Forsaken has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,

problem is as follows: I use a combination of Socket and IO::Select, quite classic really. I open a number of socket connections using this little snippet: 

sub makecon
  {
  my($self, $server, $port) = @_;
  my($iaddr, $paddr, $filehandle);
  my $proto = getprotobyname('tcp');

  unless($iaddr = inet_aton($server)) { carp "No host found: $server";
+ return; }
  $paddr = sockaddr_in($port, $iaddr);
  unless(socket($filehandle, PF_INET, SOCK_STREAM, $proto)) { carp "so
+cket failure: $!"; return; }
  unless(connect($filehandle, $paddr)) { carp "connect failure: $!"; r
+eturn; }

  return $filehandle;
  }

[download]
and then stuff the filehandle into IO::Select so I can perform the classic select loop to check which filehandles have data available. However, what I'm trying to do now is to implement a detection system for when a socket has died. Having read the documentation for IO::Select my first impression was that IO::Select->has_exception() would be the perfect tool to do so but for some reason it refuses to give me back anything, even though I am absolutely sure that the connection is dead on the other side. Any suggestions or ideas?

Remember rule one...

Replies are listed 'Best First'.
Re: Knowing when a socket connection is dead
by castaway (Parson) on Jul 29, 2005 at 10:34 UTC
    It's dead (disconnected), when can_read() indicates data available, but reading from it gives you no bytes. (See im2 code). I forget where I picked that up from, but it's worked for me. Why are you not using IO::Socket, btw?

    C.

[reply]
      If you want to be completely sure, you need to do a little more checking. At the least, you should check if $! is EINTR. In that case, you got no data because the system call was interrupted.
[reply]
Re: Knowing when a socket connection is dead
by metalgear119 (Acolyte) on Jul 29, 2005 at 12:29 UTC
    I had the same problem just recently. This is how I resolved it. 
    sub socketListen
{
    my $client = shift;            # specific client connection execut
+ed in this thread
    my $server = shift;
    my $server_port = shift;    # port number of currently connected s
+ocket
    
    # data is ready to be received over the socket #
    $client->autoflush();
            
    print "connected to port: $server_port\n" if ( $debug == 1 );
    my @recData = ();
    my $numTries = 0;                          
    
    # set connection up for non-blocking read #
    my $true=1;    
    my $rec;    
    ioctl($client,  0x8004667e, \$true) or return -1;
    
    while ( $quitThread == 0 ) {        
        # try to read data  #
        $rec = undef;
        my $res = sysread($client, $rec, $bufferSize);
        
        if ( defined($res) ) {
            if ( $res == 0 ) {
                # socket was closed remotely #
                print "received nothing on socket $server_port.\n" if 
+( $debug == 1 );
                last;
            }    
            elsif ( $res > 0 ) {
                # received message #
                print "received on socket $server_port:$rec\n" if ( $d
+ebug == 1 );
            }
        }
        else {
            # no new information came through #
            print "sleeping: received nothing on socket $server_port.\
+n" if ( $debug == 1 );
            sleep 2;
            next;
        }
        
        
    }
    
    $client->close;
    print "closing socket $server_port...\n" if ( $debug == 1 );
}

    [download]
    This was the body of a sub routine I called whenever my selector could do a "can_read" on the connection. You may want to use the "numTries" to have the connection timeout if nothing comes through over a certain time period.
[reply]
[d/l]
Re: Knowing when a socket connection is dead
by kscaldef (Pilgrim) on Jul 29, 2005 at 16:11 UTC
    You're misunderstanding what has_exception() does. It's a little used feature of sockets where "urgent" information can be sent out-of-band. You can think of it a little like how unix processes have both STDOUT and STDERR; there's this extra channel that you can use in exceptional situations.

    That said, I've never used that feature myself and I'm not sure it's really that useful.

[reply]

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