I ran across the following code the other day. The implementation definately has, some, well issues. However the behavior was a little odd (not the exact code, distilled to highlight the issue): 
sub run_cmd
{
  my $cmd = shift();

  my $in;
  my $out;
  my $err;

  my $pid = open3($in, $out, $err, $cmd);

  return ($in, $out, $err);
}

my ($in, $out, $err)  = run_cmd("some command");

while(<$out>)
{
   do stuff with $out;
}

while(<$err>)
{
   do stuff with $err;
}

[download]
The problem is that $out gets both STDERR and STDOUT of "some command".

After digging through the perldoc of Open3 and seeing the $dad_err ||= $dad_rdr line in IPC/Open3 the problem was made clear. I re-factored the code to work a bit more like Re: IPC::Open3 woes this so the code works now.

My question is, why is this desired behavior? If I wanted STDERR and STDOUT to be on the same handle I would call open 3 with the same handle for outfh and errfh right? (i.e. open3($in, $out, $out, ...) )

In reply to open3 question by amw1

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.