Beefy Boxes and Bandwidth Generously Provided by pair Networks
P is for Practical

Knowing when a socket connection is dead

by Forsaken (Friar)
on Jul 29, 2005 at 09:03 UTC ( #479302=perlquestion: print w/replies, xml ) Need Help??
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; }
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?


      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.
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 ); }
    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.
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.

Log In?

What's my password?
Create A New User
Node Status?
node history
Node Type: perlquestion [id://479302]
Approved by Tanalis
Front-paged by planetscape
and all is quiet...

How do I use this? | Other CB clients
Other Users?
Others browsing the Monastery: (5)
As of 2018-03-24 22:18 GMT
Find Nodes?
    Voting Booth?
    When I think of a mole I think of:

    Results (299 votes). Check out past polls.