3X) or newterm(3X). All except start_color return ERR if start_color has not been called, or itself returned ERR. start_color returns ERR if it cannot allocate memory for its color pair table. init_color returns ERR if the terminal type does not support assignable color values; that is, if the initialize_color (initc) capability is absent from its description. init_color returns ERR if any of its r, g, b arguments is outside the range 0-1000 inclusive. init_pair, init_color, init_extended_pair, init_extended_color, color_content, pair_content, extended_color_content, and extended_pair_content return ERR on attempts to use
    color identifiers outside the range 0-COLORS-1 inclusive, the default colors extension notwithstanding, or color pair identifiers outside the range 0-COLOR_PAIRS-1 inclusive.

NOTES
Setting a background color via a color pair identifier affects only character cells that a character write operation explicitly touches. To change the background color used when parts of a window are blanked by erasing or scrolling operations, see curs_bkgd(3X).

Windows created by ripoffline(3X) do not inherit color pair configuration applied to stdscr; they must be configured independently.

In ncurses, init_pair accepts negative foreground and background color arguments to support its use_default_colors(3X) extension, but only after the latter function has been called.

The assumption that COLOR_BLACK is the terminal's default background color can be overridden using ncurses's assume_default_colors(3X) extension.

In ncurses, each pointer passed to color_content and pair_content can be null, in which case the library ignores it, permitting the application to disregard unnecessary information.

In ncurses, each screen has a color activation flag, color palette, color pair table, and associated COLORS and COLOR_PAIRS values; start_color affects only the current screen. The SVr4 and X/Open Curses interface was not really designed with this in mind; historical implementations may use a single shared color palette.

Several caveats apply to IBM PC-compatible machines of the 80486 era and earlier with CGA/EGA/VGA video. COLOR_YELLOW was frequently converted, in the analog domain, to a shade of brown if the intensity bit was not set. To get yellow on such devices, one would combine COLOR_YELLOW with the A_BOLD attribute. The A_BLINK attribute should in theory make the background bright. This often fails to work, and even VGA controllers for which it mostly works, such as those from Paradise and compatibles, do the wrong thing when you try to set a bright “yellow” background — you get a blinking yellow foreground instead. Color RGB values are not configurable on these devices (in text mode).

  Why This Color Model?
A programmer new to curses may wonder why the library works with pairs of indexed colors instead of “direct” foreground and background RGB triples. The answer lies in the limited bandwidth between terminals and their time-sharing host machines. In the 1980s, a 9600bps serial link was considered fast. That speed typically corresponded to a data rate of 960 bytes per second. Using a single-byte character encoding, refreshing an 80×24 terminal screen took two full seconds. (Alternatively, a channel with this data payload rendered half a frame per second.) Adding data to each character cell necessary for the “direct” color model at 8 bits per color channel would add 6 bytes to every character cell, extending the full-screen refresh time to 14 seconds. Even a pair of 8-bit color values would triple the refresh time.

Moreover, the vast majority of curses applications do not demand a wide gamut of colors. Even a colorful application like the game nethack(6) renders most of its interface in monochrome by default. A text user interface employing the form(3X) or menu(3X) libraries often uses fewer than ten distinct colors at any one time. Thus, small integers suffice both to index individual colors and to pair them. Typically, each color pair is assigned to a type of user interface element, like a button or scroll bar control. Further, in the original SVr3.2 curses implementation of color and today still in ncurses's non-wide library, chtype affords few bits for encoding of the color pair identifier.

Even in the common modern scenario where a terminal emulator runs on the same host as the curses application, and available bandwidth is limited only by the speed of the system bus, efficient encoding of character cell data aids performance by minimizing copies to and from the kernel's memory space and the CPU mode switches thus demanded by use of the pseudoterminal (pty) system interface. Parsimonious data management also reduces memory cache pressure.

EXTENSIONS
The functions marked as extensions originated in ncurses, and are not found in SVr4 curses, 4.4BSD curses, or any other previous curses implementation.

PORTABILITY
Applications employing ncurses extensions should condition their use on the visibility of the NCURSES_VERSION preprocessor macro.

X/Open Curses Issue 4 describes these functions. It specifies no error conditions for them.

ncurses satisfies X/Open Curses's minimum maximums for COLORS and COLOR_PAIRS.

X/Open Curses does not specify a limit for the number of colors and color pairs which a terminal can support. However, in its use of short for the parameters, it carries over SVr4's implementation detail for the compiled terminfo database, which uses signed 16-bit numbers. ncurses provides extended versions of the functions using short parameters, allowing applications to use larger color and pair identifiers.

SVr4 curses returns ERR from pair_content if its pair argument was not initialized using init_pairs, and from color_content if the terminal does not support changing colors. ncurses does neither.

HISTORY
SVr3.2 (1987) introduced color support to curses with all of the symbols in the synopsis above except those marked as extensions. It reserved color pair 0 as the terminal's initial, “uncolored” state, and limited the number of possible color pairs to 64, because the color pair datum was encoded in six bits of a chtype.

SVr4 made only internal changes, such as moving the storage of color state from the SCREEN structure (pointed to by SP) to the TERMINAL structure (pointed to by cur_term).

Other curses implementations impose different limits on the number of colors and color pairs. PCCurses (1987-1990) provided for only 8 colors (and therefore permitted at most 8×8 = 64 color pairs). PDCurses (1992-present) initially inherited the 8-color limitation from PCCurses, but increased it to 256 in version 2.5 (2001), and widened its chtype from 16 to 32 bits. X/Open Curses (1992-present) specified a new integral type, attr_t, storing rendering attributes (see attr_on(3X)) and a color pair identifier, and a new structure type, cchar_t, to store a sequence of wide character codes separately from the character cell's attributed and color pair, allowing an increased range of color pairs. The standard specifies attr_t as a short, limiting portable values to 15 bits; negative values are invalid in System V. ncurses (1992-present), in its non-wide-character configuration, uses 8 bits of chtype for the color pair identifier.

    Version 5.3 (2002) introduced a wide-character interface, but encoded the color pair identifier with attributes in the character type.

    Since version 6 (2015), ncurses uses a separate int for the color pair identifier in a cchar_t, adding extension functions to manage the wider type. When a color pair identifier fits in 8 bits, ncurses permits manipulation of color pair identifiers with functions taking chtype arguments, even when a curses window uses wide-character cells. NetBSD curses used 6 bits for the color pair identifier from 2000 (when it first added color support) until 2004. At that point, NetBSD widened the color pair identifier to use 9 bits. As of 2025, that size is unchanged. Like ncurses before version 6, the NetBSD color pair identifier is stored in the attributes field of cchar_t, limiting the number of color pairs.

SEE ALSO
curses(3X), curs_attr(3X), curs_initscr(3X), curs_variables(3X), default_colors(3X)