Perl socket programing send method

Question

I am making socket programming for simple communication now.

I have noticed that the server is not the one I created and it works fine (given the experimental client)

In my code, recv works fine, but send does not work. Is there anything wrong with my code?

my $socket = new IO::Socket::INET (
    #PeerHost => '127.0.0.1',
    PeerHost => '192.168.0.100',
    PeerPort => '8472',
    Proto => 'tcp',
);
die "cannot connect to the server $!\n" unless $socket;
print "connected to the server\n";

while (1) {
    my $response = "";
    $socket->recv($response, 1024);
    if ($response) {
        my @test = split(//,$response);
        my ($length,$data) = unpack("N A*",$response);
        %json = json_decode($data,$length);

        switch ($json{'type'}) {
            case 1 { print "Game Start\n";}
            #case 2 { my $tmp = &my_turn(%json);} #my_turn func is return  "{'type': 0, 'point': [5, 4]}", but fail!
            #case 2 { $socket->send("{'type': 0, 'point': [5, 4]}");} # fail!
            case 2 { print $socket "{'type': 0, 'point': [5, 4]}"; print "ok\n";} # print is executed. However, the server does not receive packets
            #case 2 { $socket->send("{'type': 0, 'point': [5, 4]}");} #fail...
            case 3 { print "ACCEPT\n";}
            case 5 { print "NOPOINT\n";}
            case 6 { print "GAMEOVER\n";}
            case 7 { print "ERROR\n";}
            else {print "ERROR type : $json{'type'}\n"}
        }
    }
}

The server works fine. I checked with the example source (python code) given with the server. What am I missing?

Answer 1

• You can't assume the recv (or read) will return the entire response. You need to call it repeatedly.
• You can't assume the recv (or read) will just the response. You need to limit the size of the read of buffer the excess.
• decode_json returns a reference (not a list of key-value pairs you can assign to a hash).
• You might also have to handle encoding of the JSON string. The example below assumes UTF-8 encoding.
• JSON response to the server (case 2 in the original code) needs to include length too.

The following code should be used instead:

#!/usr/bin/perl
use strict;
use warnings;

use JSON;
use IO::Socket;

my $socket = IO::Socket::INET->new(
    PeerHost => '127.0.0.1',
    PeerPort => '22',
    Proto    => 'tcp',
) or
    die "cannot connect to the server $!\n";

print "connected to the server\n";

sub read_bytes($$) {
    my($socket, $length) = @_;
    my $result = '';
    print "ATTEMPT TO READ ${length}\n";
    while ($length > 0) {
        my $received = $socket->read($result, $length, length($result));
        die "socket error: $!\n" unless defined($received);
        die "unexpected EOF\n"   unless $received;
        $length -= $received;
    }

    print "READ '${result}'\n";
    return($result);
}

while (1) {
    my $length = unpack("N", read_bytes($socket, 4));
    my $json   = read_bytes($socket, $length);
    my $data   = JSON->new->utf8->decode($json);

    print $data->{type}, "\n";

    if ($data->{type} == 2) {
        my $response = {
            type  => 0,
            point => [5, 4],
        };
        my $resp_json = JSON->new->utf8->encode($response);
        print "JSON:   ${resp_json}\n";

        my $packet = pack('NA*', length($resp_json), $resp_json);
        print "PACKET: ", unpack('H*', $packet), "\n";

        $socket->write($packet);
    }
}

As I don't have access to your server I used sshd on my local machine, which of course does not send me a JSON. But it shows that reading works :-)

$ perl dummy.pl
connected to the server
ATTEMPT TO READ 4
READ 'SSH-'
ATTEMPT TO READ 1397966893
^C

Output for an example response to the server would be:

JSON:   {"type":0,"point":[5,4]}
PACKET: 000000187b2274797065223a302c22706f696e74223a5b352c345d7d