curs_getch

Section: Miscellaneous Library Functions (3X)
Updated:
Page Index

NAME

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

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 ch);
int has_key(int ch);

DESCRIPTION

Reading characters

The getch, wgetch, mvgetch and mvwgetch, routines read a character from the window. In no-delay mode, if no input is waiting, the value ERR is returned. In delay mode, the program waits until the system passes text through to the program. Depending on the setting of cbreak, this is after one character (cbreak mode), or after the first newline (nocbreak mode). In half-delay mode, the program waits until a character is typed or the specified timeout has been reached.

If echo is enabled, and the window is not a pad, then the character will also be echoed into the designated window according to the following rules:
.IP • 4 If the character is the current erase character, left arrow, or backspace, the cursor is moved one space to the left and that screen position is erased as if delch had been called.
.IP • 4 If the character value is any other KEY_ define, the user is alerted with a beep call.
.IP • 4 If the character is a carriage-return, and if nl is enabled, it is translated to a line-feed after echoing.
.IP • 4 Otherwise the character is simply output to the screen.

If the window is not a pad, and it has been moved or modified since the last call to wrefresh, wrefresh will be called before another character is read.

Keypad mode

If keypad is TRUE, and a function key is pressed, the token for that function key is returned instead of the raw characters:
.IP • 4 The predefined function keys are listed in <curses.h> as macros with values outside the range of 8-bit characters. Their names begin with KEY_.
.IP • 4 Other (user-defined) function keys which may be defined using define_key(3X) have no names, but also are expected to have values outside the range of 8-bit characters.

Thus, a variable intended to hold the return value of a function key must be of short size or larger.

When a character that could be the beginning of a function key is received (which, on modern terminals, means an escape character), curses sets a timer. If the remainder of the sequence does not come in within the designated time, the character is passed through; otherwise, the function key value is returned. For this reason, many terminals experience a delay between the time a user presses the escape key and the escape is returned to the program.

In ncurses, the timer normally expires after the value in ESCDELAY (see curs_variables(3X)). If notimeout is TRUE, the timer does not expire; it is an infinite (or very large) value. Because function keys usually begin with an escape character, the terminal may appear to hang in notimeout mode after pressing the escape key until another key is pressed.

Ungetting characters

The ungetch routine places ch back onto the input queue to be returned by the next call to wgetch. There is just one input queue for all windows.

Predefined key-codes

The following special keys are defined in <curses.h>.
.IP • 4 Except for the special case KEY_RESIZE, it is necessary to enable keypad for getch to return these codes.
.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 long terminfo capability names for the keys, and were defined long ago, in the 1980s.

Name	Key name

KEY_BREAK	Break key
KEY_DOWN	The four 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)	For 0 ≤ n ≤ 63
KEY_DL	Delete line
KEY_IL	Insert line
KEY_DC	Delete character
KEY_IC	Insert char or enter insert mode
KEY_EIC	Exit insert char mode
KEY_CLEAR	Clear screen
KEY_EOS	Clear to end of screen
KEY_EOL	Clear to end of line
KEY_SF	Scroll 1 line forward
KEY_SR	Scroll 1 line backward (reverse)
KEY_NPAGE	Next page
KEY_PPAGE	Previous page
KEY_STAB	Set tab
KEY_CTAB	Clear tab
KEY_CATAB	Clear all tabs
KEY_ENTER	Enter or send
KEY_SRESET	Soft (partial) reset
KEY_RESET	Reset or hard reset
KEY_PRINT	Print or copy
KEY_LL	Home down or 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 read
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_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 char key
KEY_SDL	Shifted delete line key
KEY_SELECT	Select 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 input key
KEY_SLEFT	Shifted left arrow key
KEY_SMESSAGE	Shifted message key
KEY_SMOVE	Shifted move key
KEY_SNEXT	Shifted next key
KEY_SOPTIONS	Shifted options key
KEY_SPREVIOUS	Shifted prev key
KEY_SPRINT	Shifted print key
KEY_SREDO	Shifted redo key
KEY_SREPLACE	Shifted replace key
KEY_SRIGHT	Shifted right arrow
KEY_SRSUME	Shifted resume key
KEY_SSAVE	Shifted save key
KEY_SSUSPEND	Shifted suspend key
KEY_SUNDO	Shifted undo key
KEY_SUSPEND	Suspend key
KEY_UNDO	Undo key

Keypad is arranged like this:

A1	up	A3
left	B2	right
C1	down	C3

A few of these predefined values do not correspond to a real key:
.IP • 4 KEY_RESIZE is returned when the SIGWINCH signal has been detected (see initscr(3X) and resizeterm(3X)). This code is returned whether or not keypad has been enabled.
.IP • 4 KEY_MOUSE is returned for mouse-events (see curs_mouse(3X)). This code relies upon whether or not keypad(3X) has been enabled, because (e.g., with xterm mouse prototocol) ncurses must read escape sequences, just like a function key.

Testing key-codes

The has_key routine takes a key-code value from the above list, and returns TRUE or FALSE according to whether the current terminal type recognizes a key with that value.

The library also supports these extensions:

define_key: defines a key-code for a given string (see define_key(3X)).
key_defined: checks if there is a key-code defined for a given string (see key_defined(3X)).

RETURN VALUE

All routines return the integer ERR upon failure and an integer value other than ERR (OK in the case of ungetch) upon successful completion.

ungetch: returns ERR if there is no more room in the FIFO.
wgetch: returns ERR if the window pointer is null, or if its timeout expires without having any data, or if the execution was interrupted by a signal (errno will be set to EINTR).

Functions with a "mv" prefix first perform a cursor movement using wmove, and return an error if the position is outside the window, or if the window pointer is null.

NOTES

Use of the escape key by a programmer for a single character function is discouraged, as it will cause a delay of up to one second while the keypad code looks for a following function-key sequence.

Some keys may be the same as commonly used control keys, e.g., KEY_ENTER versus control/M, KEY_BACKSPACE versus control/H. Some curses implementations may differ according to whether they treat these control keys specially (and ignore the terminfo), or use the terminfo definitions. Ncurses uses the terminfo definition. If it says that KEY_ENTER is control/M, getch will return KEY_ENTER when you press control/M.

Generally, KEY_ENTER denotes the character(s) sent by the Enter key on the numeric keypad:
.IP • 4 the terminal description lists the most useful keys,
.IP • 4 the Enter key on the regular keyboard is already handled by the standard ASCII characters for carriage-return and line-feed,
.IP • 4 depending on whether nl or nonl was called, pressing "Enter" on the regular keyboard may return either a carriage-return or line-feed, and finally
.IP • 4 "Enter or send" is the standard description for this key.

When using getch, wgetch, mvgetch, or mvwgetch, nocbreak mode (nocbreak) and echo mode (echo) should not be used at the same time. Depending on the state of the tty driver when each character is typed, the program may produce undesirable results.

Note that getch, mvgetch, and mvwgetch may be macros.

Historically, the set of keypad macros was largely defined by the extremely function-key-rich keyboard of the AT&T 7300, aka 3B1, aka Safari 4. Modern personal computers usually have only a small subset of these. IBM PC-style consoles typically support little more than KEY_UP, KEY_DOWN, KEY_LEFT, KEY_RIGHT, KEY_HOME, KEY_END, KEY_NPAGE, KEY_PPAGE, and function keys 1 through 12. The Ins key is usually mapped to KEY_IC.

PORTABILITY

The *get* functions are described in the XSI Curses standard, Issue 4. They read single-byte characters only. The standard specifies that they return ERR on failure, but specifies no error conditions.

The echo behavior of these functions on input of KEY_ or backspace characters was not specified in the SVr4 documentation. This description is adopted from the XSI Curses standard.

The behavior of getch and friends in the presence of handled signals is unspecified in the SVr4 and XSI Curses documentation. Under historical curses implementations, it varied depending on whether the operating system's implementation of handled signal receipt interrupts a read(2) call in progress or not, and also (in some implementations) depending on whether an input timeout or non-blocking mode has been set.

KEY_MOUSE is mentioned in XSI 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 is an extension first implemented for ncurses. NetBSD curses later added this extension.

Programmers concerned about portability should be prepared for either of two cases: (a) signal receipt does not interrupt getch; (b) signal receipt interrupts getch and causes it to return ERR with errno set to EINTR.

The has_key function is unique to ncurses. We recommend that any code using it be conditionalized on the NCURSES_VERSION feature macro.