Sysread() returns 0 at EOF, which I understand to mean that there is no data waiting to be read. My program seems to indicate that sysread() isn't returning until it has data to read.

#!/usr/bin/perl
# teof2 20201104        NHA
# Test eof() for IPC over socketpair filehandle.

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);
        $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: $!\n";
#$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]

When I run this I get:

= ./teof2
89: polling child
89: received <88: child started>
90: polling child
94: received <94: child wrote again>
95: polling child
96: received <E_O_F>
96: child exiting
96: child reaped, parent exiting
[download]

Note that I call sysread() at second 90, and it doesn't return until 94, when the child finally decides to write again.

If I uncomment-out the blocking(0) lines in the middle of the program, I get the following:

= ./teof2
16: polling child
16: received <15: child started>
17: polling child
Uncaught exception from user code:
    sysread: Resource temporarily unavailable
    main::readLine(GLOB(0x55a6c475a4d8)) called at ./teof2 line 63
[download]

As near as I can tell, setting nonblocking shouldn't be necessary, and it shouldn't cause a problem. What is going on here?