curs_addch(3X)

NAME
addch, waddch, mvaddch, mvwaddch, echochar, wechochar - add a curses character to a window and advance the cursor

SYNOPSIS
#include <curses.h>

int addch(const chtype ch);
int waddch(WINDOW * win, const chtype ch);
int mvaddch(int y, int x, const chtype ch);
int mvwaddch(WINDOW * win, int y, int x, const chtype ch);

int echochar(const chtype ch);
int wechochar(WINDOW * win, const chtype ch);

/* (integer) constants */
/* ... */ ACS_BLOCK;
/* ... */ ACS_BOARD;
/* ... */ ACS_BTEE;
/* ... */ ACS_BULLET;
/* ... */ ACS_CKBOARD;
/* ... */ ACS_DARROW;
/* ... */ ACS_DEGREE;
/* ... */ ACS_DIAMOND;
/* ... */ ACS_HLINE;
/* ... */ ACS_LANTERN;
/* ... */ ACS_LARROW;
/* ... */ ACS_LLCORNER;
/* ... */ ACS_LRCORNER;
/* ... */ ACS_LTEE;
/* ... */ ACS_PLMINUS;
/* ... */ ACS_PLUS;
/* ... */ ACS_RARROW;
/* ... */ ACS_RTEE;
/* ... */ ACS_S1;
/* ... */ ACS_S9;
/* ... */ ACS_TTEE;
/* ... */ ACS_UARROW;
/* ... */ ACS_ULCORNER;
/* ... */ ACS_URCORNER;
/* ... */ ACS_VLINE;
/* extensions */
/* ... */ ACS_GEQUAL;
/* ... */ ACS_LEQUAL;
/* ... */ ACS_NEQUAL;
/* ... */ ACS_PI;
/* ... */ ACS_S3;
/* ... */ ACS_S7;
/* ... */ ACS_STERLING;

DESCRIPTION

  waddch
waddch writes the curses character ch to the window win, then advances the cursor position, analogously to the standard C library's putchar(3). ncurses(3X) describes the variants of this function.

Construct a curses character from a char by assignment or typecast. Subsection “Video Attributes” of attron(3X) describes how to manipulate its attributes and color pair. (A color pair selection is not honored unless initialized; see start_color(3X).)

The object or expression ch may contain attributes and/or a color pair identifier. (A chtype can be copied from place to place using winch(3X) and waddch.) curses defines constants to aid the manipulation of character attributes; see curs_attr(3X). A ch whose character component is a space, and whose only attribute is A_NORMAL, is a blank character, and therefore combines with the window's background character; see curs_bkgd(3X).

If ch is a backspace, carriage return, line feed, or tab, the cursor moves appropriately within the window. Backspace moves the cursor one character left; at the left margin of a window, it does nothing. Carriage return moves the cursor to the left margin on the same line of the window. Line feed performs clrtoeol(3X), then advances as if from the right margin (see below). Tab advances the cursor to the next tab stop (possibly on the next line); these are placed at every eighth column by default.

    You can alter the tab interval with the TABSIZE extension; see curs_variables(3X).

If ch is any other nonprintable character, curses draws it in printable form using the same convention as unctrl(3X). Calling winch(3X) on the location of a nonprintable character does not return the character itself, but its unctrl(3X) representation.

Adding printable characters with waddch causes it to wrap at the right margin of the window. If the cursor is not at the bottom of the scrolling region and advancement occurs at the right margin, the cursor automatically wraps to the beginning of the next line. If the cursor is at the bottom of the scrolling region when advancement occurs at the right margin, and scrollok(3X) is enabled for win, the scrolling region scrolls up one line and the cursor wraps as above. Otherwise, advancement and scrolling do not occur, and waddch returns ERR.

A window's margins may coincide with the screen boundaries. This may be a problem when ncurses updates the screen to match the curses window. When their right and bottom margins coincide, ncurses employs varying strategies to handle the ways in which scrolling and wrapping behaviors combine at the lower-right corner of the terminal display by detecting and configuring the terminal's capabilities. If the terminal does not wrap its cursor to the next line when a character is written to the rightmost cell on a line — (terminfo capability auto_right_margin (am) — ncurses simply writes the character. If the terminal possesses the “auto right margin” capability, but also enables it to be configured on and off, ncurses disables auto right margin temporarily when writing a character to lower-right cell of the terminal display. If the terminal does not permit auto right margin deconfiguration, but has an “insert mode” — terminfo capabilities enter_insert_mode (smir) and exit_insert_mode (rmir) — ncurses saves the contents of the character cell preceding that at the bottom right, writes the character with which the application wishes to populate the bottom right character cell to the cell preceding it on the same line, then uses the terminal's insert mode to rewrite the contents of the saved character cell to that location, “pushing” the desired character into the bottom right cell.

  wechochar
echochar and wechochar are equivalent to calling (w)addch followed by (w)refresh on stdscr or the specified window. curses interprets these functions as a hint to its optimizer that only a single character cell in the window is being altered between refreshes; for non-control characters, a considerable performance gain may be enjoyed by employing them.

  Forms-Drawing Characters
curses defines macros starting with ACS_ that can be used with waddch to write line-drawing and other symbols to the screen. ncurses terms these forms-drawing characters. curses uses the ACS default listed below if the terminal type lacks the acs_chars (acsc) capability; that capability does not define a replacement for the character; or if the terminal type and locale configuration require Unicode to access these characters, but the library is unable to use Unicode. The “acsc char” column corresponds to how the characters are specified in the acs_chars (acsc) string capability, and the characters in it may appear on the screen if the terminal type's database entry incorrectly advertises ACS support. The name “ACS” originates in the Alternate Character Set feature of the DEC VT100 terminal.

 | ACS | acsc |
Symbol | Default | char | Glyph Name
ACS_BLOCK | # | 0 | solid square block
ACS_BOARD | # | h | board of squares
ACS_BTEE | + | v | bottom tee
ACS_BULLET | o | ~ | bullet
ACS_CKBOARD | : | a | checker board (stipple)
ACS_DARROW | v | . | arrow pointing down
ACS_DEGREE | ' | f | degree symbol
ACS_DIAMOND | + | ` | diamond
ACS_GEQUAL | > | > | greater-than-or-equal-to
ACS_HLINE | - | q | horizontal line
ACS_LANTERN | # | i | lantern symbol
ACS_LARROW | < | , | arrow pointing left
ACS_LEQUAL | < | y | less-than-or-equal-to
ACS_LLCORNER | + | m | lower left-hand corner
ACS_LRCORNER | + | j | lower right-hand corner
ACS_LTEE | + | t | left tee
ACS_NEQUAL | ! | | | not-equal
ACS_PI | * | { | greek pi
ACS_PLMINUS | # | g | plus/minus
ACS_PLUS | + | n | plus
ACS_RARROW | > | + | arrow pointing right
ACS_RTEE | + | u | right tee
ACS_S1 | - | o | scan line 1
ACS_S3 | - | p | scan line 3
ACS_S7 | - | r | scan line 7
ACS_S9 |  | s | scan line 9
ACS_STERLING | f | } | pound-sterling symbol
ACS_TTEE | + | w | top tee
ACS_UARROW | ^ | - | arrow pointing up
ACS_ULCORNER | + | l | upper left-hand corner
ACS_URCORNER | + | k | upper right-hand corner
ACS_VLINE | | | x | vertical line

RETURN VALUE
These functions return OK on success and ERR on failure.

In ncurses, these functions fail if the curses screen has not been initialized, (for functions taking a WINDOW pointer argument) win is a null pointer, wrapping to a new line is impossible because scrollok(3X) has not been called on win (or stdscr, as applicable) when a write to its bottom right location is attempted, or it is not possible to add a complete character at the cursor position.

The last may be due to different causes: conversion of a wide character to a multibyte character sequence can fail, or at least one of the bytes resulting from wide character conversion to a multibyte character sequence cannot be added to the window. See section “PORTABILITY” below regarding the use of waddch with wide characters.

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

NOTES
addch, mvaddch, mvwaddch, and echochar may be implemented as macros.

EXTENSIONS
The symbols ACS_S3, ACS_S7, ACS_LEQUAL, ACS_GEQUAL, ACS_PI, ACS_NEQUAL, and ACS_STERLING were not documented in any publicly released System V and are not standard. However, many publicly available terminfo entries include acs_chars (acsc) capabilities in which their key characters (pryz{|}) are embedded, and a second-hand list of their character descriptions has come to light, which identifies them as VT100 special characters.

The DEC Special Character and Line Drawing Set (VT100) is indexed by an ASCII character in the range 96 (`) to 126 (~). That index character is part of the definition for the curses ACS_ symbols. The VT100 special characters can be categorized in three groups: useful graphic symbols with a standard ACS_ symbol, (e.g., the line-drawing symbols), possibly useful characters (these non-standard symbols), representations of control characters (e.g., newline and vertical tabulation).

A few ACS_ symbols do not fit into DEC's VT100 scheme. The AT&T Teletype 5410v1 arrow symbols and ACS_BLOCK use indices outside the range 96 to 126. Two of the Teletype symbols use indices in that range, with different meaning versus the VT100: ACS_BOARD corresponds to the VT100 symbol for newline ACS_LANTERN corresponds to the VT100 symbol for vertical tabulation

AT&T defined ACS_ names for the most useful graphic symbols, as well as for its own. Its header file commented:

    /*
     * Standard alternate character set.  The current ACS world is
     * evolving, so we support only a widely available subset: the
     * line drawing characters from the VT100, plus a few from the
     * Teletype 5410v1.  Eventually there may be support of more
     * sophisticated ACS line drawing, such as that in the Teletype
     * 5410, the HP line drawing set, and the like.  There may be
     * support for some non line oriented characters as well.
     *
     * Line drawing ACS names are of the form ACS_trbl, where t is
     * the top, r is the right, b is the bottom, and l is the left.
     * t, r, b, and l might be B (blank), S (single), D (double), or
     * T (thick).  The subset defined here only uses B and S.
     */

Although these less-useful graphic symbols were not given names, they were used in terminfo entries. The ncurses developers invented ACS-prefixed names for them.

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.

SVr4 describes a successful return value only as “an integer value other than ERR”.

The defaults specified for forms-drawing characters apply in the POSIX locale.

  ACS Symbols
X/Open Curses states that the ACS_ definitions are char constants. Some implementations are problematic. Solaris curses, for example, defines the ACS symbols as constants; others define them as elements of an array.

    SVr4 used an array, acs_map, as does ncurses. NetBSD curses also uses an array, actually named _acs_char, with a “#define” for compatibility. HP-UX curses equates some of the ACS_ symbols to the analogous WACS_ symbols as if the ACS_ symbols were wide characters (see curs_add_wch(3X)). The misdefined symbols are the arrows and others that are not used for line drawing. X/Open Curses (Issues 2 through 7) has a typographical error for the ACS_LANTERN symbol, equating its “VT100+ Character” to “I” (capital I), while the header files for SVr4 curses and other implementations use “i” (small i).

    None of the terminal descriptions on Unix platforms use uppercase I, except for Solaris (in its terminfo entry for screen(1), apparently based on the X/Open documentation around 1995). On the other hand, its gs6300 (AT&T PC6300 with EMOTS Terminal Emulator) description uses lowercase i.

The displayed values of ACS_ constants depend on the ncurses ABI — for example, wide-character versus non-wide-character configurations (the former is capable of displaying Unicode while the latter is not), and whether the locale uses UTF-8 encoding.

In certain cases, the terminal is unable to display forms-drawing characters except by using UTF-8; see the discussion of the NCURSES_NO_UTF8_ACS environment variable in ncurses(3X).

  Character Set
X/Open Curses assumes that the parameter passed to waddch contains a single character. That character may have been more than eight bits wide in an SVr3 or SVr4 implementation, but X/Open Curses leaves the width of a non-wide character code unspecified. The standard further does not specify the internal structure of a chtype, though the use of bitwise operators to combine the character code with attributes and a color pair identifier into a chtype for passage to waddch is common. A portable application uses only the macros discussed in curs_attr(3X) to manipulate a chtype.

In ncurses, chtype holds an eight-bit character, but the library allows a multibyte character sequence to be passed via a succession of calls to waddch. Other implementations do not; a waddch call transmits exactly one character, which may be rendered in one or more screen locations depending on whether it is printable (see unctrl(3X)). Depending on the locale, ncurses inspects the byte passed in each waddch call and checks whether the latest call continues a multibyte character. When a character is complete, ncurses displays the character and advances the cursor. If the calling application interrupts the succession of bytes in a multibyte character sequence by changing the current location — for example, with wmove(3X) — ncurses discards the incomplete character.

For portability to other implementations, do not rely upon the foregoing behavior. Check whether a character can be represented as a single byte in the current locale. If it can, call either waddch or wadd_wch. If it cannot, use only wadd_wch.

HISTORY
4BSD (1980) introduced waddch and its variants.

SVr3 (1987) added the echochar and wechochar functions and most of the ACS_ constants, except for ACS_GEQUAL, ACS_LEQUAL, ACS_NEQUAL, ACS_PI, ACS_S3, ACS_S7, and ACS_STERLING.

ncurses 1.9.6 (1995) furnished the remaining ACS_ constants.

SEE ALSO
curs_add_wch(3X) describes comparable functions of the ncurses library in its wide-character configuration (ncursesw).

curses(3X), curs_addchstr(3X), curs_addstr(3X), curs_attr(3X), curs_bkgd(3X), curs_clear(3X), curs_inch(3X), curs_outopts(3X), curs_refresh(3X), curs_variables(3X), putchar(3)