I'm using a forked pipe [ open(FILE, '|-' ] and all appears to be well until I try to close the file handle, which blocks. I understand that close waits for the child process to exit before returning, but the read also appears to block even when the close has been initiated. This seems to create a deadlock situation of the child process waiting for the read to return, and the close waiting for the child to exit. The following test case may help to explain. 

our @handles;
our $message = ('xxxxxxxx');

sub child
{
        my $buffer;
        while(sysread(STDIN, $buffer, 8))
        {
                print("$$: $buffer\n");
        }
}

for($i=0; $i < 4; $i++)
{
        if(!open($handles[$i], '|-'))
        {
                # child process
                child();
                exit 0;
        }
}

for($i=0; $i < 4; $i++)
{
        syswrite($handles[$i], $message);
        syswrite($handles[$i], $message);
        syswrite($handles[$i], $message);
}

for($i=0; $i < 4; $i++)
{
        close($handles[$i]);
}

Produces the following output then freezes:

18241: xxxxxxxx
18241: xxxxxxxx
18241: xxxxxxxx
18242: xxxxxxxx
18242: xxxxxxxx
18242: xxxxxxxx
18243: xxxxxxxx
18243: xxxxxxxx
18243: xxxxxxxx
18244: xxxxxxxx
18244: xxxxxxxx
18244: xxxxxxxx

[download]
If I use an external command such as open(FILE, '| cat') it works fine, but deadlocks with the forked pipe version. Any ideas?

In reply to Forked pipe deadlock by Anonymous Monk

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.