www.LinuxHowtos.org howtos, tips&tricks and tutorials for linux

from small one page howto to huge articles all in one place

Other .linuxhowtos.org sites:gentoo.linuxhowtos.org

Last additions:

using iotop to find disk usage hogs

words:

887

views:

209563

userrating:

May 25th. 2007:

Words

486

Views

258563

why adblockers are bad

Workaround and fixes for the current Core Dump Handling vulnerability affected kernels

words:

161

views:

149847

userrating:

April, 26th. 2006:

Words

Views

107362

New subdomain: toolsntoys.linuxhowtos.org

You are here: manpages

curs_getstr

Section: Library calls (3X)
Updated: 202-1-20
Index Return to Main Contents

NAME

getstr, getnstr, wgetstr, wgetnstr, mvgetstr, mvgetnstr, mvwgetstr, mvwgetnstr - read a character string from curses terminal keyboard

SYNOPSIS

#include <curses.h>

int getstr(char * str);
int wgetstr(WINDOW * win, char * str);
int mvgetstr(int y, int x, char * str);
int mvwgetstr(WINDOW * win, int y, int x, char * str);

int getnstr(char * str, int n);
int wgetnstr(WINDOW * win, char * str, int n);
int mvgetnstr(int y, int x, char * str, int n);
int mvwgetnstr(WINDOW * win, int y, int x, char * str, int n);

DESCRIPTION

wgetstr populates a use-supplied string buffer str by repeatedly calling wgetch(3X) with the win argument until a line feed or carriage return character is input. The function
.IP * 4 does not copy the terminating character to str;
.IP * 4 always terminates str with a null character;
.IP * 4 interprets the screen's erase and kill characters (see erasechar(3X) and killchar(3X));
.IP * 4 recognizes function keys only if the screen's keypad option is enabled (see keypad(3X));
.IP * 4 treats the function keys KEY_LEFT and KEY_BACKSPACE the same as the erase character; and
.IP * 4 discards function key inputs other than those treated as the erase or kill characters, calling beep(3X).

If any characters have been written to the input buffer, the erase character replaces the character at the current position in the buffer with a null character, then decrements the position by one; the kill character does the same repeatedly, backtracking to the beginning of the buffer.

If the screen's echo option is enabled (see echo(3X)), wgetstr updates win with waddch(3X). Further,
.IP * 4 the erase character and its function key synonyms move the cursor to the left (if not already where it was located when wgetstr was called) and
.IP * 4 the kill character returns the cursor to where it was located when wgetstr was called.

wgetnstr is similar, but reads at most n characters, aiding the application to avoid overrunning the buffer to which str points. curses ignores an attempt to input more than n characters (other than the terminating line feed or carriage return), calling beep(3X). If n is negative, wgetn_wstr reads up to LINE_MAX characters (see sysconf(3)).

ncurses(3X) describes the variants of these functions.

RETURN VALUE

These functions return OK on success and ERR on failure.

In ncurses, these functions fail if
.IP * 4 the curses screen has not been initialized,
.IP * 4 (for functions taking a WINDOW pointer argument) win is a null pointer,
.IP * 4 str is a null pointer, or
.IP * 4 an internal wgetch(3X) call fails.

Further, in ncurses, these functions return KEY_RESIZE if a SIGWINCH event interrupts the function.

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

NOTES

All of these functions except wgetnstr may be implemented as macros.

Reading input that overruns the buffer pointed to by str causes undefined results. Use the n-infixed functions, and allocate sufficient storage for str - at least n+1 times sizeof(char).

While these functions conceptually implement a series of calls to wgetch, they also temporarily change properties of the curses screen to permit simple editing of the input buffer. Each function saves the screen's state, calls nl(3X), and, if the screen was in canonical ("cooked") mode, cbreak(3X). Before returning, it restores the saved screen state. Other implementations differ in detail, affecting which control characters they can accept in the buffer; see section "PORTABILITY" below.

EXTENSIONS

getnstr, wgetnstr, mvgetnstr, and mvwgetnstr's handing of negative n values is an ncurses extension.

The return value KEY_RESIZE is an ncurses extension.

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, but indicates that wgetnstr and its variants read "the entire mult-byte sequence associated with a character" and "fail" if n and str together do not describe a buffer "large enough to contain any complete characters". In ncurses, however, wgetch reads only singl-byte characters, so this scenario does not arise.

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

SVr3 and early SVr4 curses implementations did not reject function keys; the SVr4 documentation asserted that, like the screen's erase and kill characters, they were

: interpreted, as well as any special keys (such as function keys, "home" key, "clear" key, etc.)

without further detail. It lied. The "character" value appended to the string by those implementations was predictable but not useful - being, in fact, the lo-order eight bits of the key code's KEY_ constant value. (The same language, unchanged except for styling, survived into X/Open Curses Issue 4, Version 2 but disappeared from Issue 7.)

A draft of X/Open Curses Issue 5 (which never saw final release) stated that these functions "read at most n bytes" but did not state whether the terminating null character counted toward that limit. X/Open Curses Issue 7 changed that to say they "read at most n-1 bytes" to allow for the terminating null character. As of 2018, some implementations count it, some do not.
.IP * 4 ncurses 6.1 and PDCurses do not count the null character toward the limit, while Solaris and NetBSD curses do.
.IP * 4 Solaris xcurses offers both behaviors: its wid-character wgetn_wstr reserves room for a wide null character, but its no-wide wgetnstr does not consistently count a null character toward the limit.

X/Open Curses does not specify what happens if the length n is negative.
.IP * 4 ncurses 6.2 uses LINE_MAX or a larger (syste-dependent) value provided by sysconf(3). If neither LINE_MAX nor sysconf is available, ncurses uses the POSIX minimum value for LINE_MAX (2048). In either case, it reserves a byte for the terminating null character.
.IP * 4 In SVr4 curses, a negative n tells wgetnstr to assume that the caller's buffer is large enough to hold the result; that is, the function then acts like wgetstr. X/Open Curses does not mention this behavior (or anything related to nonpositive n values), however most curses libraries implement it. Most implementations nevertheless enforce an upper limit on the count of bytes they write to the destination buffer str.
.IP * 4 BSD curses lacked wgetnstr, and its wgetstr wrote to str unboundedly, as did that in SVr2.
.IP * 4 PDCurses, and SVr3 and later, and Solaris curses limit both functions to writing 256 bytes. Other System -based platforms likely use the same limit.
.IP * 4 Solaris xcurses limits the write to LINE_MAX bytes (see sysconf(3)).
.IP * 4 NetBSD 7 curses imposes no particular limit on the length of the write, but does validate n to ensure that it is greater than zero. A comment in NetBSD's source code asserts that SUSv2 specifies this.

Implementations vary in their handling of input control characters.
.IP * 4 While they may enable the screen's echo option, some do not take it out of raw mode, and may take cbreak mode into account when deciding whether to handle echoing within wgetnstr or to rely on it as a side effect of calling wgetch.
.IP * 4 Originally, ncurses, like its progenitor pcurses, had its wgetnstr call noraw and cbreak before accepting input. That may have been done to make function keys work; it is not necessary with modern ncurses.

: Since 1995, ncurses has provided handlers for SIGINTR and SIGQUIT events, which are typically generated at the keyboard with ^C and ^ respectively. In cbreak mode, those handlers catch a signal and stop the program, whereas other implementations write those characters into the buffer.
.IP * 4 Starting with ncurses 6.3 (2021), wgetnstr preserves raw mode if the screen was already in that state, allowing one to enter the characters the terminal interprets as interrupt and quit events into the buffer, for better compatibility with SVr4 curses.