Question

Is there any way to clear the STDIN buffer in Perl? A part of my program has lengthy output (enough time for someone to enter a few characters) and after that output I ask for input, but if characters were entered during the output, they are “tacked on” to whatever is entered at the input part. Here is an example of my problem:

for(my $n = 0; $n < 70000; $n++){
   print $n . "\n";
}
chomp(my $input = <STDIN>);
print $input . "\n";

The output would include any characters entered during the output from that for loop. How could I either disable STDIN or flush the STDIN buffer (or any other way to not allow extra characters to be inserted into STDIN before calling it)?

Answer 1

It looks like you can accomplish this with the Term::ReadKey module:

#!perl

use strict;
use warnings;
use 5.010;

use Term::ReadKey;

say "I'm starting to sleep...";
ReadMode 2;
sleep(10);
ReadMode 3;
my $key;
while( defined( $key = ReadKey(-1) ) ) {}
ReadMode 0;
say "Enter something:";
chomp( my $input = <STDIN> );
say "You entered '$input'";

Here’s what happens:

• ReadMode 2 means “put the input mode into regular mode but turn off echo”. This means that any keyboard banging that the user does while you’re in your computationally-expensive code won’t get echoed to the screen. It still gets entered into STDIN‘s buffer though, so…
• ReadMode 3 turns STDIN into cbreak mode, meaning STDIN kind of gets flushed after every keypress. That’s why…
• while(defined($key = ReadKey(-1))) {} happens. This is flushing out the characters that the user entered during the computationally-expensive code. Then…
• ReadMode 0 resets STDIN, and you can read from STDIN as if the user hadn’t banged on the keyboard.

When I run this code and bang on the keyboard during the sleep(10), then enter some other text after the prompt, it only prints out the text I typed after the prompt appeared.

Strictly speaking the ReadMode 2 isn’t needed, but I put it there so the screen doesn’t get cluttered up with text when the user bangs on the keyboard.