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Dmitry Karasik


IO::Lambda::Thread - wait for blocking code using threads


The module implements a lambda wrapper that allows to asynchronously wait for a blocking code. The wrapping is done so that the code is executed in another thread's context. IO::Lambda::Thread provides bidirectional communication between threads, that is based on a shared socket between parent and child threads. The socket can be also used by the caller for its own needs, if necessary ( see IO::Lambda::Message ).


    use IO::Lambda qw(:lambda);
    use IO::Lambda::Thread qw(threaded);

    lambda {
        context 0.1, threaded {
              return "hello!";
        any_tail {
            if ( @_) {
                print "done: ", $_[0]-> peek, "\n";
            } else {
                print "not yet\n";
    }-> wait;


new_thread ( $options = (), $code, $pass_socket, @param) -> ($thread, $socket)

A special replacement for thread-> create, that not only creates a thread, but also creates a socket between the parent and child threads. The socket is important for getting an asynchronous notification when the child thread has finished, because there is no portable way to get that signal otherwise. That means that this socket must be closed and the thread must be join'ed to avoid problems. For example:

    my ( $thread, $reader) = new_thread( $sub {
        my $writer = shift;
        print $writer, "Hello world!\n";
    }, 1 );
    print while <$reader>;
    $thread-> join;

Note that join is a blocking call, so one needs to be sure that the thread indeed is finished before joining it. By default, the child thread closes its side of the socket, thus making the parent side readable. However, the child code can also hijack the socket for its own needs, so if that functionality is needed, one must create an extra layer of communication that ensures that the child code is properly exited, so that the parent can reliably call join without blocking (see IO::Lambda::Message, that is destined exactly for this use).

$code is executed in another thread's context, and is passed the communication socket ( if $pass_socket is set to 1 ). $code is also passed @param. Data returned from the code can be retrieved from join.

threaded($code) :: () -> ( @results )

Creates a lambda, that executes $code in a newly created thread. The lambda finishes when the $code and the thread are finished, and returns results returned by $code.

Note, that this lambda, if terminate'd between after being started and before being finished, has no chance to wait for completion of the associated thread, and so Perl will complain. To deal with that, obtain the thread object manually and wait for the thread:

    my $l = threaded { 42 };
    $l-> start;
    $l-> terminate;

    # synchronously
    $l-> thread-> join;

    # or asynchronously
    context $l-> socket;
    readable { $l-> thread-> join };

Returns the associated thread object. Valid only for lambdas created with threaded.


Returns the associated communication socket. Valid only for lambdas created with threaded.


Unbalanced string table refcount

Threading in Perl is fragile, so errors like the following:

   Unbalanced string table refcount: (1) for "GEN1" during global

are due to some obscure Perl bugs. They are triggered, in my experience, when a child thread tries to deallocate scalars that it thinks belongs to that thread. This can be sometimes avoided with explicit cleaning up of scalars that may be visible in threads. For example, calls as



   undef $my_lambda; # or other scalars, whatever

inexplicably hush these errors.

Perl exited with active threads

Errors like this

  Perl exited with active threads:
        1 running and unjoined
        0 finished and unjoined
        0 running and detached

are triggered when child threads weren't properly joined. Make sure your lambdas are finished properly. Use env IO_LAMBDA_DEBUG=thread to find out the details.

Scalars leaked: 1

This is a known bug, f.ex. http://rt.perl.org/rt3//Public/Bug/Display.html?id=70974 suggests adding the @_ = (); construct at random places as a workaround.

panic: del_backref during global destruction

This is a known bug, http://rt.perl.org/rt3/Ticket/Display.html?id=70748 . I observed in on win32 only. No workaround is known however.


AnyEvent doesn't work with threads, so this module most probably won't work too when AnyEvent is selected for IO::Lambda::Loop.


IO::Lambda, threads.


Dmitry Karasik, <dmitry@karasik.eu.org>.