curs_getch(3X)

NAME
getch, wgetch, mvgetch, mvwgetch, ungetch, has_key - get (or push back) characters from curses terminal keyboard buffer

SYNOPSIS
#include <curses.h>

int getch(void);
int wgetch(WINDOW * win);
int mvgetch(int y, int x);
int mvwgetch(WINDOW * win, int y, int x);

int ungetch(int c);

/* extension */
int has_key(int c);

DESCRIPTION

  Reading Characters
wgetch gathers a key event from the terminal input stream associated with a curses window win. ncurses(3X) describes the variants of this function.

When input is pending, wgetch returns an integer identifying the key event; for alphanumeric and punctuation keys, the space bar, and (usually) the Backspace, Tab, Return, and Escape keys, this value corresponds to the character encoding used by the terminal. Use of the control key as a modifier, by holding it down while pressing and releasing another key, often results in a distinct code. The behavior of other keys depends on whether win is in keypad mode; see subsection “Keypad Mode” below.

If no input is pending, then if the no-delay flag is set in the window (see nodelay(3X)), the function returns ERR; otherwise, curses waits until the terminal has input. If cbreak(3X) or raw(3X) has been called, this happens after curses reads one key event. If nocbreak(3X) or noraw(3X) has been called, it occurs when curses reads a newline. (Because the terminal's canonical or “cooked” mode is line-buffered, multiple wgetch calls may then be necessary to empty the input queue.) If halfdelay(3X) has been called, curses waits until input is available or the specified delay elapses.

If echo(3X) has been called, and the window is not a pad, curses writes the returned character c to the window (at the cursor position) per the following rules. If c matches the terminal's erase character (see erasechar(3X)), and the cursor is not at the window's leftmost column, the cursor moves leftward one position and the new position is erased as if wmove(3X) and then wdelch(3X) were called. When the window's keypad mode is enabled (see below), KEY_LEFT and KEY_BACKSPACE are handled the same way. curses writes any other c to the window, as with wechochar(3X). If the window win has been moved or modified since the last call to wrefresh(3X), curses calls wrefresh on it.

If c is a carriage return and nl(3X) has been called, wgetch returns the character code for line feed instead.

  Keypad Mode
Call keypad(3X) on a window to configure keypad mode when reading input from it. In keypad mode, curses treats key strokes not from the alphabetic section of the keyboard (those corresponding to the ECMA-6 character set — see ascii(7) — optionally modified by either the control or shift keys) as function keys. (In curses, the term “function key” includes but is not limited to keycaps engraved with “F1”, “PF1”, and so on.) If a window is in keypad mode, wgetch translates these key strokes to a numeric code corresponding to the KEY_ symbols listed in subsection “Key Codes” below. If the window is not in keypad mode, the input queue populates with the characters of the function key's string capability (see terminfo(5)), which the application must collect individually with multiple wgetch calls. The curses.h header file declares many function keys whose names begin with KEY_; these object-like macros have integer values outside the range of eight-bit character codes. In ncurses, define_key(3X) configures user-defined function keys; they have no names, but are also expected to have integer values outside the range of eight-bit character codes.

A variable intended to hold a function key code must thus be of type short or larger.

Most terminals one encounters follow the ECMA-48 standard insofar as their function keys produce character sequences prefixed with the escape character ESC. This fact implies that curses cannot distinguish a user's press of the escape key (or any prefix common to multiple function key definitions) from the beginning of a function key's character sequence without waiting to see if, and how soon, further input arrives. If the escape sequence matches a string capability defining a function key for the terminal type (such as key_home (khome) or key_up (kuu1)), wgetch returns the function key code corresponding to the unique sequence defined by the terminal. If the escape sequence matches no function keys defined for the terminal type, call wgetch repeatedly to obtain the codes of the individual characters of the sequence, in the order they occurred in the input. If wgetch cannot decide the validity of the input as a function key because it has not read enough characters to disambiguate it, the function waits until it has this information or the escape delay elapses. Configure the escape delay with the global variable ESCDELAY, an extension (see section “EXTENSIONS” below), or the environment variable of the same name (see section “ENVIRONMENT” of ncurses(3X)), also an extension.

Consequently, a user of a curses application that employs keypad mode may experience a pause or “hang” after pressing the escape key while curses collects sufficient characters to disambiguate the input. If the window is in “no time-out” mode, the escape delay is effectively infinite; see notimeout(3X). In the event of such a pause, further typing “awakens” curses.

  Ungetting Characters
ungetch places c into the input queue to be returned by the next call to wgetch. A single input queue serves all windows associated with the screen.

  Key Codes
The header file curses.h defines the following function key codes. Except for the special case of KEY_RESIZE, a window's keypad mode must be enabled for wgetch to read these codes from it. Not all of these are necessarily supported on any particular terminal. The naming convention may seem obscure, with some apparent misspellings (such as “RSUME” for “resume”); the names correspond to the terminfo capability names for the keys, and were standardized before the IBM PC/AT keyboard layout achieved a dominant position in industry.

    Symbol | Key name
    KEY_BREAK | Break key
    KEY_DOWN | Arrow keys
    KEY_UP |
    KEY_LEFT |
    KEY_RIGHT |
    KEY_HOME | Home key (upward+left arrow)
    KEY_BACKSPACE | Backspace
    KEY_F0 | Function keys; space for 64 keys is reserved
    KEY_F(n) | Function key n where 0 ≤ n ≤ 63
    KEY_DL | Delete line
    KEY_IL | Insert line
    KEY_DC | Delete character
    KEY_IC | Insert character/Enter insert mode
    KEY_EIC | Exit insert character mode
    KEY_CLEAR | Clear screen
    KEY_EOS | Clear to end of screen
    KEY_EOL | Clear to end of line
    KEY_SF | Scroll one line forward
    KEY_SR | Scroll one line backward (reverse)
    KEY_NPAGE | Next page/Page up
    KEY_PPAGE | Previous page/Page down
    KEY_STAB | Set tab
    KEY_CTAB | Clear tab
    KEY_CATAB | Clear all tabs
    KEY_ENTER | Enter/Send
    KEY_SRESET | Soft (partial) reset
    KEY_RESET | (Hard) reset
    KEY_PRINT | Print/Copy
    KEY_LL | Home down/Bottom (lower left)
    KEY_A1 | Upper left of keypad
    KEY_A3 | Upper right of keypad
    KEY_B2 | Center of keypad
    KEY_C1 | Lower left of keypad
    KEY_C3 | Lower right of keypad
    KEY_BTAB | Back tab key
    KEY_BEG | Beg(inning) key
    KEY_CANCEL | Cancel key
    KEY_CLOSE | Close key
    KEY_COMMAND | Cmd (command) key
    KEY_COPY | Copy key
    KEY_CREATE | Create key
    KEY_END | End key
    KEY_EXIT | Exit key
    KEY_FIND | Find key
    KEY_HELP | Help key
    KEY_MARK | Mark key
    KEY_MESSAGE | Message key
    KEY_MOUSE | Mouse event occurred
    KEY_MOVE | Move key
    KEY_NEXT | Next object key
    KEY_OPEN | Open key
    KEY_OPTIONS | Options key
    KEY_PREVIOUS | Previous object key
    KEY_REDO | Redo key
    KEY_REFERENCE | Ref(erence) key
    KEY_REFRESH | Refresh key
    KEY_REPLACE | Replace key
    KEY_RESIZE | Screen resized
    KEY_RESTART | Restart key
    KEY_RESUME | Resume key
    KEY_SAVE | Save key
    KEY_SELECT | Select key
    KEY_SUSPEND | Suspend key
    KEY_UNDO | Undo key
    KEY_SBEG | Shifted beginning key
    KEY_SCANCEL | Shifted cancel key
    KEY_SCOMMAND | Shifted command key
    KEY_SCOPY | Shifted copy key
    KEY_SCREATE | Shifted create key
    KEY_SDC | Shifted delete character key
    KEY_SDL | Shifted delete line key
    KEY_SEND | Shifted end key
    KEY_SEOL | Shifted clear line key
    KEY_SEXIT | Shifted exit key
    KEY_SFIND | Shifted find key
    KEY_SHELP | Shifted help key
    KEY_SHOME | Shifted home key
    KEY_SIC | Shifted insert key
    KEY_SLEFT | Shifted left arrow key
    KEY_SMESSAGE | Shifted message key
    KEY_SMOVE | Shifted move key
    KEY_SNEXT | Shifted next object key
    KEY_SOPTIONS | Shifted options key
    KEY_SPREVIOUS | Shifted previous object key
    KEY_SPRINT | Shifted print key
    KEY_SREDO | Shifted redo key
    KEY_SREPLACE | Shifted replace key
    KEY_SRIGHT | Shifted right arrow key
    KEY_SRSUME | Shifted resume key
    KEY_SSAVE | Shifted save key
    KEY_SSUSPEND | Shifted suspend key
    KEY_SUNDO | Shifted undo key

Many keyboards feature a nine-key directional pad.

    A1 | up | A3
    left | B2 | right
    C1 | down | C3

Two of the symbols in the list above do not correspond to a physical key. wgetch returns KEY_RESIZE, even if the window's keypad mode is disabled, if ncurses has handled a SIGWINCH signal since wgetch was called; see initscr(3X) and resizeterm(3X). wgetch returns KEY_MOUSE to indicate that a mouse event is pending collection; see curs_mouse(3X). Receipt of this code requires a window's keypad mode to be enabled, because to interpret mouse input (as with xterm(1)'s mouse protocol), ncurses must read an escape sequence, as with a function key.

  Testing Key Codes
In ncurses, has_key returns a Boolean value indicating whether the terminal type recognizes its parameter as a key code value. See also define_key(3X) and key_defined(3X).

RETURN VALUE
wgetch returns a key code identifying the key event as described above, which may include KEY_RESIZE or KEY_MOUSE indicating non-key events, or ERR on failure. wgetch fails if its timeout expires without any data arriving, which cannot happen if nodelay(3X) is in effect on the window.

In ncurses, wgetch also fails if the curses screen has not been initialized, (for functions taking a WINDOW pointer argument) win is a null pointer, the terminal input stream is closed, in which case errno is set to EBADF, or execution was interrupted by a signal, in which case errno is set to EINTR.

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

ungetch returns OK on success and ERR on failure. In ncurses, ungetch fails if the curses screen has not been initialized, or there is no more room in the input queue.

has_key returns TRUE or FALSE.

NOTES
getch, mvgetch, and mvwgetch may be implemented as macros.

curses discourages assignment of the ESC key to a discrete function by the programmer because the library requires a delay while it awaits the potential remainder of a terminal escape sequence.

Some key strokes are indistinguishable from control characters; for example, KEY_ENTER may be the same as ^M, and KEY_BACKSPACE may be the same as ^H or ^?. Consult the terminfo entry for the terminal type to determine whether this is the case; see infocmp(1). Some curses implementations, including ncurses, honor the terminfo key definitions; others treat such control characters specially.

curses distinguishes the Enter keys in the alphabetic and numeric keypad sections of a keyboard because (most) terminals do. KEY_ENTER refers to the key on the numeric keypad and, like other function keys, is reliably recognized only if the window's keypad mode is enabled. The terminfo key_enter (kent) capability describes the character (sequence) sent by the Enter key of a terminal's numeric (or similar) keypad. “Enter or send” is X/Open Curses's description of this key.

curses treats the Enter or Return key in the alphabetic section of the keyboard differently. It usually produces a control code for carriage return (^M) or line feed (^J). Depending on the terminal mode (raw, cbreak, or canonical), and whether nl(3X) or nonl(3X) has been called, wgetch may return either a carriage return or line feed upon an Enter or Return key stroke.

Use of wgetch with echo(3X) and neither cbreak(3X) nor raw(3X) is not well-defined.

Historically, the list of key code macros above was influenced by the keyboard of the AT&T 7300 (also known variously as the “3B1”, “Safari 4”, and “UNIX PC”), a 1985 machine rich in function keys. Today's computer keyboards are based on that of the IBM PC/AT and tend to have fewer. A curses application can expect such a keyboard to transmit key codes KEY_UP, KEY_DOWN, KEY_LEFT, KEY_RIGHT, KEY_HOME, KEY_END, KEY_PPAGE (Page Up), KEY_NPAGE (Page Down), KEY_IC (Insert), KEY_DC (Delete), KEY_A1, KEY_A3, KEY_B2, KEY_C1, KEY_C3, and KEY_F(n) for 1 ≤ n ≤ 12.

EXTENSIONS
In ncurses, when a window's “no time-out” mode is not set, the ESCDELAY variable configures the duration of the timer used to disambiguate a function key character sequence from a series of key strokes beginning with their common prefix (such as ESC) when typed by the user; see curs_variables(3X).

has_key is an ncurses extension, and is 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.

Except as noted in section “EXTENSIONS” above, 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”.

wgetch reads only single-byte characters.

The echo behavior of these functions on input of KEY_ or backspace characters is not documented in SVr4 curses.

The behavior of wgetch in the presence of signal handlers is not documented in SVr4 curses and is unspecified by X/Open Curses. In historical curses implementations, it varied depending on whether the operating system's dispatch of a signal to a handler interrupted a read(2) call in progress, and also (in some implementations) whether an input timeout or non-blocking mode had been set. A portable curses application prepares for two cases: (a) signal receipt does not interrupt wgetch; and (b) signal receipt interrupts wgetch and causes it to return ERR with errno set to EINTR.

KEY_MOUSE is mentioned in X/Open Curses, along with a few related terminfo capabilities, but no higher-level functions use the feature. The implementation in ncurses is an extension.

KEY_RESIZE and has_key are extensions first implemented for ncurses. By 2022, PDCurses and NetBSD curses had added them along with KEY_MOUSE.

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

SVr3 (1987) added ungetch.

ncurses 1.9.9g (1996) furnished the has_key extension.

SEE ALSO
ECMA-6 “7-bit coded Character Set” <https://ecma-international.org/publications-and-standards/standards/ecma-6/>

ECMA-48 “Control Functions for Coded Character Sets” <https://ecma-international.org/publications-and-standards/standards/ecma-48/>

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

curses(3X), curs_addch(3X), curs_inopts(3X), curs_mouse(3X), curs_move(3X), curs_outopts(3X), curs_refresh(3X), curs_variables(3X), resizeterm(3X), ascii(7)