Daemon problem with sleep()

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

Hi Monks! I'm about to write script to be used as a daemon. It's purpose is to check for a file every second and process it, if present. I use Daemon::Simple from CPAN to daemonize my script. Now my Problem is, when I use sleep() in my main loop no outputs at all are made (I write a logfile) - if I remove the sleep() statement, of course, the logfile is filled very rapidly. I just don't get it... what is the problem? Here's the code

use Daemon::Simple;
$prog_name = 'some_prog';

## Initialize Daemon ##
my $homedir = '/';
my $pidfile = "/var/run/$prog_name.pid";
my $command = $ARGV[0];
Daemon::Simple::init($command,$homedir,$pidfile);

## Daemon script ##
  open LOG, ">>$log_file";   # $log_file contains an absolute path
  print LOG "$prog_name started.";


  ## Daemon Loop ##
  while (1) {
    print LOG "before sleep...";
    sleep(1);
    print LOG "after sleep...";
  }


  close LOG;
__END__
[download]

Could anyone help out?

Comment on Daemon problem with sleep() Download Code

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Re: Daemon problem with sleep() by almut (Canon) on Dec 17, 2008 at 12:08 UTC
It might be just a buffering problem. Are you sure you have waited long enough for the IO buffer (typically around 4k) to fill up? Try flushing: `use IO::Handle; ... while (1) { print LOG "before sleep..."; LOG->flush(); sleep(1); print LOG "after sleep..."; }` [download] (Also see Re: Troubles with do...while loop and sleep (line- vs. block-buffered) — or the classic "Suffering from Buffering") (Update: moved `LOG->flush()` before the sleep — thanks ikegami)	[reply] [d/l] [select]
Re^2: Daemon problem with sleep() by MichaelMeyer (Novice) on Dec 17, 2008 at 12:38 UTC
Hi almut, you were right, it was a buffering problem with explicit flushing now it works. Thanks for the help!	[reply]
Re^3: Daemon problem with sleep() by targetsmart (Curate) on Dec 17, 2008 at 14:28 UTC
Even after proper flushing, don't forget to rotate your log file properly using log4perl or log::agent modules, otherwise your log file may reach the maximum allowed file size by the OS and soon it may lead to catastrophic situations.	[reply]
Re: Daemon problem with sleep() by zentara (Cardinal) on Dec 17, 2008 at 12:13 UTC
Or instead of autoflushing, use a \n to force the flush. `## Daemon Loop ## while (1) { print LOG "before sleep...\n"; sleep(1); print LOG "after sleep...\n"; }` [download] I'm not really a human, but I play one on earth Remember How Lucky You Are	[reply] [d/l]
Re^2: Daemon problem with sleep() by almut (Canon) on Dec 17, 2008 at 12:26 UTC
I think newlines wouldn't help here, as the LOG handle isn't 'interactive' (such as when connected to a tty). Virtually anything else is by default block-buffered (as opposed to line-buffered in the interactive case).	[reply]
Re^2: Daemon problem with sleep() by ikegami (Patriarch) on Dec 17, 2008 at 14:13 UTC
\n only flushes when writing to STDOUT and only when it's connected to a terminal. That's not the case here. Autoflushing can be turned on using `use IO::Handle qw( ); LOG->autoflushing(1);` [download]	[reply] [d/l]
Re^3: Daemon problem with sleep() by Lawliet (Curate) on Dec 17, 2008 at 19:38 UTC
Is that the same as `$\|++;`? I'm so adjective, I verb nouns! chomp; # nom nom nom	[reply] [d/l]
Re^4: Daemon problem with sleep() by ikegami (Patriarch) on Dec 17, 2008 at 19:53 UTC