Safe fork() construct

This snippet shows how to (properly) fork() in Perl. Don't forget to use wait() to collect your children after they're done...

Update:: Thanks to Coruscate for catching the runaway $

use strict;
use warnings;
 
use constant FORK_WAIT => 2;
use constant MAX_ATTEMPTS => 10;
our $attempt = 0;
 
my $pid = undef;
 
while (not defined ($pid = fork())) {
    die "Too many failed fork() attempts: $!\n"
        if ++$attempt > MAX_ATTEMPTS;
    warn "fork() failed: $!\n";
    sleep FORK_WAIT;
}
 
if ($pid) {
 
# I'm the father
 
} else {
 
 # I'm the son
 
}
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Re: Safe fork() construct by rob_au (Abbot) on Feb 09, 2003 at 21:23 UTC
I wrote a similar construct for an updated version of Proc::Daemon - The updated code has not been released under the Proc::Daemon namespace as yet however as I am waiting to hear back from Earl Hood, the maintainer of Proc::Daemon, with his comments on the proposed code changes. The updated `Fork` subroutine code follows: sub Fork { my $pid; my $retry = 0; # The following loop structure is responsible for performing a f +ork safely - # If the core fork function returns a valid process identificati +on value, # this is returned and this subroutine is exited. If however, a +n error # occurs, an attempt is made to retry the fork, if the error mes +sage is the # result of no additional processes. This is attempted $RETRY t +imes after # sleeping for $SLEEP seconds before execution is abandoned. FORK: { if ( defined( $pid = fork ) ) { return $pid; } elsif ( $! =~ /No more process/i ) { if ( ++$retry > ( $RETRY \|\| 3 ) ) { croak( __PACKAGE__, '::Fork - Cannot fork process, ret +ry count exceeded - ', $! ); } sleep ( $SLEEP \|\| 5 ); redo FORK; } else { croak( __PACKAGE__, '::Fork - Cannot fork process - ', $! +); } } } [download] `perl -le 'print+unpack("N",pack("B32","00000000000000000000001000101111"))'`	[reply] [d/l] [select]

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Re: Safe fork() construct
by rob_au (Abbot) on Feb 09, 2003 at 21:23 UTC

Proc::Daemon

The updated Fork subroutine code follows:

sub Fork {
    my $pid;
    my $retry = 0;

    #   The following loop structure is responsible for performing a f
+ork safely - 
    #   If the core fork function returns a valid process identificati
+on value, 
    #   this is returned and this subroutine is exited.  If however, a
+n error 
    #   occurs, an attempt is made to retry the fork, if the error mes
+sage is the 
    #   result of no additional processes.  This is attempted $RETRY t
+imes after 
    #   sleeping for $SLEEP seconds before execution is abandoned.

    FORK: {
        if ( defined( $pid = fork ) ) {
            return $pid;
        } elsif ( $! =~ /No more process/i ) {
            if ( ++$retry > ( $RETRY || 3 ) ) {
                croak( __PACKAGE__, '::Fork - Cannot fork process, ret
+ry count exceeded - ', $! );
            }

            sleep ( $SLEEP || 5 );
            redo FORK;
        } else {
            croak( __PACKAGE__, '::Fork - Cannot fork process - ', $! 
+);
        }
    }
}
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