You seem to be mixing concepts.

First, read open again. If you use a piped open, it will return the PID of the child process:

my $pid = open my $fh, "| $some_program"
    or die "Can't run '$some_program': $!\n";
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Second, you will only need IPC::Open2 to write and read to/from the child process, but in your script I don't see you're attempting to read. open2 will also return a PID, and it will handle all that fork stuff and plumbing the pipes for you, so you don't need to fork.

Third: if you open a filehandle in the child after forking, it is visible in the child only. If you open it before forking, the file will be open in both parent and child:

#!/usr/bin/perl -w

use strict;

open(O,'>', "$$.tmp");
select O; $| = 1; # make output unbuffered
print STDOUT "file is $$.tmp\n";

$SIG{'CHLD'} = \&reap;
if ( (my $pid = fork()) == 0 ) {
    sleep 1;
    print "child ($$) here\n";
    close O;
    exit 0;
} else { 
    print "parent ($$) here\n";
    sleep 2;
    print "parent ($$) waking up..\n";
}   

sub reap {
    my $pid = wait;
    print "reaped PID $pid\n";
}
__END__
file is 4782.tmp
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$ cat 4782.tmp
parent (4782) here
child (4783) here
reaped PID 4783
parent (4782) waking up..
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Note that you must set $| = 1 for your filehandle to make output unbuffered if you want to keep the output of parent and child in order. Or better use IO::File and set autoflush FH 1.

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

I don't need to read from the process, just write to it. I know open2() returns PID. I am specifically using open2() because it returns a PID and I am able to write to the process. But maybe I can do that like you said.

I know I can open the handle globally (ie; before I fork), but I couldn't figure out how to work it into the code I wrote so that the flow works out to how it should.

Thing is, I need to be able to write to the binary process, whilst also checking to see if it exists, if it doesn't, I need to restart it.

If the binary process terminates for some reason, it must be restarted. But if I want to actually end the binary process, I need to kill off the parent(s). I couldn't see that this would work with what I had done unless I just send it a catchable SIGNAL. Of course if one were to send a SIGKILL signal to the parent, it wouldn't be catchable. Sure it will terminate it, but the binary process would still remain.

I will try some stuff and re-post it.

And here is a solution :

------------------------------------

use POSIX ":sys_wait_h"; # Didn't do this before.
my $spid;
print "Launching \"$actual_prog\" as UID($uid)/GID($gid)\n";

    unless (my $pid=fork()) {
        # Re-direct $program's STDERR to a file on the hard disk
        open(STDERR,">> $logfile"); 
        $spid = open2(*SREAD, *SWRITE, "$program @prog_options");
        while(1) {
            if($got_sig==1) {
                print STDOUT "GOT SIGNAL on $0\n";
                print SWRITE "Hey there server process\n";
                sleep 0.5;
                kill('KILL',$spid);
                exit(0);
            }else{
                    my $der = waitpid($spid,WNOHANG);
                    # If return val < 0. It ended ...
                    if($der==-1) {
                        print STDOUT "Time to re-launch\n";
                        open(STDERR,">> $logfile");
                        $spid = open2(*SREAD, *SWRITE, "$program @prog
+_options");
                    }
            }
        }
    }
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------------------------------------

Obviously I am only using one Perl process to manage the spawned binary process now. I also specifically included sys_wait_h : use POSIX ":sys_wait_h";, which I hadn't done before. Now the following works fine :

my $der = waitpid($spid,WNOHANG);
                    if($der==-1) {
                        print STDOUT "Time to re-launch\n";
                        open(STDERR,">> $logfile");
                        $spid = open2(*SREAD, *SWRITE, "$program @prog
+_options");
                    }
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and I can do other stuff whilst the child process state hasn't changed (ie; actually non-blocking waitpid()).

This works fine for me. But I liked my other code. Oh well. Whatever works. I should probably re-read over waitpid() and how Perl implements it.