Ubuntu Manpage: start_color, has_colors, can_change_color, init_pair, init_color, init_extended

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 - curses color
       manipulation routines

SYNOPSIS

       #include <curses.h>

       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);

       /* extensions */
       void reset_color_pairs(void);

       int COLOR_PAIR(int n);
       PAIR_NUMBER(attrs);

DESCRIPTION

Overview
curses supports color attributes on terminals with that capability. To use these routines
start_color must be called, usually right after initscr. Colors are always used in pairs
(referred to as color-pairs). A color-pair consists of a foreground color (for
characters) and a background color (for the blank field on which the characters are
displayed). A programmer initializes a color-pair with the routine init_pair. After it
has been initialized, COLOR_PAIR(n) can be used to convert the pair to a video attribute.

If a terminal is capable of redefining colors, the programmer can use the routine
init_color to change the definition of a color. The routines has_colors and
can_change_color return TRUE or FALSE, depending on whether the terminal has color
capabilities and whether the programmer can change the colors. The routine color_content
allows a programmer to extract the amounts of red, green, and blue components in an
initialized color. The routine pair_content allows a programmer to find out how a given
color-pair is currently defined.

Color Rendering
The curses library combines these inputs to produce the actual foreground and background
colors shown on the screen:

• per-character video attributes (e.g., via waddch),

• the window attribute (e.g., by wattrset), and

• the background character (e.g., wbkgdset).

Per-character and window attributes are usually set by a parameter containing video
attributes including a color pair value. Some functions such as wattr_set use a separate
parameter which is the color pair number.

The background character is a special case: it includes a character value, just 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, 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 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

       In <curses.h> the following macros are defined.  These are the standard colors (ISO-6429).
       curses also assumes that COLOR_BLACK is the default background color for all terminals.

             COLOR_BLACK
             COLOR_RED
             COLOR_GREEN
             COLOR_YELLOW
             COLOR_BLUE
             COLOR_MAGENTA
             COLOR_CYAN
             COLOR_WHITE

       Some terminals support more than the eight (8) “ANSI” colors.  There are no standard names
       for those additional colors.

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.

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
legal, 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)).

• legal color pair values 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
The has_colors routine requires no arguments. It returns TRUE if the terminal can
manipulate colors; otherwise, it returns FALSE. This routine facilitates writing
terminal-independent programs. For example, a programmer can use it to decide whether to
use color or some other video attribute.

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 legal color pair value. 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 legal 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 legal color value; default colors are not allowed here.
(See the section Colors 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 and
their red, green, and blue components to 32767 on modern hardware. The extension
init_extended_color uses ints for the color value and for setting the red, green, and blue
components, allowing a larger number of colors to be supported.

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 legal 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 legal 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.

PAIR_NUMBER
PAIR_NUMBER(attrs) extracts the color value from its attrs parameter and returns it as a
color pair number.

COLOR_PAIR
Its inverse 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.

RETURN VALUE

       The routines can_change_color and has_colors return TRUE or FALSE.

       All other routines return the integer ERR upon failure and an OK (SVr4 specifies only  “an
       integer value other than ERR”) upon successful completion.

       X/Open  defines no error conditions.  SVr4 does document some error conditions which apply
       in general:

       •   This implementation will return ERR on attempts to use color values outside the  range
           0  to  COLORS-1  (except for the default colors extension), or use color pairs outside
           the range 0 to COLOR_PAIRS-1.

           Color values used in init_color must be in the range 0 to 1000.

           An error is returned from all functions if the terminal has not been initialized.

           An error is returned from secondary functions such as init_pair if start_color was not
           called.

       •   SVr4  does much the same, except that it returns ERR from pair_content if the pair was
           not initialized using init_pairs and it returns ERR from color_content if the terminal
           does not support changing colors.

           This implementation does not return ERR for either case.

       Specific functions make additional checks:

          init_color
               returns  an  error  if  the  terminal  does not support this feature, e.g., if the
               initialize_color capability is absent from the terminal description.

          start_color
               returns an error if the color table cannot be allocated.

NOTES

       In the ncurses implementation, there is a separate color activation flag,  color  palette,
       color  pairs  table,  and  associated  COLORS  and COLOR_PAIRS counts for each screen; the
       start_color function only affects the current  screen.   The  SVr4/XSI  interface  is  not
       really  designed with this in mind, and historical implementations may use a single shared
       color palette.

       Setting an implicit background color via a color pair 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 bkgd(3NCURSES).

       Several caveats apply on  older  x86  machines  (e.g.,  i386,  i486)  with  VGA-compatible
       graphics:

       •   COLOR_YELLOW  is  actually  brown.   To get yellow, use COLOR_YELLOW combined with the
           A_BOLD attribute.

       •   The A_BLINK attribute should in theory cause the background to go bright.  This  often
           fails to work, and even some cards for which it mostly works (such as the 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 settable.

HISTORY

SVr3.2 introduced color support to curses in 1987.

SVr4 made internal changes, e.g., moving the storage for the color state from SP (the
SCREEN structure) to cur_term (the TERMINAL structure), but provided the same set of
library functions.

SVr4 curses limits the number of color pairs to 64, reserving color pair zero (0) as the
terminal's initial uncolored state. This limit arises because the color pair information
is a bitfield in the chtype data type (denoted by A_COLOR).

Other implementations of curses had different limits:

• PCCurses (1987-1990) provided for only eight (8) colors.

• PDCurses (1992-present) inherited the 8-color limitation from PCCurses, but changed
this to 256 in version 2.5 (2001), along with changing chtype from 16-bits to 32-bits.

• X/Open Curses (1992-present) added a new structure cchar_t to store the character,
attributes and color-pair values, allowing increased range of color-pairs. Both
color-pairs and color-values used a signed short, limiting values to 15 bits.

• ncurses (1992-present) uses eight bits for A_COLOR in chtype values.

Version 5.3 provided a wide-character interface (2002), but left color-pairs as part
of the attributes-field.

Since version 6 (2015), ncurses uses a separate int for color-pairs in the cchar_t
values. When those color-pair values fit in 8 bits, ncurses allows color-pairs to be
manipulated via the functions using chtype values.

• NetBSD curses used 6 bits from 2000 (when colors were first supported) until 2004. At
that point, NetBSD changed to use 10 bits. As of 2021, that size is unchanged. Like
ncurses before version 6, the NetBSD color-pair information is stored in the
attributes field of cchar_t, limiting the number of color-pairs by the size of the
bitfield.

PORTABILITY

       This implementation satisfies XSI Curses's minimum maximums for COLORS and COLOR_PAIRS.

       The  init_pair  routine  accepts  negative  values  of  foreground and background color to
       support the use_default_colors(3X) extension, but only if  that  routine  has  been  first
       invoked.

       The  assumption  that COLOR_BLACK is the default background color for all terminals can be
       modified using the assume_default_colors(3X) extension.

       This implementation checks the pointers, e.g., for the values  returned  by  color_content
       and pair_content, and will treat those as optional parameters when null.

       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.  This implementation provides extended versions  of  those  functions  which  use
       short parameters, allowing applications to use larger color- and pair-numbers.

       The reset_color_pairs function is an extension of ncurses.