Keyboard input/Flush the keyboard buffer

Flush the keyboard buffer. This reads characters from the keyboard input and discards them until there are no more currently buffered, and then allows the program to continue.
The program must not wait for users to type anything.

Ada

<lang Ada>with Ada.Text_IO; procedure Flushtest is

  use Text_IO;

begin

  Put_Line ("Type anything for 2 s");
  delay 2.0;

Flush_Input:

  declare
     Ch   : Character;
     More : Boolean;
  begin
     loop
        Get_Immediate (Ch, More);
        exit when not More;
     end loop;
  end Flush_Input;
  New_Line;
  Put_Line ("Okay, thanks. Here is some input from you:");
  Put_Line (Get_Line);

end Flushtest;</lang>

BASIC

Applesoft BASIC

Locomotive Basic

<lang locobasic>10 CLEAR INPUT</lang>

(Only available in BASIC 1.1 though, i.e. not on the CPC 464.)

ZX Spectrum Basic

There is no need to flush keyboard buffer in Spectrum since key presses are not buffered. If a key is currently pressed, the following waits until key is released. <lang basic>10 IF INKEY$ <> "" THEN GO TO 10</lang>

BBC BASIC

<lang bbcbasic> *FX 15,1</lang> Strictly speaking *FX 15,1 is an Operating System command, but it is emulated in BBC BASIC for Windows. Alternatively the keyboard buffer may be flushed as follows: <lang bbcbasic> REPEAT UNTIL INKEY(0)=-1</lang>

C

Library: POSIX

Code lifted from Keyboard input/Obtain a Y or N response: <lang c>#include <stdio.h>

include <stdio.h>
include <termios.h>
include <unistd.h>
include <fcntl.h>
include <ctype.h>

void set_mode(int want_key) { static struct termios old, new; if (!want_key) { tcsetattr(STDIN_FILENO, TCSANOW, &old); return; }

tcgetattr(STDIN_FILENO, &old); new = old; new.c_lflag &= ~(ICANON); tcsetattr(STDIN_FILENO, TCSANOW, &new); }

int get_key() { int c = 0; fd_set fs;

FD_ZERO(&fs); FD_SET(STDIN_FILENO, &fs); select(STDIN_FILENO + 1, &fs, 0, 0, 0);

if (FD_ISSET(STDIN_FILENO, &fs)) { c = getchar(); set_mode(0); } return c; }

int main() { int c = 0; while (c != 'n') { set_mode(1);

/* flush pending input so we won't format the hardrive because user accidentally typed 'y' before we even prompted */ tcflush(STDIN_FILENO, TCIFLUSH);

printf("Show this prompt again [Yes/No/Ignore you]? "); fflush(stdout);

switch(c = tolower(get_key())) { case 'y': putchar('\n'); break;

case 'n': printf("\nDone\n"); break;

case 'i': puts("\nI'll ignore keys for 5 seconds"); sleep(5); putchar('\n'); break; default: puts("\nAssume that was the cat."); } }

return 0; }</lang>

D

<lang d>extern (C) {

   void _STI_conio();
   void _STD_conio();
   int kbhit();
   int getch();

}

void main() {

   void flushKB() {
       while (kbhit()) getch();
   }

   _STI_conio();

   flushKB();

   _STD_conio();

}</lang>

DCL

<lang>$ wait 0::10 ! gives us 10 seconds to get keystrokes into the type-ahead buffer $ on control_y then $ goto clean $ set terminal /noecho $ loop: read /prompt="" /time=0 sys$command /error=clean buffer $ goto loop $ clean: $ set terminal /echo</lang>

Output:

$ @flush_the_keyboard_buffer  ! ignores/discards keystrokes for 10 seconds
$

ERRE
<lang ERRE>
!$KEY
..........
REPEAT
   GET(K$)
UNTIL K$=""
..........
</lang>
Note: Test after K$ can be replaced with LEN(K$)=0.

Euphoria
<lang Euphoria>while get_key()!=-1 do
end while</lang>

Go
Library: Curses
<lang go>package main

import (
    "log"

    gc "code.google.com/p/goncurses"
)

func main() {
    _, err := gc.Init()
    if err != nil {
        log.Fatal("init:", err)
    }
    defer gc.End()
    gc.FlushInput()
}</lang>

Icon and Unicon
The following solution works in both Icon and Unicon.
<lang unicon>procedure flushKB()
    while kbhit() do getch()   # flush input
end</lang>

Nim
Library: POSIX
<lang nim>const TCIFLUSH: cint = 0
proc tcflush(fd, queue_selector: cint): cint {.header: "termios.h".}

discard tcflush(cint(getFileHandle(stdin)), TCIFLUSH)</lang>

Oforth
<lang Oforth>System.Console flush</lang>

Perl
<lang perl>use Term::ReadKey;
ReadMode 'restore';    # Flush the keyboard and returns input stream to initial state
# ReadMode 0;            # Numerical equivalent of keyboard restore (move comment marker to use instead)

# A more complete example for use in keyboard handler programming.
# We should also check we are being used in an interactive context (not done here).

use Term::ReadKey;
ReadMode 'cbreak';

# Flush the keyboard in terminal character break mode
while (defined ReadKey -1) {
  # Do nothing
}

# Don't forget to restore the readmode, when we are finished using the keyboard
ReadMode 'restore';</lang>

PicoLisp
<lang PicoLisp>(while (key 10))</lang>

PowerShell
The following uses the special $Host variable which points to an instance of the PowerShell host application. Since the host's capabilities may vary this may not work in all PowerShell hosts. In particular, this works in the console host, but not in the PowerShell ISE.
<lang powershell>while ($Host.UI.RawUI.KeyAvailable) {
    $Host.UI.RawUI.ReadKey() | Out-Null
}</lang>

PureBasic
<lang PureBasic>While Inkey(): Wend</lang>

Python
<lang Python>def flush_input():
    try:
        import msvcrt
        while msvcrt.kbhit():
            msvcrt.getch()
    except ImportError:
        import sys, termios
        termios.tcflush(sys.stdin, termios.TCIOFLUSH)
</lang>

Racket
Using stty to get the terminal into raw mode.

<lang racket>
#lang racket
(define-syntax-rule (with-raw body ...)
  (let ([saved #f])
    (define (stty x) (system (~a "stty " x)) (void))
    (dynamic-wind (λ() (set! saved (with-output-to-string (λ() (stty "-g"))))
                       (stty "raw -echo opost"))
                  (λ() body ...)
                  (λ() (stty saved)))))

(with-raw
  (printf "Keys pressed from now will be ignored\n")
  (sleep 2)
  (let loop () (when (char-ready?) (read-char) (loop))) ; flush input
  (printf "Now press a key which will not be ignored\n")
  (printf "You pressed ~a\n" (read-char)))
</lang>

REXX
This will work for Regina:
<lang rexx>call dropbuf</lang>
This will work for CMS REXX, PC/REXX, Personal REXX, and TSO REXX:
<lang rexx>'DROPBUF'</lang>

Ruby
Each terminal device has an input queue for keyboard input. We can either flush this input queue, or read it empty.

Ruby 1.9.3 adds a new library 'io/console', providing IO#iflush to flush and discard the input queue. If its IO object is not a terminal, it raises an error, perhaps Errno::ENODEV.

Works with: Ruby version 1.9.3
<lang ruby>require 'io/console'
$stdin.iflush</lang>

The other option uses IO#read_nonblock to read the input, without any blocking or waiting. This has a caveat: if the terminal uses the canonical input mode, IO reads only entire lines; and if the input queue contains part of a line, IO#read_nonblock cannot discard this last partial line!

<lang ruby>loop { $stdin.read_nonblock(256) } rescue nil</lang>

The complete solution calls IO#iflush, or turns off canonical input mode and calls IO#read_nonblock.

<lang ruby>class IO
  def discard_input
    icanon = false
    if tty?
      begin
        # With Ruby 1.9.3, simply call IO#iflush.
        require 'io/console'
        return iflush
      rescue LoadError
        # Try to run stty(1) to check if this terminal uses
        # canonical input mode. Acts like `stty -a`, but redirects
        # stdin from tty. Works with Ruby 1.8, no Process#spawn.
        r, w, pid = nil
        begin
          r, w = IO.pipe
          pid = fork do
            IO.for_fd(0).reopen(self)  # stdin from tty
            IO.for_fd(1).reopen(w)     # stdout to pipe
            exec 'stty', '-a'
          end
          w.close; w = nil
          icanon = (not r.read.include? "-icanon")
        rescue
          # stty(1) only works with Unix clones.
        ensure
          pid and Process.wait pid
          w and w.close
          r and r.close
        end
      end
    end

    if icanon
      # Turn off canonical input mode.
      pid = nil
      begin
        pid = fork do
          IO.for_fd(0).reopen(self)  # stdin from tty
          exec 'stty', '-icanon'
        end
      ensure
        pid and Process.wait pid
      end
    end

    # Discard input.
    loop { $stdin.read_nonblock(256) } rescue nil

    if icanon
      # Turn on canonical input mode.
      pid = nil
      begin
        pid = fork do
          IO.for_fd(0).reopen(self)  # stdin from tty
          exec 'stty', 'icanon'
        end
      ensure
        pid and Process.wait pid
      end
    end

    nil
  end
end</lang>

<lang ruby># Demonstration: discard input, then input a line from user.
puts 'Type anything for 2 seconds.'
sleep 2
$stdin.discard_input
print 'Enter a line? '
if line = $stdin.gets
then print 'Got line. ', line
else puts 'No line!'
end</lang>

Scala
<lang scala>def flush() { out.flush() }</lang>
Seed7
The Seed7 library keybd.s7i
defines the file KEYBOARD,
which provides functions like keypressed
and getc.

<lang seed7>while keypressed(KEYBOARD) do
  ignore(getc(KEYBOARD));
end while;</lang>

Tcl
<lang tcl># No waiting for input
fconfigure stdin -blocking 0
# Drain the data by not saving it anywhere
read stdin

# Flip back into blocking mode (if necessary)
fconfigure stdin -blocking 1</lang>

Vedit macro language
<lang vedit>Key_Purge()</lang>

XPL0
<lang XPL0>code OpenI=13;
OpenI(0)</lang>