My version will spawn N number of children immediately and then spawn one more every time an already existing child dies until a job queue is exhausted. It will then wait for every existing child to die. Oh, it also prints a message before spawning a new child.

Notice I never install a SIGCHLD handler. That is because the parent will process them itself. Also, the waitpids are done non-blocking. The sleep makes sure we do not loop too tightly.

As splinky said, if you call a subroutine, make sure it exits or you call exit after it returns. The original code I took this from actually exec'd and so I didn't worry about it.

$num_children is a command line arg, not shown here so you don't have to see my lame mp3 ripping code. It, obviously, controls how many child procs will be spawned.

# Interesting stuff has happened to load the job queue and par
# the command line, and whatever else you want to happen.  
sub SPAWN {
    $counter++;
    if ( $cpid = fork ) {
        $tracker{$cpid} = $ref->[1];
    }
    else {
    # This can be a call like this, or
    my_sub( @_ );
    exit;
    # You can exec some external proc like
    # exec "/bin/ls -FC /var";
    }
}

my $dpid = 0;

while( @job_q ) {
    my $ref = shift @job_q;
    #--
    # Initial loop to spawn children
    #--
    if ( $counter < $num_children ) {
        &SPAWN( $ref );
    }
    else {
        do {
            sleep 5;
            $dpid = waitpid( -1, 1 );
        } until $dpid;
        last if ( $dpid == -1 );  # No children left
        printf "%s has finished, spawning next child (%d left)\n", $tr
+acker{$dpi
d}, scalar @job_q;
        &SPAWN( $ref );
    }
}

do {    
    sleep 5;
    $dpid = waitpid( -1, 1 );
    printf("%s has finished\n", $tracker{$dpid}) if(defined( $tracker{
+$dpid} )
    );
} until $dpid == -1;
[download]

I will warn you this isn't the most robust code. I haven't had problems with it, but it is something only I use. It is a slightly different solution to the problem though.

mikfire

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