Question

I have a simple script that should bind to an SMSC and listen for incoming messages. The problem I’m having is that it will time out if it doesn’t receive any messages.

Here is the script:

#!/usr/bin/perl

use Net::SMPP;
use Data::Dumper;
$Net::SMPP::trace = 1;

$smpp = Net::SMPP->new_receiver('--removed--',
                                port => '--removed--',
                                system_id => '--removed--',
                                password => '--removed--',
                                ) or die;

while (1)
{       
    $pdu = $smpp->read_pdu() or die;

    print "Received #$pdu->{seq} $pdu->{cmd}:". Net::SMPP::pdu_tab->{$pdu->{cmd}}{cmd} ."\n";
    print "From: $pdu->{source_addr}\nTo: $pdu->{destination_addr}\nData: $pdu->{data}\n";
    print "Messsage: $pdu->{short_message}\n\n";
}

Here’s the error I’m getting:

premature eof reading from socket at /usr/lib/perl5/site_perl/5.8.8/Net/SMPP.pm line 2424.
$VAR1 = undef;

And here’s the relevant sub from SMPP.pm:

sub read_hard {
    my ($me, $len, $dr, $offset) = @_;
    while (length($$dr) < $len+$offset) {
        my $n = length($$dr) - $offset;
        eval {
            local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
            alarm ${*$me}{enquire_interval} if ${*$me}{enquire_interval};
            warn "read $n/$len enqint(${*$me}{enquire_interval})" if $trace>1;
            while (1) {
                $n = $me->sysread($$dr, $len-$n, $n+$offset);
                next if $! =~ /^Interrupted/;
                last;
            }
            alarm 0;
        };
        if ($@) {
            warn "ENQUIRE $@" if $trace;
            die unless $@ eq "alarm\n";   # propagate unexpected errors
            $me->enquire_link();   # Send a periodic ping
        } else {
            if (!defined($n)) {
                warn "error reading header from socket: $!";
                ${*$me}{smpperror} = "read_hard I/O error: $!";
                ${*$me}{smpperrorcode} = 1;
                return undef;
            }
            #if ($n == 0) { last; }
            if (!$n) {
                warn "premature eof reading from socket";
                ${*$me}{smpperror} = "read_hard premature eof";
                ${*$me}{smpperrorcode} = 2;
                return undef;
                #return 0;
            }
        }
    }
    #warn "read complete";
    return 1;
}

In the sub, the if statement it’s hitting is the one where $n is 0 or undef.

My guess is that the socket is timing out and disconnecting. How can I keep the listener up indefinitely?

In addition, this listener blocks while waiting for a pdu. Is there a way to listen without blocking?

I’m a Telecom Engineer who does programming on the side, and I’ve gone through all the material I could find but couldn’t find an answer.

Answer 1

It looks as if the sysread() call simply returns 0. It can do that only, if the connection status is known to be disconnected. Since your side did not disconnect or timeout, i would deduce that the remote side disconnected. If a timeout would have occured on your side, you should not have been able to see the premature eof... message.

So, you are already ‘keeping the listener up indefinitely‘, since you do not set the enquire_interval option.

Regarding ‘Is there a way to listen without blocking?‘ the description section describes asynchronous mode at the end: Module can also be used asynchronously by specifying async=>1 to the constructor. You have to implement the data polling then yourself.