In Perl, how can I block for a bunch of system calls to finish?

Question

I'm doing a bunch of system calls that I want to run in parallel:

system(" sleep 5 && echo step 1 done &");
system(" sleep 3 && echo step 2 done &");
system(" sleep 7 && echo step 3 done &");

// block here

How can I block the program flow until all the system calls are finished?

Answer 1

Fork a child process to perform each job, and in the parent, wait for those processes to finish before exiting.

See perldoc perlfork, perldoc -f fork and perldoc -f waitpid.

(The exact code for this is left as an exercise for the reader -- everyone should have to write this from scratch at least once, to understand the details involved. There are lots of examples on this site as well.)

Answer 2

what about running each one of the system call from a different thread and join on the threads

Answer 3

The easiest way to do this is probably to fork a new child process for each system call, and then wait for them to finish. Here's a simplified example:

my @commands = ( "sleep 5 && echo step 1 done",
                 "sleep 3 && echo step 2 done",
                 "sleep 7 && echo step 3 done" );

my @pids;
foreach my $cmd( @commands ) {
    my $pid = fork;
    if ( $pid ) {
        # parent process
        push @pids, $pid;
        next;
    }

    # now we're in the child
    system( $cmd );
    exit;            # terminate the child
}

wait for @pids;   # wait for each child to terminate

print "all done.\n";