—

              
##---------------------------------------------------------------------------##
##  File:
##      @(#) Daemon.pm 1.2 99/04/17 01:02:34
##  Author:
##      Earl Hood       earlhood@usa.net
##---------------------------------------------------------------------------##
##  Copyright (C) 1997-1999     Earl Hood, earlhood@usa.net
##      All rights reserved.
##
##  This program is free software; you can redistribute it and/or
##  modify it under the same terms as Perl itself.
##---------------------------------------------------------------------------##
package Proc::Daemon;
use strict;
use vars qw( $VERSION @ISA @EXPORT_OK );
use Exporter;
@ISA = qw( Exporter );
$VERSION = "0.02";
@EXPORT_OK = qw( Fork OpenMax );
##---------------------------------------------------------------------------##
use Carp;
use POSIX;
##---------------------------------------------------------------------------##
##      Fork(): Try to fork if at all possible.  Function will croak
##      if unable to fork.
##
sub Fork {
    my($pid);
    FORK: {
        if (defined($pid = fork)) {
            return $pid;
        } elsif ($! =~ /No more process/) {
            sleep 5;
            redo FORK;
        } else {
            croak "Can't fork: $!";
        }
    }
}
##---------------------------------------------------------------------------##
##      OpenMax(): Return the maximum number of possible file descriptors.
##      If sysconf() does not give us value, we punt with our own value.
##
sub OpenMax {
    my $openmax = POSIX::sysconf( &POSIX::_SC_OPEN_MAX );
    (!defined($openmax) || $openmax < 0) ? 64 : $openmax;
}
##---------------------------------------------------------------------------##
##      Init(): Become a daemon.
##
sub Init {
    my $oldmode = shift || 0;
    my($pid, $sess_id, $i);
    ## Fork and exit parent
    if ($pid = Fork) { exit 0; }
    ## Detach ourselves from the terminal
    croak "Cannot detach from controlling terminal"
        unless $sess_id = POSIX::setsid();
    ## Prevent possibility of acquiring a controling terminal
    if (!$oldmode) {
        $SIG{'HUP'} = 'IGNORE';
        if ($pid = Fork) { exit 0; }
    }
    ## Change working directory
    chdir "/";
    ## Clear file creation mask
    umask 0;
    ## Close open file descriptors
    foreach $i (0 .. OpenMax) { POSIX::close($i); }
    ## Reopen stderr, stdout, stdin to /dev/null
    open(STDIN,  "+>/dev/null");
    open(STDOUT, "+>&STDIN");
    open(STDERR, "+>&STDIN");
    $oldmode ? $sess_id : 0;
}
*init = \&Init;
##---------------------------------------------------------------------------##
1;
__END__
HideShow 125 lines of Pod
=head1 NAME
Proc::Daemon - Run Perl program as a daemon process
=head1 SYNOPSIS
    use Proc::Daemon;
    Proc::Daemon::Init;
=head1 DESCRIPTION
This module contains the routine B<Init> which can be called by
a perl program to initialize itself as a daemon.  A daemon is a
process that runs in the background with no controlling terminal.
Generally servers (like FTP and HTTP servers) run as daemon processes.
However, do not make the mistake that a daemon == server.
The B<Proc::Daemon::Init> function does the following:
=over 4
=item 1
Forks a child and exits the parent process.
=item 2
Becomes a session leader (which detaches the program from
the controlling terminal).
=item 3
Forks another child process and exits first child.  This prevents
the potential of acquiring a controlling terminal.
=item 4
Changes the current working directory to "/".
=item 5
Clears the file creation mask.
=item 6
Closes all open file descriptors.
=back
You will notice that no logging facility, or other functionality
is performed.  B<Proc::Daemon::Init> just performs the main steps
to initialize a program as daemon.  Since other funtionality can vary
depending on the nature of the program, B<Proc::Daemon> leaves
the implementation of other desired functionality to the
caller, or other module/library (like B<Sys::Syslog>).
There is no meaningful return value B<Proc::Daemon::Init>.  If an
error occurs in B<Init> so it cannot perform the above steps, than
it croaks with an error message.  One can prevent program termination
by using eval.
=head1 OTHER FUNCTIONS
B<Proc::Daemon> also defines some other functions.  These functions
can be imported into the callers name space if the function names
are specified during the B<use> declaration:
=head2 Fork
B<Fork> is like the built-in B<fork>, but will try to fork if at all
possible, retrying if necessary.  If not possible, B<Fork> will
croak.
=head2 OpenMax
B<OpenMax> returns the maximum file descriptor number.
If undetermined, 64 will be returned.
=head1 NOTES
=over 4
=item *
B<Proc::Daemon::init> is still available for backwards capatibilty.
However, it will not perform the double fork, and will return the
session ID.
=back
=head1 AUTHOR
Earl Hood, earlhood@usa.net
http://www.oac.uci.edu/indiv/ehood/
=head1 CREDITS
Implementation of B<Proc::Daemon> derived from the following sources:
=over 4
=item *
B<Advanced Programming in the UNIX Environment>, by W. Richard Stevens.
Addison-Wesley, Copyright 1992.
=item *
B<UNIX Network Progamming>, Vol 1, by W. Richard Stevens.
Prentice-Hall PTR, Copyright 1998.
=back
=head1 DEPENDENCIES
B<Proc::Daemon> depends on the following modules: B<Carp>, B<POSIX>.
=head1 SEE ALSO
L<POSIX>,
L<Sys::Syslog>
=cut