Question

I have this simple perl daemon:

#!/usr/bin/perl

use strict;
use warnings;
use Proc::Daemon;

Proc::Daemon::Init;

my $continue = 1;

$SIG{TERM} = sub { $continue = 0 };
$SIG{USR1} = sub { do_process(1) };

# basic daemon                                                                                    

boxesd_log("started boxesd");

while ($continue) {
    do_process(0);
    sleep(30);
}

boxesd_log("finished boxesd");

exit(0);

# required subroutines                                                                            

sub do_process {
    my ($notified) = @_;
    boxesd_log("doing it $notified");
}

But there is something that is not working right.

When the daemon starts, it logs every 30 seconds without the notification as expected:

Sat Oct 30 21:05:47 2010 doing it 0
Sat Oct 30 21:06:17 2010 doing it 0
Sat Oct 30 21:06:47 2010 doing it 0

The problem comes when I send the USR1 signal to the process using kill -USR1 xxxxx. The output is not what I expect:

Sat Oct 30 21:08:25 2010 doing it 1
Sat Oct 30 21:08:25 2010 doing it 0

I get two continuous entries, one from the signal handling subroutine and another form the ever running loop. It seems as if the sleep gets interrupted whenever the USR1 signal is received.

What is going on?

Answer 1

The sleep is getting interrupted in the sense that your program will handle the incoming signal, but the loop will continue (going back to sleep) until a TERM signal is received. This is documented behaviour for the sleep() function:

May be interrupted if the process receives a signal such as “SIGALRM”.

Note that if you need to sleep for 30 seconds, even if interrupted by a signal, you can determine the number of seconds slept and then sleep again for the remainder:

while (1)
{
    my $actual = sleep(30);
    sleep(30 - $actual) if $actual < 30;
}

PS. You can read more about signal handling in perldoc perlipc, and in virtually any unix programming book by W. Richard Stevens. 🙂