process - How can I make fork in Perl in different scripts?

Question

I have a process in Perl that creates another one with the system command, I leave it on memory and I pass some variables like this:

my $var1 = "Hello";
my $var1 = "World";
system "./another_process.pl $var1 $var2 &";

But the system command only returns the result, I need to get the PID. I want to make something like fork. What should I do? How can I make something like fork but in diferent scripts?

Thanks in advance!

Answer 1

Perl has a fork function.

See perldoc perlfaq8 - How do I start a process in the background?

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(contributed by brian d foy)

There's not a single way to run code in the background so you don't have to wait for it to finish before your program moves on to other tasks. Process management depends on your particular operating system, and many of the techniques are in perlipc. Several CPAN modules may be able to help, including IPC::Open2 or IPC::Open3 , IPC::Run , Parallel::Jobs , Parallel::ForkManager , POE , Proc::Background , and Win32::Process .

There are many other modules you might use, so check those namespaces for other options too. If you are on a Unix-like system, you might be able to get away with a system call where you put an & on the end of the command:

    system("cmd &")

You can also try using fork, as described in perlfunc (although this is the same thing that many of the modules will do for you).

STDIN, STDOUT, and STDERR are shared

Both the main process and the backgrounded one (the "child" process) share the same STDIN, STDOUT and STDERR filehandles. If both try to access them at once, strange things can happen. You may want to close or reopen these for the child. You can get around this with opening a pipe (see open) but on some systems this means that the child process cannot outlive the parent.

Signals

You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is sent when you write to a filehandle whose child process has closed (an untrapped SIGPIPE can cause your program to silently die). This is not an issue with system("cmd&").

Zombies

You have to be prepared to "reap" the child process when it finishes. $SIG{CHLD} = sub { wait }; $SIG{CHLD} = 'IGNORE'; You can also use a double fork. You immediately wait() for your first child, and the init daemon will wait() for your grandchild once it exits.

unless ($pid = fork) {
        unless (fork) { 
            exec "what you really wanna do";
            die "exec failed!";
  }

        exit 0;
    }

    waitpid($pid, 0);

See Signals in perlipc for other examples of code to do this. Zombies are not an issue with system("prog &").system("prog &").