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

hi, I have written a daemon example. But i dont know why it is not running. When i debugged using print statment, it is exiting after the (open STDIN, '/dev/null' or die "Can't read /dev/null: $!";) statement in the daemonize method. Can you please help me 
#!/usr/bin/perl
# daemonize the program
&daemonize;
$| = 1;


while(1)
{
        print "\nIn the While\n";
        open(MYFILE,'>>temp.txt');
        print MYFILE "Hello Sri\n";
        close(MYFILE);
}


sub daemonize {
    chdir '/' or die "Can't chdir to /: $!";
    open STDIN, '/dev/null'   or die "Can't read /dev/null: $!";
    open STDOUT, '>>/dev/null' or die "Can't write to /dev/null: $!";
    open STDERR, '>>/dev/null' or die "Can't write to /dev/null: $!";
    defined(my $pid = fork)   or die "Can't fork: $!";
    exit if $pid;
    setsid                    or die "Can't start a new session: $!";
    umask 0;
}

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Edit: Code tags added by Corion

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Re: Perl daemon exiting without warning
by cdarke (Prior) on Feb 11, 2011 at 09:43 UTC
    It would have helped if you used <code> ... </code> tags:
    #!/usr/bin/perl 
# daemonize the program 

print "Parent pid: $$\n";    # I added this
&daemonize; 

$| = 1; 
while(1) { 
    print "\nIn the While\n"; 
    open(MYFILE,'>>temp.txt'); 
    print MYFILE "Hello Sri\n"; 
    close(MYFILE); 
} 

sub daemonize { 
    chdir '/' or die "Can't chdir to /: $!"; 
    open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; 
    open STDOUT, '>>/dev/null' or die "Can't write to /dev/null: $!"; 
    open STDERR, '>>/dev/null' or die "Can't write to /dev/null: $!"; 
    defined(my $pid = fork) or die "Can't fork: $!"; 
    exit if $pid; 
    
    setsid or die "Can't start a new session: $!"; 
    umask 0; 
}

    [download]
    Works for me:
    /home/user1> ./gashd.pl
Parent pid: 7300
/home/user1> ps
  PID TTY          TIME CMD
 7190 pts/2    00:00:00 bash
 7301 pts/2    00:00:02 gashd.pl
 7315 pts/2    00:00:00 ps

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    Do you have any error messages?

    Update: I was wrong, it is not working. My mistake was in copying your code, which does not have:
    use warnings;
use strict;

    [download]
    which points out that setsid is a bare-word. What you need is POSIX::setsid
    In fact the ps output should have shown me that it was not creating a new session. Here is a version, which also logs the error when trying to open the file.
    #!/usr/bin/perl 
# daemonize the program 
use warnings;
use strict;
use POSIX ('setsid');
use Sys::Syslog qw(:standard);

print "Parent pid: $$\n";    # I added this
&daemonize;

$| = 1;
while(1) {
    print "\nIn the While\n";
    if (!open(MYFILE,'>>temp.txt')) {
        my $err = $!;   # In case Sys::Syslog resets it
        # Log pid with messages, set our facility to be LOCAL0
        openlog($0, 'pid', 'LOG_LOCAL0');

        syslog('LOG_ERR', "Unable to open temp.txt: $err");
        closelog();
    }
    print MYFILE "Hello Sri\n";
    close(MYFILE);
}

sub daemonize {
    chdir '/' or die "Can't chdir to /: $!";
    open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
    open STDOUT, '>>/dev/null' or die "Can't write to /dev/null: $!";
    open STDERR, '>>/dev/null' or die "Can't write to /dev/null: $!";
    defined(my $pid = fork) or die "Can't fork: $!";
    exit if $pid;

    #setsid() or die "Can't start a new session: $!"; 
    POSIX::setsid();
    umask 0;
}

    [download]
    Check /var/log/messages (by default) for syslog messages, under an ordinary user I get:
    Feb 11 11:08:44 yogi ./gashd.pl[10467]: Unable to open temp.txt: Permi
+ssion denied

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Re: Perl daemon exiting without warning
by ikegami (Patriarch) on Feb 11, 2011 at 05:47 UTC

    I think your analysis of the problem is incorrect. I think successfully daemonised a program that repeatedly tries to write to /temp.txt and fails for permission reasons.

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      There is permission for writing the file temp.txt. i debugged the daemon, it is exiting after the " open STDIN, '/dev/null' " statement.
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        If you're saying it's exiting immediately after that open, I don't believe you. Did you perhaps trace the code by printing to STDOUT?

        I find it very unlikely that you have permission to create and write to /temp.txt (and that you intended to do so). Are you running as root?

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