Question

I’m reading How do we capture CTRL ^ C – Perl Monks, but I cannot seem to get the right info to help with my problem.

The thing is – I have an infinite loop, and ‘multiline’ printout to terminal (I’m aware I’ll be told to use ncurses instead – but for short scripts, I’m more comfortable writing a bunch of printfs). I’d like to trap Ctrl-C in such a way, that the script will terminate only after this multiline printout has finished.

The script is (Ubuntu Linux 11.04):

#!/usr/bin/env perl
use strict;
use warnings;

use Time::HiRes;

binmode(STDIN);   # just in case
binmode(STDOUT);   # just in case


# to properly capture Ctrl-C - so we have all lines printed out
# unfortunately, none of this works:
my $toexit = 0;
$SIG{'INT'} = sub {print "EEEEE";  $toexit=1; };
#~ $SIG{INT} = sub {print "EEEEE";  $toexit=1; };
#~ sub REAPER { # http://www.perlmonks.org/?node_id=436492
        #~ my $waitedpid = wait;
        #~ # loathe sysV: it makes us not only reinstate
        #~ # the handler, but place it after the wait
        #~ $SIG{CHLD} = \&REAPER;
        #~ print "OOOOO";
    #~ }
#~ $SIG{CHLD} = \&REAPER;
#~ $SIG{'INT'} = 'IGNORE';



# main

# http://stackoverflow.com/questions/14118/how-can-i-test-stdin-without-blocking-in-perl
use IO::Select;
my $fsin = IO::Select->new();
$fsin->add(\*STDIN);


my ($cnt, $string);
$cnt=0;
$string = "";

while (1) {
  $string = ""; # also, re-initialize
  if ($fsin->can_read(0)) { # 0 timeout
    $string = <STDIN>;
  }
  $cnt += length($string);

  printf "cnt: %10d\n", $cnt;
  printf "cntA: %10d\n", $cnt+1;
  printf "cntB: %10d\n", $cnt+2;
  print "\033[3A"; # in bash - go three lines up
  print "\033[1;35m"; # in bash - add some color
  if ($toexit) { die "Exiting\n" ; } ;
}

Now, if I run this, and I press Ctrl-C, I either get something like this (note, the _ indicates position of terminal cursor after script has terminated):

MYPROMPT$ ./test.pl
cnEEEEEcnt:          0
MYPROMPT$ _
cntB:          2
Exiting

or:

MYPROMPT$ ./test.pl
cncnt:          0
MYPROMPT$ _
cntB:          2
Exiting

… however, I’d like to get:

MYPROMPT$ ./test.pl
cncnt:          0
cntA:          1
cntB:          2
Exiting
MYPROMPT$ _

Obviously, handlers are running – but not quite in the timing (or order) I expect them to. Can someone clarify how do I fix this, so I get the output I want?

Many thanks in advance for any answers,
Cheers!

Answer 1

Hmmm… seems solution was easier than I thought 🙂 Basically, the check for “trapped exit” should run after the lines are printed – but before the characters for “go three lines up” are printed; that is, that section should be:

  printf "cnt: %10d\n", $cnt;
  printf "cntA: %10d\n", $cnt+1;
  printf "cntB: %10d\n", $cnt+2;
  if ($toexit) { die "Exiting\n" ; } ;
  print "\033[3A"; # in bash - go three lines up
  print "\033[1;35m"; # in bash - add some color

… and then the output upon Ctrl-C seems to be like:

MYPROMPT$ ./test.pl 
cnt:          0
^CcntA:          1
cntB:          2
Exiting
MYPROMPT$  _

Well, hope this may help someone,
Cheers!