a "unixish" implementation of BrowserUk's idea

Nice!++

the backspace cancellation trick would only work under rare circumstances (AFAICT) — simply reading from stdin would, for example, not treat the backspaces in any special way.

I added a little extra diagnostics. a) When the parent detects that the child entered an input state, it sends it a string "hello\n"; b) when the child enters an input state, it prints out what it receives:

#! perl -slw
use strict;
use threads;
use threads::shared;

our $N //= 12;
our $I //= 0;

my $cmd = qq[$^X -E"\$|++; sleep $N; print 'Kid got ', scalar <STDIN> 
+if $I; sleep 2;say 'Kid done'"];

my $timeout = time() + 10;
my $inInputState :shared = 0;

my $pid = open CMD, '|-', $cmd or die $!;
my $old = select CMD; $|++; select $old;
my $timedOut = 0;

async {
    $inInputState = 1 if printf CMD " \b"x2048;
}->detach;

Win32::Sleep 10 until !kill 0, $pid
    or $timedOut = time() > $timeout
    or $inInputState
;

if( $timedOut ) {
    print "Command timed out";
    kill 3, $pid;
}
if( $inInputState ) {
    print "Child waiting for input";
    print CMD 'hello';
}
else {
    print "Kid never entered input state";
}

print 'Parent done';
[download]

The upshot shows that on Windows at least, even though the child (perl in this case) only uses a standard read from stdin scalar <STDIN>, the CRT provides line-editing that allows the backspace cancellation to work:

C:\test>detectChildInputState -I=0 -N=2
Kid done
Kid never entered input state
Parent done

C:\test>detectChildInputState -I=1 -N=2
Child waiting for input
Parent done
Kid got hello
Kid done

C:\test>detectChildInputState -I=0 -N=10
Command timed out
Kid never entered input state
Parent done

C:\test>detectChildInputState -I=1 -N=10
Child waiting for input
Parent done
Kid got hello
Kid done

C:\test>detectChildInputState -I=0 -N=12
Command timed out
Kid never entered input state
Parent done

C:\test>detectChildInputState -I=1 -N=12
Command timed out
Kid never entered input state
Parent done
[download]

But I guess what you are saying is that under *nix, the standard CRT input routines don't provided line editing facilities, unless the program in question uses a readline(3) library or similar.

Would nulls be an workable alternative? Or DC1/DC3 (XON/XOFF)?