curs_attr(3X) Library calls curs_attr(3X)

attr_get, wattr_get, attr_set, wattr_set, attr_off, wattr_off, attr_on, wattr_on, attroff, wattroff, attron, wattron, attrset, wattrset, chgat, wchgat, mvchgat, mvwchgat, color_set, wcolor_set, standend, wstandend, standout, wstandout - manipulate attributes of character cells in curses windows

#include <curses.h>
int attr_get(attr_t *attrs, short *pair, void *opts);
int wattr_get(WINDOW *win, attr_t *attrs, short *pair, void *opts);
int attr_set(attr_t attrs, short pair, void *opts);
int wattr_set(WINDOW *win, attr_t attrs, short pair, void *opts);
int attr_off(attr_t attrs, void *opts);
int wattr_off(WINDOW *win, attr_t attrs, void *opts);
int attr_on(attr_t attrs, void *opts);
int wattr_on(WINDOW *win, attr_t attrs, void *opts);
int attroff(int attrs);
int wattroff(WINDOW *win, int attrs);
int attron(int attrs);
int wattron(WINDOW *win, int attrs);
int attrset(int attrs);
int wattrset(WINDOW *win, int attrs);
int chgat(int n, attr_t attr, short pair, const void *opts);
int wchgat(WINDOW *win,
      int n, attr_t attr, short pair, const void *opts);
int mvchgat(int y, int x,
      int n, attr_t attr, short pair, const void *opts);
int mvwchgat(WINDOW *win, int y, int x,
      int n, attr_t attr, short pair, const void *opts);
int color_set(short pair, void* opts);
int wcolor_set(WINDOW *win, short pair, void* opts);
int standend(void);
int wstandend(WINDOW *win);
int standout(void);
int wstandout(WINDOW *win);

These routines manipulate the current attributes of the named window, which then apply to all characters that are written into the window with waddch, waddstr and wprintw. Attributes are a property of the character, and move with the character through any scrolling and insert/delete line/character operations. To the extent possible, they are displayed as appropriate modifications to the graphic rendition of characters put on the screen.

These routines do not affect the attributes used when erasing portions of the window. See curs_bkgd(3X) for functions which modify the attributes used for erasing and clearing.

There are two sets of functions:

  • functions for manipulating the window attributes and color: wattr_set and wattr_get.
  • functions for manipulating only the window attributes (not color): wattr_on and wattr_off.

The wattr_set function sets the current attributes of the given window to attrs, with color specified by pair.

Use wattr_get to retrieve attributes for the given window.

Use attr_on and wattr_on to turn on window attributes, i.e., values OR'd together in attr, without affecting other attributes. Use attr_off and wattr_off to turn off window attributes, again values OR'd together in attr, without affecting other attributes.

The X/Open window attribute routines which set or get, turn on or off are extensions of older routines which assume that color pairs are OR'd into the attribute parameter. These newer routines use similar names, because X/Open simply added an underscore (_) for the newer names.

The int datatype used in the legacy routines is treated as if it is the same size as chtype (used by addch(3X)). It holds the common video attributes (such as bold, reverse), as well as a few bits for color. Those bits correspond to the A_COLOR symbol. The COLOR_PAIR macro provides a value which can be OR'd into the attribute parameter. For example, as long as that value fits into the A_COLOR mask, then these calls produce similar results:

attrset(A_BOLD | COLOR_PAIR(pair));
attr_set(A_BOLD, pair, NULL);

However, if the value does not fit, then the COLOR_PAIR macro uses only the bits that fit. For example, because in ncurses A_COLOR has eight (8) bits, then COLOR_PAIR(259) is 4 (i.e., 259 is 4 more than the limit 255).

The PAIR_NUMBER macro extracts a pair number from an int (or chtype). For example, the input and output values in these statements would be the same:

int value = A_BOLD | COLOR_PAIR(input);
int output = PAIR_NUMBER(value);

The attrset routine is a legacy feature predating SVr4 curses but kept in X/Open Curses for the same reason that SVr4 curses kept it: compatibility.

The remaining attr* functions operate exactly like the corresponding attr_* functions, except that they take arguments of type int rather than attr_t.

There is no corresponding attrget function as such in X/Open Curses, although ncurses provides getattrs (see curs_legacy(3X)).

The routine chgat changes the attributes of a given number of characters starting at the current cursor location of stdscr. It does not update the cursor and does not perform wrapping. A character count of -1 or greater than the remaining window width means to change attributes all the way to the end of the current line. The wchgat function generalizes this to any window; the mvwchgat function does a cursor move before acting.

In these functions, the color pair argument is a color pair index (as in the first argument of init_pair, see curs_color(3X)).

The routine color_set sets the current color of the given window to the foreground/background combination described by the color pair parameter.

The routine standout is the same as attron(A_STANDOUT). The routine standend is the same as attrset(A_NORMAL) or attrset(0), that is, it turns off all attributes.

X/Open Curses does not mark these “restricted”, because

  • they have well established legacy use, and
  • there is no ambiguity about the way the attributes might be combined with a color pair.

The following video attributes, defined in <curses.h>, can be passed to the routines attron, attroff, and attrset, or OR'd with the characters passed to addch (see curs_addch(3X)).

Name Description
A_NORMAL Normal display (no highlight)
A_STANDOUT Best highlighting mode of the terminal
A_UNDERLINE Underlining
A_REVERSE Reverse video
A_BLINK Blinking
A_DIM Half bright
A_BOLD Extra bright or bold
A_PROTECT Protected mode
A_INVIS Invisible or blank mode
A_ALTCHARSET Alternate character set
A_ITALIC Italics (non-X/Open extension)
A_CHARTEXT Bit-mask to extract a character
A_COLOR Bit-mask to extract a color (legacy routines)

These video attributes are supported by attr_on and related functions (which also support the attributes recognized by attron, etc.):

Name Description
WA_HORIZONTAL Horizontal highlight
WA_LEFT Left highlight
WA_LOW Low highlight
WA_RIGHT Right highlight
WA_TOP Top highlight
WA_VERTICAL Vertical highlight

The return values of many of these routines are not meaningful (they are implemented as macro-expanded assignments and simply return their argument). The SVr4 manual page claims (falsely) that these routines always return 1.

All routines return the integer OK on success, or ERR on failure.

X/Open Curses does not specify any error conditions.

This implementation

  • returns an error if the window pointer is null.
  • returns an error if the color pair parameter for wcolor_set is outside the range 0..COLOR_PAIRS-1.
  • does not return an error if either of the parameters of wattr_get used for retrieving attribute or color pair values is NULL.

Functions prefixed with “mv” first perform cursor movement and fail if the position (y, x) is outside the window boundaries.

These functions may be macros:

attroff, wattroff, attron, wattron, attrset, wattrset, standend and standout.

Color pair values can only be OR'd with attributes if the pair number is less than 256. The alternate functions such as color_set can pass a color pair value directly. However, ncurses ABI 4 and 5 simply OR this value within the alternate functions. You must use ncurses ABI 6 to support more than 256 color pairs.

This implementation provides the A_ITALIC attribute for terminals which have the enter_italics_mode (sitm) and exit_italics_mode (ritm) capabilities. Italics are not mentioned in X/Open Curses. Unlike the other video attributes, A_ITALIC is unrelated to the set_attributes capabilities. This implementation makes the assumption that exit_attribute_mode may also reset italics.

Each of the functions added by XSI Curses has a parameter opts, which X/Open Curses still (after more than twenty years) documents as reserved for future use, saying that it should be NULL. This implementation uses that parameter in ABI 6 for the functions which have a color pair parameter to support extended color pairs:

  • For functions which modify the color, e.g., wattr_set and wattr_on, if opts is set it is treated as a pointer to int, and used to set the color pair instead of the short pair parameter.
  • For functions which retrieve the color, e.g., wattr_get, if opts is set it is treated as a pointer to int, and used to retrieve the color pair as an int value, in addition to retrieving it via the standard pointer to short parameter.
  • For functions which turn attributes off, e.g., wattr_off, the opts parameter is ignored except except to check that it is NULL.

These functions are described in X/Open Curses, Issue 4. The standard defined the dedicated type for highlights, attr_t, which was not defined in SVr4 curses. The functions taking attr_t arguments were not supported under SVr4.

Very old versions of this library did not force an update of the screen when changing the attributes. Use touchwin to force the screen to match the updated attributes.

X/Open Curses states that whether the traditional functions attron/attroff/attrset can manipulate attributes other than A_BLINK, A_BOLD, A_DIM, A_REVERSE, A_STANDOUT, or A_UNDERLINE is “unspecified”. Under this implementation as well as SVr4 curses, these functions correctly manipulate all other highlights (specifically, A_ALTCHARSET, A_PROTECT, and A_INVIS).

X/Open Curses added these entry points:

attr_get, attr_on, attr_off, attr_set, wattr_on, wattr_off, wattr_get, wattr_set

The new functions are intended to work with a new series of highlight macros prefixed with WA_. The older macros have direct counterparts in the newer set of names:

Name Description
WA_NORMAL Normal display (no highlight)
WA_STANDOUT Best highlighting mode of the terminal
WA_UNDERLINE Underlining
WA_REVERSE Reverse video
WA_BLINK Blinking
WA_DIM Half bright
WA_BOLD Extra bright or bold
WA_ALTCHARSET Alternate character set

X/Open Curses does not assign values to these symbols, nor does it state whether or not they are related to the similarly-named A_NORMAL, etc.:

  • X/Open Curses specifies that each pair of corresponding A_ and WA_-using functions operates on the same current-highlight information.
  • However, in some implementations, those symbols have unrelated values.
For example, the Solaris xpg4 (X/Open) curses declares attr_t to be an unsigned short integer (16-bits), while chtype is a unsigned integer (32-bits). The WA_ symbols in this case are different from the A_ symbols because they are used for a smaller datatype which does not represent A_CHARTEXT or A_COLOR.
In this implementation (as in many others), the values happen to be the same because it simplifies copying information between chtype and cchar_t variables.
Because ncurses's attr_t can hold a color pair (in the A_COLOR field), a call to wattr_on, wattr_off, or wattr_set may alter the window's color. If the color pair information in the attribute parameter is zero, no change is made to the window's color.
This is consistent with SVr4 curses; X/Open Curses does not specify this.

The X/Open Curses extended conformance level adds new highlights A_HORIZONTAL, A_LEFT, A_LOW, A_RIGHT, A_TOP, A_VERTICAL (and corresponding WA_ macros for each). As of August 2013, no known terminal provides these highlights (i.e., via the sgr1 capability).

X/Open Curses is largely based on SVr4 curses, adding support for “wide-characters” (not specific to Unicode). Some of the X/Open differences from SVr4 curses address the way video attributes can be applied to wide-characters. But aside from that, attrset and attr_set are similar. SVr4 curses provided the basic features for manipulating video attributes. However, earlier versions of curses provided a part of these features.

As seen in 2.8BSD, curses assumed 7-bit characters, using the eighth bit of a byte to represent the standout feature (often implemented as bold and/or reverse video). The BSD curses library provided functions standout and standend which were carried along into X/Open Curses due to their pervasive use in legacy applications.

Some terminals in the 1980s could support a variety of video attributes, although the BSD curses library could do nothing with those. System V (1983) provided an improved curses library. It defined the A_ symbols for use by applications to manipulate the other attributes. There are few useful references for the chronology.

Goodheart's book UNIX Curses Explained (1991) describes SVr3 (1987), commenting on several functions:

  • the attron, attroff, attrset functions (and most of the functions found in SVr4 but not in BSD curses) were introduced by System V,
  • the alternate character set feature with A_ALTCHARSET was added in SVr2 and improved in SVr3 (by adding acs_map[]),
  • start_color and related color-functions were introduced by System V.3.2,
  • pads, soft-keys were added in SVr3, and

Goodheart did not mention the background character or the cchar_t type. Those are respectively SVr4 and X/Open features. He did mention the A_ constants, but did not indicate their values. Those were not the same in different systems, even for those marked as System V.

Different Unix systems used different sizes for the bit-fields in chtype for characters and colors, and took into account the different integer sizes (32-bit versus 64-bit).

This table showing the number of bits for A_COLOR and A_CHARTEXT was gleaned from the curses header files for various operating systems and architectures. The inferred architecture and notes reflect the format and size of the defined constants as well as clues such as the alternate character set implementation. A 32-bit library can be used on a 64-bit system, but not necessarily the reverse.

Year System Arch Color Char Notes
1992 Solaris 5.2 32 6 17 SVr4 curses
1992 HP-UX 9 32 no 8 SVr2 curses
1992 AIX 3.2 32 no 23 SVr2 curses
1994 OSF/1 r3 32 no 23 SVr2 curses
1995 HP-UX 10.00 32 6 16 SVr3 curses_colr
1995 HP-UX 10.00 32 6 8 SVr4, X/Open curses
1995 Solaris 5.4 32/64 7 16 X/Open curses
1996 AIX 4.2 32 7 16 X/Open curses
1996 OSF/1 r4 32 6 16 X/Open curses
1997 HP-UX 11.00 32 6 8 X/Open curses
2000 U/Win 32/64 7/31 16 uses chtype


Regarding HP-UX,

  • HP-UX 10.20 (1996) added support for 64-bit PA-RISC processors in 1996.
  • HP-UX 10.30 (1997) marked “curses_colr” obsolete. That version of curses was dropped with HP-UX 11.30 in 2006.

Regarding OSF/1 (and Tru64),

  • These used 64-bit hardware. Like ncurses, the OSF/1 curses interface is not customized for 32-bit and 64-bit versions.
  • Unlike other systems which evolved from AT&T code, OSF/1 provided a new implementation for X/Open curses.

Regarding Solaris,

  • The initial release of Solaris was in 1992.
  • The xpg4 (X/Open) curses was developed by MKS from 1990 to 1995. Sun's copyright began in 1996.
  • Sun updated the X/Open curses interface after 64-bit support was introduced in 1997, but did not modify the SVr4 curses interface.

Regarding U/Win,

  • Development of the curses library began in 1991, stopped in 2000.
  • Color support was added in 1998.
  • The library uses only chtype (no cchar_t).

Once X/Open curses was adopted in the mid-1990s, the constraint of a 32-bit interface with many colors and wide-characters for chtype became a moot point. The cchar_t structure (whose size and members are not specified in X/Open Curses) could be extended as needed.

Other interfaces are rarely used now:

BSD curses was improved slightly in 1993/1994 using Keith Bostic's modification to make the library 8-bit clean for nvi(1). He moved standout attribute to a structure member.
The resulting 4.4BSD curses was replaced by ncurses over the next ten years.
U/Win is rarely used now.

curses(3X), curs_addch(3X), curs_addstr(3X), curs_bkgd(3X), curs_printw(3X), curs_variables(3X)

2024-04-27 ncurses 6.5