curs_addch.3x - Man Page

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.

If advancement occurs at the right margin,

• the cursor automatically wraps to the beginning of the next line, then,
• if it was at the bottom of the scrolling region, and if scrollok(3X) is enabled for win, the scrolling region scrolls up one line.

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 current line of the window.
• Line feed does a clrtoeol(3X), then advances as if from the right margin.
• Tab advances the cursor to the next tab stop (possibly on the next line); these are placed at every eighth column by default. Alter the tab interval with the Tabsize extension; see curs_variables(3X).

If ch is any other nonprintable character, it is drawn 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.

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.) See curs_attr(3X) for values of predefined constants that can be usefully “or”ed with characters.

echochar and wechochar are equivalent to calling (w)addch followed by (w)refresh. curses interprets these functions as a hint that only a single character is being output; for non-control characters, a considerable performance gain may be enjoyed by employing them.

curses defines macros starting with ACS_ that can be used with waddch to write line-drawing and other special characters to the screen. ncurses terms these forms-drawing characters. The ACS default listed below is used if the acs_chars (acsc) terminfo capability does not define a terminal-specific replacement for it, or if the terminal and locale configuration requires 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.

SVr4 and other versions of curses implement the TABSIZE variable, but X/Open Curses does not specify it; see curs_variables(3X).

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.

  This implementation uses an array, acs_map, as did SVr4 curses. NetBSD 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.

Some ACS 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. However, many publicly available terminfo entries include acsc capabilities in which their key characters (pryz{|}) are embedded, and a second-hand list of their character descriptions has come to light. The ncurses developers invented ACS-prefixed names for them.

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).

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 bit operations 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(3X).
• If it cannot, use only wadd_wch(3X).