Re: Capturing errors from 3-arg pipe open in ActivePerl 5.020

From the same document a few paragraphs up from the General Examples section:

Open returns nonzero on success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess.

Even if the command isn't on your system the pipe is opening a subprocess and returning the pid. Here's what I see on Windows 8.

use warnings;
use strict;

print "here it goes...\n";
my $return_val = open(my $article, "-|", "caesar <article");
#    or die "Can't start caesar: $!";  
print "tried it and got [$return_val]\n";
[download]

c:\usr\pm>openprog.pl > openprog.txt 2>&1
here it goes...
The system cannot find the file specified.
tried it and got [14048]
[download]

Update

use warnings;
use strict;
use Win32::Process::List;

$!=0;
print "here it goes...<$!>\n";
my $return_val = open(my $article, "-|", "caesar <article");
#    or die "Can't start caesar: $!";  
print "tried it and got [$return_val] <$!>\n";

print "="x75,"\n";
my %list = Win32::Process::List->new()->GetProcesses();        #return
+s the hashes with PID and process name
foreach my $key ( keys %list ) {
    # $list{$key} is now the process name and $key is the PID
    print sprintf("%30s has PID %15s", $list{$key}, $key) . "\n" if $k
+ey == $return_val;
}
print "="x75,"\n";
[download]

here it goes...<>
The system cannot find the file specified.
tried it and got [8664] <Inappropriate I/O control operation>
======================================================================
+=====
                       cmd.exe has PID            8664
======================================================================
+=====
[download]

Comment on Re: Capturing errors from 3-arg pipe open in ActivePerl 5.020 Select or Download Code

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Re^2: Capturing errors from 3-arg pipe open in ActivePerl 5.020 by ateague (Monk) on Nov 16, 2015 at 17:50 UTC
Sorry, I feel a bit dense here, but what do I do with the PID once I have it? I get a PID regardless of whether or not the command succeeded. pipe.pl `#!/usr/bin/perl use 5.018; use strict; use warnings; my $pid = open (my $ARTICLE, "-\|", "caesar") or die "Can't start caesa +r: $!\n$^E"; my $read = <$ARTICLE>; say "[$read][$pid]";` [download] Results: `perl pipe.pl 'caesar' is not recognized as an internal or external command, operable program or batch file. Use of uninitialized value $read in concatenation (.) or string at pip +e.pl line 9. [][1236]` [download]	[reply] [d/l] [select]
Re^3: Capturing errors from 3-arg pipe open in ActivePerl 5.020 by Lotus1 (Vicar) on Nov 16, 2015 at 18:53 UTC
You asked in the OP what is different about your command and why isn't the 'or die' clause working in your open command. I provided documentation that shows if you open a pipe it returns the pid instead of just non-zero or zero like with opening a file. That is the difference and the answer to your question. I wasn't telling you to do anything with the pid, only that it is the return value and the reason why your code didn't work as expected. I don't think this example should be included in the documentation for open since it doesn't work as expected for pipes.	[reply]