in reply to Re^2: poorly documented behaviour of readline() and IO::Select
in thread poorly documented behaviour of readline() and IO::Select

sysread won't always give you the amount of bytes you request when reading from something that isn't a plain file.

See Re: A suicidal parent OR death of a forking server for a newline terminated solution, and here's a length-prefix adaptation: 

#!/usr/bin/perl

use strict;
use warnings;

use IO::Socket::INET qw( );
use IO::Select       qw( );


sub process_msg {
    my ($client, $msg) = @_;
    chomp $msg;
    print "$client->{host} said '$msg'\n";
}


sub process_msgs {
    my ($client) = @_;
    our $buf;  local *buf  = \($client->{buf});
    our $want; local *want = \($client->{want});

    for (;;) {
        if ($want) {
            return if length($buf) < $want;
            my $msg = substr($buf, 0, $want, '');
            $want = 0;
            process_msg($client, $msg);
        } else {
            return if length($buf) < 8;
            $want = 0+substr($buf, 0, 8, '');
        }
    }
}


my $server = IO::Socket::INET->new(
    ...
) or die("Couldn't create server socket: $!\n");

my $select = IO::Select->new($server);

my %clients;

while (my @ready = $select->can_read) {
    for my $fh (@ready) {
        if ($fh == $server) {
            my $client_sock = $server->accept;
            my $host = $client_sock->peerhost;
            print "[Accepted connection from $host]\n";
            $select->add($client_sock);
            $clients{fileno($client_sock)} = {
                host => $host,
                buf  => '',
                want => 0,
            };
        }
        else {
            my $client = $clients{fileno($fh)};
            our $buf;  local *buf  = \($client->{buf});
            our $want; local *want = \($client->{want});

            my $rv = sysread($fh, $buf, 64*1024, length($buf));
            if (!$rv) {
                my $host = $client->{host}fh->peerhost;
                if (defined($rv)) {
                    print "[Error reading from host $host]\n";
                } else {
                    print "[Connection from $host terminated]\n";
                }
                process_msgs($client);
                print "Incomplete message received from $host]\n"
                    if $want || length($buf);
                delete $clients{fileno($fh)};
                $sel->remove($fh);
                next;
            }

            process_msgs($client);
        }
    }
}

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Re^4: poorly documented behaviour of readline() and IO::Select
by ikegami (Patriarch) on Jun 09, 2010 at 19:19 UTC

    Instead of implementing a limited cooperative multitasking system using select, it's simpler to use threads or Coro.

    The following is the Coro equivalent of the code in the parent post. 

    #!/usr/bin/perl

use strict;
use warnings;

use Coro             qw( async );
use IO::Socket::INET qw( );

sub _read {
    my $rv = read($_[0], $_[1], $_[2], $_[3]||0);
    die("Read error\n") if !defined($rv);
    return if !$rv;
    die("Premature end of file\n") if $rv != $_[1];
}

sub client {
    my ($sock) = @_;

    my $host = $client->peerhost;
    print "[Accepted connection from $host]\n";

    if (defined(eval {
        for (;;) {
            _read($sock, 8, my $buf = '')
                or last;
            my $length = 0+$buf
                or next;
            _read($sock, $length, my $msg = '')
                or die("Premature end of file\n");
            print "$host said '$msg'\n";
        }
    })) {
        print "[Connection from $host terminated]\n";
    } else {
        print "[Connection from $host terminated: $@]\n";
    }
}

my $server = IO::Socket::INET->new(
    ...
) or die("Couldn't create server socket: $!\n");

async(\&client, $server->accept) while 1;

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