sysread returning 0 means "connection was closed".
sysread returning undef means "connection is still open but no data is available at this time, or some other error".

Your code wasn't doing what you documented.

#!/usr/bin/perl


use diagnostics;
use strict 'subs';
use strict 'refs';
use Socket;
use IO::Handle;

my $child;      #filehandle to child process
my $parent;     #filehandle to parent process
my $pid;        #Process ID of child process

#       r e a d L i n e
# This expects to read one line (\n-terminated, but the newline is cho
+mped).
# Returns the line, which will be empty if no characters are waiting t
+o be read.
sub readLine {
    my ($fh) = @_;
    my $buf = '';
    my $offset = 0;
    my $cnt = 0;
    my $stat;
    do {
        $stat = sysread $fh, $buf, 4096-$offset, $offset;
#        die "sysread: $!\n"  unless defined($stat);
        return '' unless defined($stat); ############ DOING WHAT YOU D
+OCUMENTED
        $offset += $stat;
    } while ($stat  and  "\n" ne substr($buf, -1));
    chomp $buf;
    #print STDERR time%100, " #readLine returning <$buf>\n";
    return $buf;
}                                               #readLine()

#       w r i t e L i n e
# Writes a buffer to the filehandle.
sub writeLine {
    my ($fh, $buf) = @_;
    my $offset = 0;
    my $remaining = length($buf);
    do {
        my $stat = syswrite($fh, $buf, $remaining, $offset);
        die "syswrite: $!\n"  unless defined($stat);
        $remaining -= $stat;
        $offset += $stat;
    } while (0 < $remaining);
}                                               #writeLine()


socketpair($child, $parent, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
                                              or die "socketpair: $!";
$child->autoflush(1);
$parent->autoflush(1);
$child->blocking(0)  or  die "blocking: $!\n"; ######### CHANGED
#$parent->blocking(0)  or  die "blocking: $!\n";
if ($pid = fork()) {                    #parent
    close $parent  or die "close: $!\n";
    while (1) {
        sleep 1;
        print STDOUT time%100, ": polling child\n";
        if (my $line = readLine($child)) {
            print STDOUT time%100, ": received <$line>\n";
            if ('E_O_F' eq $line) {
                my $stat = wait;
                die "wait returned $stat\n"  unless $stat == $pid;
                print STDOUT time%100, ": child reaped, parent exiting
+\n";
                exit 0;
            }
        } else {
            print STDOUT time%100, ": no input from child yet\n";
        }
    }
} else {                                #child
    die "cannot fork: $!"  unless defined $pid;
    close $child or die "close: $!\n";
    writeLine($parent, time%100 . ": child started\n");
    sleep 6;
    writeLine($parent, time%100 . ": child wrote again\n");
    sleep 2;
    writeLine($parent, "E_O_F\n");      #causes termination
    close $parent  or die "close: $!\n";
    print STDOUT time%100, ": child exiting\n";
    exit;
}
[download]

Outputs:

51: polling child
51: received <50: child started>
52: polling child
52: no input from child yet
53: polling child
53: no input from child yet
54: polling child
54: no input from child yet
55: polling child
55: no input from child yet
56: polling child
56: received <56: child wrote again>
57: polling child
57: no input from child yet
58: child exiting
58: polling child
58: received <E_O_F>
58: child reaped, parent exiting
[download]

Is this what you wanted?

---

---

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! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

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:
	&		&
	<		&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.