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curs_getch

Section: Library calls (3X)
Updated: 202-1-11
Index Return to Main Contents

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 keyboard 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 n-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 lin-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.
.IP * 4 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.
.IP * 4 curses writes any other c to the window, as with wechochar(3X).
.IP * 4 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 ECM-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 escape sequence, which the application must collect individually with multiple wgetch calls.
.IP * 4 The curses.h header file declares many function keys whose names begin with KEY_; these objec-like macros have integer values outside the range of eigh-bit character codes.
.IP * 4 In ncurses, use-defined function keys are configured with define_key(3X); they have no names, but are also expected to have integer values outside the range of eigh-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 ECM-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 (assuming it sends ESC) from the beginning of a function key's character sequence without waiting to see if, and how soon, further input arrives.
.IP * 4 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.
.IP * 4 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.
.IP * 4 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 tim-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.
.IP * 4 Except for the special case of KEY_RESIZE, a window's keypad mode must be enabled for wgetch to read these codes from it.
.IP * 4 Not all of these are necessarily supported on any particular terminal.
.IP * 4 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	Arrow keys
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 nin-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.
.IP * 4 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).
.IP * 4 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 no-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
.IP * 4 the curses screen has not been initialized,
.IP * 4 (for functions taking a WINDOW pointer argument) win is a null pointer, or
.IP * 4 execution was interrupted by a signal, in which case the library sets errno 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
.IP * 4 the curses screen has not been initialized, or
.IP * 4 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.
.IP * 4 The terminfo key_enter (kent) capability describes the character (sequence) sent by the Enter key of a terminal's numeric (or similar) keypad.
.IP * 4 "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.
.IP * 4 It usually produces a control code for carriage return (^M) or line feed (^J).
.IP * 4 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 wel-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 tim-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 ESC 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 singl-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 no-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 highe-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.