Select return 0 on a private SCTP socket

Question

This is related to the question: SCTP with Multihoming as a replacement for replacement in TCP

I have a simple echo client / concurrent server application that works fine using TCP. I could transfer the file to stdin on the client, and the client will get all the data back, call selectwhich will return 1, indicating that the socket was readable, after which the read request will return 0, indicating EOF / FIN. Then the client will exit. All is well.

However, identical applications through SCTP cause problems. The only change was made from IPPROTO_TCP to IPPROTO_SCTP. Fork server, echo back data, the child exits and receives from the parent. The client receives all the data, but after that it selectcontinues to return 0 descriptors, ready (without a timeout, I added that it will hang forever).

What is happening in the world?

Here is the code for the client:

#!/usr/bin/perl -w
use strict;
use Socket;

# forward declaration
sub logmsg;

my $remote = shift || "localhost";
my $port = 9877;

($port) = $port =~ /^(\d+)$/ or die "invalid port";

my $iaddr = inet_aton($remote) || die "no host: $remote";
my $paddr = sockaddr_in($port, $iaddr);
my $proto = getprotobyname('sctp');

my $sockfd;

socket($sockfd, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect($sockfd, $paddr) || die "connect: $!";

str_cli($sockfd);

exit(0);

#----- subs down here ------#

sub str_cli {
    my $sockfd = shift;

    my ($n, $buff, $stdineof, $s);
    my $rin = '';

    $stdineof = 0;
    while ( 1 ) {
        if ($stdineof == 0) {
            vec($rin, fileno(STDIN), 1) = 1;
        }
        vec($rin, fileno($sockfd), 1) = 1; 

        my $nfound = select($rin, undef, undef, 1.0);
        if ($nfound < 0) {
            next if $!{EINTR};
            die "select: $!";
        } else { print "\$nfound == $nfound\n"; }

        if (vec($rin, fileno($sockfd), 1) == 1) { # socket readable
            print "trying to read from sockfd\n";
            $n = sysread($sockfd, $buff, 1024);
            if (!defined($n) || $n < 0) {
                # resume if sysread() returned because a signal was received
                next if $!{EINTR};
                die "sysread: $!";
            } elsif($n == 0) {
                if ($stdineof == 1) { return; } # normal termination
                else { die "str_cli: server terminated prematurely"; }
            }
            writen(*STDOUT, $buff);
        }

        if (vec($rin, fileno(STDIN), 1) == 1) { # stdin readable
            $n = sysread(STDIN, $buff, 1024);
            if (!defined($n) || $n < 0) {
                # resume if sysread() returned because a signal was received
                next if $!{EINTR};
                die "sysread: $!";
            } elsif($n == 0) {
                $stdineof = 1;
                if (!defined($s = shutdown($sockfd, SHUT_WR)) 
                    || $s == 0) { die("shutdown: $!"); }
                vec($rin, fileno(STDIN), 1) = 0;
                next;
            }
            writen($sockfd, $buff);
        }
    }
}

sub writen {
    my ($connfd, $buff) = @_;
    my $nleft = length($buff);
    my $total = 0;
    my $nwritten = 0;
    while ($nleft) {
        if (($nwritten = syswrite($connfd, $buff, $nleft, $total)) <= 0) {
            # resume if syswrite() returned because a signal was received
            # 0 indicates an error in this context
            next if $!{EINTR};
            die "syswrite: $!";
        }
        $nleft -= $nwritten;
        $total += $nwritten;
    }
}

sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }

Remember that this works great on TCP. I'm on Ubuntu 9.04 with all the necessary sctp packages.

Answer 1

You rely on a semi-closed TCP state that is not available in SCTP.

TCP, shutdown($sockfd, SHUT_WR) FIN, , . SCTP , SCTP, . .