smackdab has asked for the wisdom of the Perl Monks concerning the following question:

I read the Ram book and tried to modify a sample...

I want to fork off some small number of programs and display the ouput in my main window. I tried using fork and `file.exe` together but it just hung. The example below works if you don't call an external program and just pass a "string".

I am running Win32 and using the "new" fork()...

use IO::Handle;    
pipe(READER, WRITER);               
WRITER->autoflush(1);

if ($pid = fork) {
    close WRITER;
    my $buf = "";
    while (<READER>)
        { $buf .= $_; }
    chomp($buf);
    print "Parent Pid $$ just read this:\n$buf\n";
    close READER;
    waitpid($pid, 0);
} else {
    die "cannot fork: $!" unless defined $pid;
    close READER;
    
#    my $out = `ping 555-1212`;
    print WRITER "Child Pid $$ is sending this\n";
    print PARENT_WRT $out;
    close WRITER;
    exit;
}

[download]
Is it because of Win32 or is there a more standard way of doing this?

thanks !!!

Replies are listed 'Best First'.
Re: Forked up
by tachyon (Chancellor) on Oct 09, 2001 at 07:09 UTC
[reply]
      Thanks, I tried the examples and they exhibit the same problem...just add:

      $buf=`ping 127.0.0.1`;

      to the "#this is child" block...

      I don't know stdout/stdin...but wonder about it...

      thanks!

[reply]

        Windows was not designed with forking in mind. I get the same problem using backticks - they work fine in the parent but hang in the child. Probably relates to the fork() implementation/emulation on Windows. Solution - don't use backtics in child. This works fine using the Net::Ping module. 

        use Net::Ping;

pipe (FROM_CHILD, TO_PARENT) or die "Oops Pipe: $!\n";
pipe (FROM_PARENT, TO_CHILD) or die "Oops Pipe: $!\n";
select((select(TO_CHILD),  $|++ )[0]); # set autoflush
select((select(TO_PARENT), $|++ )[0]); # set autoflush
$|++;
my $result;

# let's fork
if ( my $pid = fork ) {
    # this is parent
    print "Sending to kid\n";
    sleep 1;
    print TO_CHILD "perlmonks.org\n";  
    chomp( my $child_says = <FROM_CHILD> );
    print "Child says '$child_says' to parent\n";
    $result = $child_says;
    waitpid( $pid, 0 );
} else {
    # this is child
    die "Can't fork: $!\n" unless defined $pid;
    print "Waiting to get info from parent\n";
    chomp( my $parent_says = <FROM_PARENT> );
    print "Parent says '$parent_says' to child\n";
    my $p = Net::Ping->new();
    my $ping = $p->ping($parent_says);
    print "Net::Ping in child returned: $ping\n";
    $p->close();
    print TO_PARENT "Child pid $$ replies:$ping\n";
    exit;
}


close FROM_CHILD; close TO_CHILD;
close FROM_PARENT; close TO_PARENT;

print "Parent got $result\n";

        [download]

        cheers

        tachyon

        s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

[reply]
[d/l]
Re: Forked up
by CubicSpline (Friar) on Oct 09, 2001 at 07:08 UTC
    I'm relatively new, but it doesn't seem to me that you want to be using fork. perldoc: func describes fork() as: "fork - create a new process just like this one".

    Perhaps what you want is perldoc: system which lets you call other executable code and wait for that code to complete.

[reply]
      That's right. Do a fork, then do a system (or backticks) so that you can wait for the external program while still running the main program.
[reply]
[d/l]

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