perlquestion
azadian
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.
<code>
#!/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 $pid; #Process ID of child process
# r e a d L i n e
# This expects to read one line (\n-terminated, but the newline is chomped).
# Returns the line, which will be empty if no characters are waiting to be read.
sub readLine {
my ($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;
}
</code>
When I run this I get:
<code>= ./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
</code>
<p>
Note that I call sysread() at second 90, and it doesn't return until 94, when the child finally decides to write again.
</p>
If I uncomment-out the blocking(0) lines in the middle of the program, I get the following:
<code>
= ./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
</code>
<p>
As near as I can tell, setting nonblocking shouldn't be necessary, and it shouldn't cause a problem. What is going on here?
</p>