Win32: Redirect STDOUT to socket

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

Dear monks, I'm having trouble redirecting STDOUT to a socket. I'm using ActiveState Perl v5.10.0 built for MSWin32-x86-multi-thread.

When $bug_on = 0; this is the output:

got_it> line1
got_it> line2
got_it> line3
123
456
[download]

When $bug_on = 1; I expect to see:

got_it> 123
got_it> 456
[download]

but it just stalls (on the readline?). I've seen howto redirect STDOUT to $socket but can't seem to figure out what's wrong. I'm stuck. Thanks in advance!

UPDATE #1: Thanks to all who have replied. I updated the code to add in 2 lines of code, adding the suggestion from ikegami.

Now when $bug_on = 1; I get:

999
got_it> 777
got_it> 888
123
456
[download]

At this point, after what BrowserUk mentioned, I'm merely curious as to what Perl is really doing internally.

From observation, I think that the STDOUT redirection is not persistent into the exec, and that exec resets STDOUT to the usual because it doesn't know any better. Is this assumption correct?

If so, then how does Perl process backticks to return the STDOUT of a process? All I really want is to manually capture STDOUT of another process without qx() (for my education :). No Tim Toady for backticks in Win32?

UPDATE #2: Big thanks to BrowserUk! So I dived a little deeper and I think I'm drowning badly in Win32 waters... I tried using Win32::Process::Create to spawn a process that will inherit parent filehandles, but the output is still the same as before:

999
got_it> 777
got_it> 888
123
456
[download]

I don't think the filehandles are inherited. Any ideas, anyone?

use strict;
use warnings;

use Socket;

# UPDATE #2: added 2 lines below
use Win32::Process;
use Win32;

my $bug_on = 1;

sub _pipe_from_fork ($) {
    my $pid;

    # emulate pipe()
    my $WRITE;
    socketpair($_[0], $WRITE, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
        and shutdown($_[0], 1)
        and shutdown($WRITE, 0)
        or return undef;

    if (defined($pid = fork()))
    {
        unless ($pid)
        {
            if ($bug_on)
            {
                # UPDATE #1: added 1 line below
                close(STDOUT);

                # XXX: BUG
                open(STDOUT, ">&", $WRITE) or die;
            }
            else
            {
                print $WRITE "$_\n" for qw(line1 line2 line3);
            }
        }
    }

    return $pid;
}

my $pid = _pipe_from_fork(my $READ);
die unless defined($pid);

if ($pid)
{
    print "got_it> $_" while <$READ>;
}
else
{
    # UPDATE #1: added 1 line below
    print `perl -le "print 777; print 888; print STDERR 999"`;

    # UPDATE #2: commented exec line and added Win32::Process::Create(
+)
    #exec qw(perl -le), 'print 123; print 456' or die;

    Win32::Process::Create(
        my $obj,
        'C:\\Perl\\bin\\perl.exe',
        'C:\\Perl\\bin\\perl.exe -le "print 123; print 456"',

        # does not inherit filehandles?
        1,

        NORMAL_PRIORITY_CLASS,
        "."
    );

}
[download]

Comment on Win32: Redirect STDOUT to socket Select or Download Code

Replies are listed 'Best First'.
Re: Win32: Redirect STDOUT to socket by BrowserUk (Patriarch) on Feb 09, 2009 at 10:01 UTC
Using: ActiveState Perl v5.10.0 built for MSWin32-x86-multi-thread. and socketpair($_[0], $WRITE, AF_UNIX, SOCK_STREAM, PF_UNSPEC) and fork Almost certainly isn't going to work. Consider using Win32::SocketPair, but trying to use fork to create a deamon isn't going to work because fork is emulated using threads on Win32. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply]
Re^2: Win32: Redirect STDOUT to socket by repellent (Priest) on Feb 09, 2009 at 17:50 UTC
Thanks, BrowserUk. Yes, I know the path of `fork`ing on Win32 is fraught with doom :) Just curious at this point: how does Perl do `qx()` in Win32? We can't emulate it another way? Please see my OP update.	[reply] [d/l] [select]
Re^3: Win32: Redirect STDOUT to socket by BrowserUk (Patriarch) on Feb 09, 2009 at 18:46 UTC
how does Perl do qx() in Win32? There is a parameter `bInheritHandles` on CreateProcess() that needs to be set for spawned processes to inherit their parents open handles. That is a part of what qx// does. You can access it using Win32::CreateProcess(). Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l]
Re^4: Win32: Redirect STDOUT to socket by repellent (Priest) on Feb 10, 2009 at 07:34 UTC
Re^5: Win32: Redirect STDOUT to socket by BrowserUk (Patriarch) on Feb 10, 2009 at 14:58 UTC
Some notes below your chosen depth have not been shown here
Re: Win32: Redirect STDOUT to socket by juster (Friar) on Feb 09, 2009 at 07:54 UTC
What you have is a buffering problem. You have to turn autoflushing on for the new STDOUT file handle you make: `use IO::Handle; # up top ... open STDOUT, '>&', $WRITE or die "failed redirect to STDOUT: $!"; STDOUT->autoflush(1); ...` [download] Or you can use the perlvar `$\|`, aka `$OUTPUT_AUTOFLUSH` `use English qw(-no_match_vars); ... open STDOUT, '>&', $WRITE or die "failed redirect to STDOUT: $!"; $OUTPUT_AUTOFLUSH = 1; ...` [download] I almost mentioned the select builtin but it gave different output, interestingly!	[reply] [d/l] [select]
Re^2: Win32: Redirect STDOUT to socket by repellent (Priest) on Feb 09, 2009 at 08:24 UTC
Doesn't work for me. After trying your suggestions, it still stalls. I have tried placing after `open`: `use IO::Handle; STDOUT->autoflush(1);` [download] and even `select((select(STDOUT), $\| = 1)[0]);` [download] to no avail.	[reply] [d/l] [select]
Re: Win32: Redirect STDOUT to socket by ikegami (Patriarch) on Feb 09, 2009 at 09:56 UTC
`close(STDOUT);` before the `open` makes it work (but I don't know why).	[reply] [d/l] [select]
Re^2: Win32: Redirect STDOUT to socket by repellent (Priest) on Feb 09, 2009 at 17:46 UTC
Thanks, ikegami! It works for within the current "process" only. Please see my update.	[reply]