Debugging forking scripts in screen

harleypig has asked for the wisdom of the Perl Monks concerning the following question:

I've got the following subroutine working relatively well for debugging forking processes in a screen session.

I'd like to submit it to whoever is maintaining the perl5db.pl file, or at least to rt.cpan.org. But before I do that, I'd like to make it as robust as possible.

I wasn't sure where else to put this--and since this is a request for the collective wisdom to evaluate the code this seemed as best a place as any. Please let me know if you think this can go better elsewhere.

Please note the following:

Detecting whether we are actually under screen takes place elsewhere in perl5db.pl.
If you exit the parent process before the child process(es) you're next debugging fork will be screwed up. I don't know why

Here's the subroutine:

sub DB::get_fork_TTY {

  require File::Temp or return '';

  my ( $FH, $filename ) = File::Temp::tempfile( UNLINK => 1 );

  system( qq{screen -t 'Child $$' sh -c "tty > $filename ; sleep 10000
+00"} ) == 0
    or return '';

  chomp ( my $tty = <$FH> );

  return $tty;
}
[download]

And the test code I've been using it on:

#!/usr/bin/perl -w

use strict;

sub DB::get_fork_TTY {

  require File::Temp or return '';

  my ( $FH, $filename ) = File::Temp::tempfile( UNLINK => 1 );

  system( qq{screen -t 'Child $$' sh -c "tty > $filename ; sleep 10000
+00"} ) == 0
    or return '';

  chomp ( my $tty = <$FH> );

  return $tty;
}

if ( my $child = fork ) {

  $DB::single = 1;
  print "We're in the parent! ($child)\n";

} else {

  die "Can't fork" unless defined $child;
  $DB::single = 1;
  print "We're in the child!\n";

}

exit;
[download]

Added check for failed fork as suggested

Harley J Pig

Comment on Debugging forking scripts in screen Select or Download Code

Replies are listed 'Best First'.
Re: Debugging forking scripts in screen by kyle (Abbot) on Feb 09, 2007 at 02:43 UTC
You should check whether fork actually worked. When it fails, it returns `undef`, and your script will think it succeeded, and that it is the child. The perlipc page has an example I'm paraphrasing: `if ( my $pid = fork() ) { # parent } else { die "Can't fork: $!" if ( ! defined $pid ); # child }` [download]	[reply] [d/l] [select]
Re^2: Debugging forking scripts in screen by harleypig (Monk) on Feb 09, 2007 at 06:00 UTC
Thanks. I know about that ... the second code block was what I used to check the get_fork_TTY subroutine. It's not the script that I'm debugging. Harley J Pig	[reply]


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