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Resizing IPC::Msg queue size

by Jeppe (Monk)
on Jul 08, 2004 at 09:13 UTC ( #372716=perlquestion: print w/ replies, xml ) Need Help??
Jeppe has asked for the wisdom of the Perl Monks concerning the following question:

Fellow perl followers!

I am despairing. I am trying to use IPC::Msg for an interprocess message queue, but I cannot seem to make the queues large enough!

So - how do I resize a message queue created from IPC::Msg?

Comment on Resizing IPC::Msg queue size
Re: Resizing IPC::Msg queue size
by gellyfish (Monsignor) on Jul 08, 2004 at 09:24 UTC

    You need to use the set method of IPC::Msg to set the qbytes attribute of the queue.


      Unfortunately, it fails. Given the code below, I get
      [jps@secanaws jps]$ perl Can't call method "pack" on an undefined value at /usr/lib/perl5/5.6.1 +/i386-linux/IPC/ line 78.
      If I remove the set call, everything seems to work except the queue being too small. What am I missing?
      #!/usr/bin/perl use IPC::SysV qw(IPC_PRIVATE IPC_CREAT IPC_NOWAIT); use IPC::Msg; use Data::Dumper; use strict; use warnings; my $msgtype = 1; my $message = "hello nr "; my $messages = 2000; my $buf; my $msg = new IPC::Msg('24h', IPC_CREAT); $msg->set('qbytes' => 32768); for (my $i = 0; $i < $messages; $i++) { my $rv = $msg->snd($msgtype, pack("L a*",$msgtype,$message . $i), IPC_NOWAIT); print "rv of $i is $rv\n" } while ($msg->rcv($buf,256, $msgtype, IPC_NOWAIT)) { print "$buf\n"; }

        Hmm, that appears to work fine with 5.8.1 - admittedly the program doesn't do what I think it should though.


        Looking at the code near the line on which the error was reported, we find this set method in 5.6.1:

        # IPC/ lines 61-79 sub set { my $self = shift; my $ds; if(@_ == 1) { $ds = shift; } else { croak 'Bad arg count' if @_ % 2; my %arg = @_; my $ds = $self->stat or return undef; my($key,$val); $ds->$key($val) while(($key,$val) = each %arg); } msgctl($$self,IPC_SET,$ds->pack); }

        There is a bug in this code: on line 11 (which is line 71 of the original file), my $ds creates a new $ds variable which goes out of scope at the end of the else block. That is why the final line has an undefined value for $ds when it tries to invoke $ds->pack.

        Removing the redundant "my" will fix that problem by ensuring that (as intended) the outer $ds variable is the one used, and indeed in later perls you'll find that exactly this change has been made: replacing

        my $ds = $self->stat
        $ds = $self->stat
        at line 71.

        If you don't have permission to make this fix to the code directly, and you can't convince the system administrator to apply the fix for you, there are a couple of ways you can get around it. One way is subclassing to replace the buggy routine:

        { package IPC::Msg::Bugfix; our @ISA = qw/ IPC::Msg /; sub set { # corrected version of set here ... } } # in your code my $msg = new IPC::Msg::Bugfix('24h', IPC_CREAT); # and continue as before

        An alternative is to recode to avoid the path with the bug in it:

        # avoiding buggy multiple-arg set() # $msg->set('qbytes' => 32768); my $ds = $msg->stat or die "stat: $!"; $ds->qbytes(32768); $msg->set($ds);

        (Please note, I haven't tested any of this code.)


Re: Resizing IPC::Msg queue size
by Jeppe (Monk) on Jul 09, 2004 at 11:30 UTC
    And now - my final finding:

    Setting a larger value for kernel.msgmnb (on Linux) does the trick. That's all. You find it under /proc/sys/kernel/msgmnb - and remember to add it to your sysctl.conf to!

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